Merge branch 'feature/NetworkMultiplayerCooperate' into develop

.copilot-rules.md

@@ -0,0 +1,168 @@
+# Copilot Rules — Spacetris (SDL3 / C++20)
+
+These rules define **non-negotiable constraints** for all AI-assisted changes.
+They exist to preserve determinism, performance, and architecture.
+
+If these rules conflict with `.github/copilot-instructions.md`,
+**follow `.github/copilot-instructions.md`.**
+
+---
+
+## Project Constraints (Non-Negotiable)
+
+- Language: **C++20**
+- Runtime: **SDL3** + **SDL3_ttf**
+- Build system: **CMake**
+- Dependencies via **vcpkg**
+- Assets must use **relative paths only**
+- Deterministic gameplay logic is mandatory
+
+Do not rewrite or refactor working systems unless explicitly requested.
+
+---
+
+## Repo Layout & Responsibilities
+
+- Core gameplay loop/state: `src/Game.*`
+- Entry point: `src/main.cpp`
+- Text/TTF: `src/Font.*`
+- Audio: `src/Audio.*`, `src/SoundEffect.*`
+- Effects: `src/LineEffect.*`, `src/Starfield*.cpp`
+- High scores: `src/Scores.*`
+- Packaging: `build-production.ps1`
+
+When adding a module:
+- Place it under `src/` (or an established subfolder)
+- Register it in `CMakeLists.txt`
+- Avoid circular includes
+- Keep headers minimal
+
+---
+
+## Build & Verification
+
+Prefer existing scripts:
+
+- Debug: `cmake --build build-msvc --config Debug`
+- Release:
+  - Configure: `cmake -S . -B build-release -DCMAKE_BUILD_TYPE=Release`
+  - Build: `cmake --build build-release --config Release`
+- Packaging (Windows): `./build-production.ps1`
+
+Before finalizing changes:
+- Debug build must succeed
+- Packaging must succeed if assets or DLLs are touched
+
+Do not introduce new build steps unless required.
+
+---
+
+## Coding & Architecture Rules
+
+- Match local file style (naming, braces, spacing)
+- Avoid large refactors
+- Prefer small, testable helpers
+- Avoid floating-point math in core gameplay state
+- Game logic must be deterministic
+- Rendering code must not mutate game state
+
+---
+
+## Rendering & Performance Rules
+
+- Do not allocate memory per frame
+- Do not load assets during rendering
+- No blocking calls in render loop
+- Visual effects must be time-based (`deltaTime`)
+- Rendering must not contain gameplay logic
+
+---
+
+## Threading Rules
+
+- SDL main thread:
+  - Rendering
+  - Input
+  - Game simulation
+- Networking must be **non-blocking** from the SDL main loop
+  - Either run networking on a separate thread, or poll ENet frequently with a 0 timeout
+  - Never wait/spin for remote inputs on the render thread
+- Cross-thread communication via queues or buffers only
+
+---
+
+## Assets, Fonts, and Paths
+
+- Runtime expects adjacent `assets/` directory
+- `FreeSans.ttf` must remain at repo root
+- New assets:
+  - Go under `assets/`
+  - Must be included in `build-production.ps1`
+
+Never hardcode machine-specific paths.
+
+---
+
+## AI Partner (COOPERATE Mode)
+
+- AI is **supportive**, not competitive
+- AI must respect sync timing and shared grid logic
+- AI must not “cheat” or see hidden future pieces
+- AI behavior must be deterministic per seed/difficulty
+
+---
+
+## Networking (COOPERATE Network Mode)
+
+Follow `docs/ai/cooperate_network.md`.
+If `network_cooperate_multiplayer.md` exists, keep it consistent with the canonical doc.
+
+Mandatory model:
+- **Input lockstep**
+- Transmit inputs only (no board state replication)
+
+Determinism requirements:
+- Fixed tick (e.g. 60 Hz)
+- Shared RNG seed
+- Deterministic gravity, rotation, locking, scoring
+
+Technology:
+- Use **ENet**
+- Do NOT use SDL_net or TCP-only networking
+
+Architecture:
+- Networking must be isolated (e.g. `src/network/NetSession.*`)
+- Game logic must not care if partner is local, AI, or network
+
+Robustness:
+- Input delay buffer (4–6 ticks)
+- Periodic desync hashing
+- Graceful disconnect handling
+
+Do NOT implement:
+- Rollback
+- Full state sync
+- Server-authoritative sim
+- Matchmaking SDKs
+- Versus mechanics
+
+---
+
+## Agent Behavior Rules (IMPORTANT)
+
+- Always read relevant markdown specs **before coding**
+- Treat markdown specs as authoritative
+- Do not invent APIs
+- Do not assume external libraries exist
+- Generate code **file by file**, not everything at once
+- Ask before changing architecture or ownership boundaries
+
+---
+
+## When to Ask Before Proceeding
+
+Ask the maintainer if unclear:
+- UX or menu flow decisions
+- Adding dependencies
+- Refactors vs local patches
+- Platform-specific behavior

CMakeLists.txt

@@ -28,6 +28,7 @@ find_package(SDL3_ttf CONFIG REQUIRED)
 find_package(SDL3_image CONFIG REQUIRED)
 find_package(cpr CONFIG REQUIRED)
 find_package(nlohmann_json CONFIG REQUIRED)
+find_package(unofficial-enet CONFIG REQUIRED)
 
 set(TETRIS_SOURCES
     src/main.cpp
@@ -46,6 +47,7 @@ set(TETRIS_SOURCES
     src/graphics/renderers/RenderManager.cpp
     src/persistence/Scores.cpp
     src/network/supabase_client.cpp
+    src/network/NetSession.cpp
     src/graphics/effects/Starfield.cpp
     src/graphics/effects/Starfield3D.cpp
     src/graphics/effects/SpaceWarp.cpp
@@ -57,6 +59,7 @@ set(TETRIS_SOURCES
     src/audio/Audio.cpp
     src/gameplay/effects/LineEffect.cpp
     src/audio/SoundEffect.cpp
+    src/video/VideoPlayer.cpp
     src/ui/MenuLayout.cpp
     src/ui/BottomMenu.cpp
     src/app/BackgroundManager.cpp
@@ -66,6 +69,7 @@ set(TETRIS_SOURCES
     src/states/LoadingManager.cpp
     # State implementations (new)
     src/states/LoadingState.cpp
+    src/states/VideoState.cpp
     src/states/MenuState.cpp
     src/states/OptionsState.cpp
     src/states/LevelSelectorState.cpp
@@ -160,10 +164,17 @@ if(APPLE)
     endif()
 endif()
 
-target_link_libraries(spacetris PRIVATE SDL3::SDL3 SDL3_ttf::SDL3_ttf SDL3_image::SDL3_image cpr::cpr nlohmann_json::nlohmann_json)
+target_link_libraries(spacetris PRIVATE SDL3::SDL3 SDL3_ttf::SDL3_ttf SDL3_image::SDL3_image cpr::cpr nlohmann_json::nlohmann_json unofficial::enet::enet)
+
+find_package(FFMPEG REQUIRED)
+if(FFMPEG_FOUND)
+    target_include_directories(spacetris PRIVATE ${FFMPEG_INCLUDE_DIRS})
+    target_link_directories(spacetris PRIVATE ${FFMPEG_LIBRARY_DIRS})
+    target_link_libraries(spacetris PRIVATE ${FFMPEG_LIBRARIES})
+endif()
 
 if (WIN32)
-    target_link_libraries(spacetris PRIVATE mfplat mfreadwrite mfuuid)
+    target_link_libraries(spacetris PRIVATE mfplat mfreadwrite mfuuid ws2_32 winmm)
 endif()
 if(APPLE)
     # Needed for MP3 decoding via AudioToolbox on macOS
@@ -194,6 +205,7 @@ endif()
 target_include_directories(spacetris PRIVATE
     ${CMAKE_SOURCE_DIR}/src
     ${CMAKE_SOURCE_DIR}/src/audio
+    ${CMAKE_SOURCE_DIR}/src/video
     ${CMAKE_SOURCE_DIR}/src/gameplay
     ${CMAKE_SOURCE_DIR}/src/graphics
     ${CMAKE_SOURCE_DIR}/src/persistence

docs/ai/cooperate_network.md

@@ -0,0 +1,271 @@
+# Spacetris — COOPERATE Mode  
+## Network Multiplayer (2 PLAYER – NETWORK)  
+### VS Code Copilot AI Agent Prompt
+
+You are integrating **online cooperative multiplayer** into an existing **C++ / SDL3 game** called **Spacetris**.
+
+This feature extends the existing **COOPERATE mode** to support:
+- Local 2 players
+- Human + AI
+- **Human + Human over network (NEW)**
+
+The networking solution must be **deterministic, lightweight, and stable**.
+
+---
+
+## 1. High-Level Goal
+
+Add **COOPERATE 2 PLAYER (NETWORK)** mode where:
+- Two players play together over the internet
+- Each player controls one half of the shared grid
+- A line clears only when both halves are filled
+- Gameplay remains identical to local COOPERATE mode
+
+---
+
+## 2. Technology Constraints
+
+- Language: **C++**
+- Engine: **SDL3**
+- Networking: **ENet (UDP with reliability)**
+- No engine rewrite
+- No authoritative server logic required (co-op only)
+
+SDL3 is used ONLY for:
+- Rendering
+- Input
+- Timing
+
+Networking is a **separate layer**.
+
+---
+
+## 3. Network Model (MANDATORY)
+
+### Use **Input Lockstep Networking**
+
+#### Core idea:
+- Both clients run the same deterministic simulation
+- Only **player inputs** are sent over the network
+- No board state is transmitted
+- Both simulations must remain identical
+
+This model is ideal for Tetris-like games.
+
+---
+
+## 4. Determinism Requirements (CRITICAL)
+
+To ensure lockstep works:
+
+- Fixed simulation tick (e.g. 60 Hz)
+- Identical RNG seed for both clients
+- Deterministic piece generation (bag system)
+- No floating-point math in core gameplay
+- Same gravity, rotation, lock-delay logic
+- Identical line clear and scoring rules
+
+Before networking:
+- Input recording + replay must produce identical results
+
+---
+
+## 5. Network Topology
+
+### Host / Client Model (Initial Implementation)
+
+- One player hosts the game
+- One player joins
+- Host is authoritative for:
+  - RNG seed
+  - start tick
+  - game settings
+
+This is sufficient and fair for cooperative gameplay.
+
+---
+
+## 6. Network Library
+
+Use **ENet** for:
+- Reliable, ordered UDP packets
+- Low latency
+- Simple integration with C++
+
+Do NOT use:
+- SDL_net
+- TCP-only networking
+- High-level matchmaking SDKs
+
+---
+
+## 7. Network Packet Design
+
+### Input Packet (Minimal)
+
+```cpp
+struct InputPacket {
+    uint32_t tick;
+    uint8_t buttons; // bitmask
+};
+````
+
+Button bitmask example:
+
+* bit 0 → move left
+* bit 1 → move right
+* bit 2 → rotate
+* bit 3 → soft drop
+* bit 4 → hard drop
+* bit 5 → hold
+
+Packets must be:
+
+* Reliable
+* Ordered
+* Small
+
+---
+
+## 8. Tick & Latency Handling
+
+### Input Delay Buffer (RECOMMENDED)
+
+* Add fixed delay: **4–6 ticks**
+* Simulate tick `T` using inputs for `T + delay`
+* Prevents stalls due to latency spikes
+
+Strict lockstep without buffering is NOT recommended.
+
+---
+
+## 9. Desync Detection (IMPORTANT)
+
+Every N ticks (e.g. once per second):
+
+* Compute a hash of:
+
+  * Both grid halves
+  * Active pieces
+  * RNG index
+  * Score / lines / level
+* Exchange hashes
+* If mismatch:
+
+  * Log desync
+  * Stop game or mark session invalid
+
+This is required for debugging and stability.
+
+---
+
+## 10. Network Session Architecture
+
+Create a dedicated networking module:
+
+```
+/network
+  NetSession.h
+  NetSession.cpp
+```
+
+Responsibilities:
+
+* ENet host/client setup
+* Input packet send/receive
+* Tick synchronization
+* Latency buffering
+* Disconnect handling
+
+SDL main loop must NOT block on networking.
+
+---
+
+## 11. Integration with Existing COOPERATE Logic
+
+* COOPERATE grid logic stays unchanged
+* SyncLineRenderer remains unchanged
+* Scoring logic remains unchanged
+* Network layer only injects **remote inputs**
+
+Game logic should not know whether partner is:
+
+* Local human
+* AI
+* Network player
+
+---
+
+## 12. UI Integration (Menu Changes)
+
+In COOPERATE selection screen, add a new button:
+
+```
+[ LOCAL CO-OP ]   [ AI PARTNER ]   [ 2 PLAYER (NETWORK) ]
+```
+
+### On selecting 2 PLAYER (NETWORK):
+
+* Show:
+
+  * Host Game
+  * Join Game
+* Display join code or IP
+* Confirm connection before starting
+
+---
+
+## 13. Start Game Flow (Network)
+
+1. Host creates session
+2. Client connects
+3. Host sends:
+
+   * RNG seed
+   * start tick
+   * game settings
+4. Both wait until agreed start tick
+5. Simulation begins simultaneously
+
+---
+
+## 14. Disconnect & Error Handling
+
+* If connection drops:
+
+  * Pause game
+  * Show “Reconnecting…”
+  * After timeout:
+
+    * End match or switch to AI (optional)
+* Never crash
+* Never corrupt game state
+
+---
+
+## 15. What NOT to Implement
+
+* ❌ Full state synchronization
+* ❌ Prediction / rollback
+* ❌ Server-authoritative gameplay
+* ❌ Complex matchmaking
+* ❌ Versus mechanics
+
+This is cooperative, not competitive.
+
+---
+
+## 16. Acceptance Criteria
+
+* Two players can complete COOPERATE mode over network
+* Gameplay matches local COOPERATE exactly
+* No noticeable input lag under normal latency
+* Desync detection works
+* Offline / disconnect handled gracefully
+* SDL3 render loop remains smooth
+
+---
+
+## 17. Summary for Copilot
+
+Integrate networked cooperative multiplayer into Spacetris using SDL3 + C++ with ENet. Implement input lockstep networking with deterministic simulation, fixed tick rate, input buffering, and desync detection. Add a new COOPERATE menu option “2 PLAYER (NETWORK)” that allows host/join flow. Networking must be modular, non-blocking, and transparent to existing gameplay logic.

settings.ini

@@ -14,7 +14,7 @@ SmoothScroll=1
 UpRotateClockwise=0
 
 [Player]
-Name=P2
+Name=GREGOR
 
 [Debug]
 Enabled=1

src/app/TetrisApp.cpp

@@ -49,6 +49,9 @@
 #include "graphics/ui/Font.h"
 #include "graphics/ui/HelpOverlay.h"
 
+#include "network/CoopNetButtons.h"
+#include "network/NetSession.h"
+
 #include "persistence/Scores.h"
 
 #include "states/LevelSelectorState.h"
@@ -57,6 +60,7 @@
 #include "states/MenuState.h"
 #include "states/OptionsState.h"
 #include "states/PlayingState.h"
+#include "states/VideoState.h"
 #include "states/State.h"
 
 #include "ui/BottomMenu.h"
@@ -259,6 +263,12 @@ struct TetrisApp::Impl {
     double moveTimerMs = 0.0;
     double p1MoveTimerMs = 0.0;
     double p2MoveTimerMs = 0.0;
+
+    // Network coop fixed-tick state (used only when ctx.coopNetEnabled is true)
+    double coopNetAccMs = 0.0;
+    uint32_t coopNetCachedTick = 0xFFFFFFFFu;
+    uint8_t coopNetCachedButtons = 0;
+    uint32_t coopNetLastHashSentTick = 0xFFFFFFFFu;
     double DAS = 170.0;
     double ARR = 40.0;
     SDL_Rect logicalVP{0, 0, LOGICAL_W, LOGICAL_H};
@@ -301,11 +311,21 @@ struct TetrisApp::Impl {
     std::unique_ptr<StateManager> stateMgr;
     StateContext ctx{};
     std::unique_ptr<LoadingState> loadingState;
+    std::unique_ptr<VideoState> videoState;
     std::unique_ptr<MenuState> menuState;
     std::unique_ptr<OptionsState> optionsState;
     std::unique_ptr<LevelSelectorState> levelSelectorState;
     std::unique_ptr<PlayingState> playingState;
 
+    // Startup fade-in overlay (used after intro video).
+    bool startupFadeActive = false;
+    float startupFadeAlpha = 0.0f; // 0..1 black overlay strength
+    double startupFadeClockMs = 0.0;
+    static constexpr double STARTUP_FADE_IN_MS = 650.0;
+
+    // Intro video path.
+    std::string introVideoPath = "assets/videos/spacetris_intro.mp4";
+
     int init();
     void runLoop();
     void shutdown();
@@ -662,7 +682,11 @@ int TetrisApp::Impl::init()
     };
     ctx.requestFadeTransition = requestStateFade;
 
+    ctx.startupFadeActive = &startupFadeActive;
+    ctx.startupFadeAlpha = &startupFadeAlpha;
+
     loadingState = std::make_unique<LoadingState>(ctx);
+    videoState = std::make_unique<VideoState>(ctx);
     menuState = std::make_unique<MenuState>(ctx);
     optionsState = std::make_unique<OptionsState>(ctx);
     levelSelectorState = std::make_unique<LevelSelectorState>(ctx);
@@ -672,6 +696,20 @@ int TetrisApp::Impl::init()
     stateMgr->registerOnEnter(AppState::Loading, [this](){ loadingState->onEnter(); loadingStarted.store(true); });
     stateMgr->registerOnExit(AppState::Loading, [this](){ loadingState->onExit(); });
 
+    stateMgr->registerHandler(AppState::Video, [this](const SDL_Event& e){ if (videoState) videoState->handleEvent(e); });
+    stateMgr->registerOnEnter(AppState::Video, [this]() {
+        if (!videoState) return;
+        const bool ok = videoState->begin(renderer, introVideoPath);
+        if (!ok) {
+            SDL_LogWarn(SDL_LOG_CATEGORY_APPLICATION, "Intro video unavailable; skipping to Menu");
+            state = AppState::Menu;
+            stateMgr->setState(state);
+            return;
+        }
+        videoState->onEnter();
+    });
+    stateMgr->registerOnExit(AppState::Video, [this](){ if (videoState) videoState->onExit(); });
+
     stateMgr->registerHandler(AppState::Menu, [this](const SDL_Event& e){ menuState->handleEvent(e); });
     stateMgr->registerOnEnter(AppState::Menu, [this](){ menuState->onEnter(); });
     stateMgr->registerOnExit(AppState::Menu, [this](){ menuState->onExit(); });
@@ -823,7 +861,7 @@ void TetrisApp::Impl::runLoop()
                         Settings::instance().setSoundEnabled(SoundEffectManager::instance().isEnabled());
                     }
                     const bool helpToggleKey =
-                        (e.key.scancode == SDL_SCANCODE_F1 && state != AppState::Loading && state != AppState::Menu);
+                        (e.key.scancode == SDL_SCANCODE_F1 && state != AppState::Loading && state != AppState::Video && state != AppState::Menu);
                     if (helpToggleKey)
                     {
                         showHelpOverlay = !showHelpOverlay;
@@ -1149,12 +1187,31 @@ void TetrisApp::Impl::runLoop()
             }
         }
 
+        // State transitions can be triggered from render/update (e.g. menu network handshake).
+        // Keep our cached `state` in sync every frame, not only when events occur.
+        state = stateMgr->getState();
+
         Uint64 now = SDL_GetPerformanceCounter();
         double frameMs = double(now - lastMs) * 1000.0 / double(SDL_GetPerformanceFrequency());
         lastMs = now;
         if (frameMs > 100.0) frameMs = 100.0;
         gameplayBackgroundClockMs += frameMs;
 
+        if (startupFadeActive) {
+            if (startupFadeClockMs <= 0.0) {
+                startupFadeClockMs = STARTUP_FADE_IN_MS;
+                startupFadeAlpha = 1.0f;
+            }
+            startupFadeClockMs -= frameMs;
+            if (startupFadeClockMs <= 0.0) {
+                startupFadeClockMs = 0.0;
+                startupFadeAlpha = 0.0f;
+                startupFadeActive = false;
+            } else {
+                startupFadeAlpha = float(std::clamp(startupFadeClockMs / STARTUP_FADE_IN_MS, 0.0, 1.0));
+            }
+        }
+
         auto clearChallengeStory = [this]() {
             challengeStoryText.clear();
             challengeStoryLevel = 0;
@@ -1309,6 +1366,10 @@ void TetrisApp::Impl::runLoop()
 
                 if (game->isPaused()) {
                     // While paused, suppress all continuous input changes so pieces don't drift.
+                    if (ctx.coopNetEnabled && ctx.coopNetSession) {
+                        ctx.coopNetSession->poll(0);
+                        ctx.coopNetStalled = false;
+                    }
                     coopGame->setSoftDropping(CoopGame::PlayerSide::Left, false);
                     coopGame->setSoftDropping(CoopGame::PlayerSide::Right, false);
                     p1MoveTimerMs = 0.0;
@@ -1318,6 +1379,17 @@ void TetrisApp::Impl::runLoop()
                     p2LeftHeld = false;
                     p2RightHeld = false;
                 } else {
+                    const bool coopNetActive = ctx.coopNetEnabled && ctx.coopNetSession;
+
+                    // If we just entered network co-op, reset per-session fixed-tick bookkeeping.
+                    if (coopNetActive && coopNetCachedTick != 0xFFFFFFFFu && ctx.coopNetTick == 0u) {
+                        coopNetAccMs = 0.0;
+                        coopNetCachedTick = 0xFFFFFFFFu;
+                        coopNetCachedButtons = 0;
+                        coopNetLastHashSentTick = 0xFFFFFFFFu;
+                        ctx.coopNetStalled = false;
+                    }
+
                     // Define canonical key mappings for left and right players
                     const SDL_Scancode leftLeftKey = SDL_SCANCODE_A;
                     const SDL_Scancode leftRightKey = SDL_SCANCODE_D;
@@ -1327,7 +1399,194 @@ void TetrisApp::Impl::runLoop()
                     const SDL_Scancode rightRightKey = SDL_SCANCODE_RIGHT;
                     const SDL_Scancode rightDownKey = SDL_SCANCODE_DOWN;
 
-                    if (!coopVsAI) {
+                    if (coopNetActive) {
+                        // Network co-op: fixed tick lockstep.
+                        // Use a fixed dt so both peers simulate identically.
+                        static constexpr double FIXED_DT_MS = 1000.0 / 60.0;
+                        static constexpr uint32_t HASH_INTERVAL_TICKS = 60; // ~1s
+
+                        ctx.coopNetSession->poll(0);
+
+                        // If the connection drops during gameplay, abort back to menu.
+                        if (ctx.coopNetSession->state() == NetSession::ConnState::Disconnected ||
+                            ctx.coopNetSession->state() == NetSession::ConnState::Error) {
+                            const std::string reason = (ctx.coopNetSession->state() == NetSession::ConnState::Error && !ctx.coopNetSession->lastError().empty())
+                                                           ? (std::string("NET ERROR: ") + ctx.coopNetSession->lastError())
+                                                           : std::string("NET DISCONNECTED");
+                            SDL_LogWarn(SDL_LOG_CATEGORY_APPLICATION, "[NET COOP] %s", reason.c_str());
+
+                            ctx.coopNetUiStatusText = reason;
+                            ctx.coopNetUiStatusRemainingMs = 6000.0;
+                            ctx.coopNetEnabled = false;
+                            ctx.coopNetStalled = false;
+                            ctx.coopNetDesyncDetected = false;
+                            ctx.coopNetTick = 0;
+                            ctx.coopNetPendingButtons = 0;
+                            if (ctx.coopNetSession) {
+                                ctx.coopNetSession->shutdown();
+                                ctx.coopNetSession.reset();
+                            }
+
+                            // Ensure we don't remain paused due to a previous net stall/desync.
+                            if (game) {
+                                game->setPaused(false);
+                            }
+                            state = AppState::Menu;
+                            stateMgr->setState(state);
+                            continue;
+                        }
+
+                        coopNetAccMs = std::min(coopNetAccMs + frameMs, FIXED_DT_MS * 8.0);
+
+                        auto buildLocalButtons = [&]() -> uint8_t {
+                            uint8_t b = 0;
+                            if (ctx.coopNetLocalIsLeft) {
+                                if (ks[leftLeftKey]) b |= coopnet::MoveLeft;
+                                if (ks[leftRightKey]) b |= coopnet::MoveRight;
+                                if (ks[leftDownKey]) b |= coopnet::SoftDrop;
+                            } else {
+                                if (ks[rightLeftKey]) b |= coopnet::MoveLeft;
+                                if (ks[rightRightKey]) b |= coopnet::MoveRight;
+                                if (ks[rightDownKey]) b |= coopnet::SoftDrop;
+                            }
+                            b |= ctx.coopNetPendingButtons;
+                            ctx.coopNetPendingButtons = 0;
+                            return b;
+                        };
+
+                        auto applyButtonsForSide = [&](CoopGame::PlayerSide side,
+                                                       uint8_t buttons,
+                                                       bool& leftHeldPrev,
+                                                       bool& rightHeldPrev,
+                                                       double& timer) {
+                            const bool leftHeldNow = coopnet::has(buttons, coopnet::MoveLeft);
+                            const bool rightHeldNow = coopnet::has(buttons, coopnet::MoveRight);
+                            const bool downHeldNow = coopnet::has(buttons, coopnet::SoftDrop);
+
+                            coopGame->setSoftDropping(side, downHeldNow);
+
+                            int moveDir = 0;
+                            if (leftHeldNow && !rightHeldNow) moveDir = -1;
+                            else if (rightHeldNow && !leftHeldNow) moveDir = +1;
+
+                            if (moveDir != 0) {
+                                if ((moveDir == -1 && !leftHeldPrev) || (moveDir == +1 && !rightHeldPrev)) {
+                                    coopGame->move(side, moveDir);
+                                    timer = DAS;
+                                } else {
+                                    timer -= FIXED_DT_MS;
+                                    if (timer <= 0.0) {
+                                        coopGame->move(side, moveDir);
+                                        timer += ARR;
+                                    }
+                                }
+                            } else {
+                                timer = 0.0;
+                            }
+
+                            if (coopnet::has(buttons, coopnet::RotCW)) {
+                                coopGame->rotate(side, +1);
+                            }
+                            if (coopnet::has(buttons, coopnet::RotCCW)) {
+                                coopGame->rotate(side, -1);
+                            }
+                            if (coopnet::has(buttons, coopnet::HardDrop)) {
+                                SoundEffectManager::instance().playSound("hard_drop", 0.7f);
+                                coopGame->hardDrop(side);
+                            }
+                            if (coopnet::has(buttons, coopnet::Hold)) {
+                                coopGame->holdCurrent(side);
+                            }
+
+                            leftHeldPrev = leftHeldNow;
+                            rightHeldPrev = rightHeldNow;
+                        };
+
+                        const char* roleStr = ctx.coopNetIsHost ? "HOST" : "CLIENT";
+
+                        int safetySteps = 0;
+                        bool advancedTick = false;
+                        ctx.coopNetStalled = false;
+                        while (coopNetAccMs >= FIXED_DT_MS && safetySteps++ < 8) {
+                            const uint32_t tick = ctx.coopNetTick;
+
+                            if (coopNetCachedTick != tick) {
+                                coopNetCachedTick = tick;
+                                coopNetCachedButtons = buildLocalButtons();
+                                if (!ctx.coopNetSession->sendLocalInput(tick, coopNetCachedButtons)) {
+                                    SDL_LogWarn(SDL_LOG_CATEGORY_APPLICATION,
+                                                "[NET COOP][%s] sendLocalInput failed (tick=%u)",
+                                                roleStr,
+                                                tick);
+                                }
+                            }
+
+                            auto remoteButtonsOpt = ctx.coopNetSession->getRemoteButtons(tick);
+                            if (!remoteButtonsOpt.has_value()) {
+                                if (!ctx.coopNetStalled) {
+                                    SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION,
+                                                "[NET COOP][%s] STALL begin waitingForTick=%u",
+                                                roleStr,
+                                                tick);
+                                }
+                                ctx.coopNetStalled = true;
+                                break; // lockstep stall
+                            }
+
+                            const uint8_t remoteButtons = remoteButtonsOpt.value();
+                            const bool localIsLeft = ctx.coopNetLocalIsLeft;
+
+                            if (localIsLeft) {
+                                applyButtonsForSide(CoopGame::PlayerSide::Left, coopNetCachedButtons, p1LeftHeld, p1RightHeld, p1MoveTimerMs);
+                                applyButtonsForSide(CoopGame::PlayerSide::Right, remoteButtons, p2LeftHeld, p2RightHeld, p2MoveTimerMs);
+                            } else {
+                                applyButtonsForSide(CoopGame::PlayerSide::Right, coopNetCachedButtons, p2LeftHeld, p2RightHeld, p2MoveTimerMs);
+                                applyButtonsForSide(CoopGame::PlayerSide::Left, remoteButtons, p1LeftHeld, p1RightHeld, p1MoveTimerMs);
+                            }
+
+                            coopGame->tickGravity(FIXED_DT_MS);
+                            coopGame->updateVisualEffects(FIXED_DT_MS);
+
+                            if ((tick % HASH_INTERVAL_TICKS) == 0 && coopNetLastHashSentTick != tick) {
+                                coopNetLastHashSentTick = tick;
+                                const uint64_t hash = coopGame->computeStateHash();
+                                if (!ctx.coopNetSession->sendStateHash(tick, hash)) {
+                                    SDL_LogWarn(SDL_LOG_CATEGORY_APPLICATION,
+                                                "[NET COOP][%s] sendStateHash failed (tick=%u hash=0x%016llX)",
+                                                roleStr,
+                                                tick,
+                                                (unsigned long long)hash);
+                                }
+                                auto rh = ctx.coopNetSession->takeRemoteHash(tick);
+                                if (rh.has_value() && rh.value() != hash) {
+                                    ctx.coopNetDesyncDetected = true;
+                                    ctx.coopNetUiStatusText = "NET DESYNC";
+                                    ctx.coopNetUiStatusRemainingMs = 8000.0;
+                                    SDL_LogError(SDL_LOG_CATEGORY_APPLICATION,
+                                                 "[NET COOP][%s] DESYNC detected at tick=%u local=0x%016llX remote=0x%016llX",
+                                                 roleStr,
+                                                 tick,
+                                                 (unsigned long long)hash,
+                                                 (unsigned long long)rh.value());
+                                    game->setPaused(true);
+                                }
+                            }
+
+                            ctx.coopNetTick++;
+                            advancedTick = true;
+                            coopNetAccMs -= FIXED_DT_MS;
+                        }
+
+                        if (advancedTick) {
+                            if (ctx.coopNetStalled) {
+                                SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION,
+                                            "[NET COOP][%s] STALL end atTick=%u",
+                                            roleStr,
+                                            ctx.coopNetTick);
+                            }
+                            ctx.coopNetStalled = false;
+                        }
+                    } else if (!coopVsAI) {
                         // Standard two-player: left uses WASD, right uses arrow keys
                         handleSide(CoopGame::PlayerSide::Left, p1LeftHeld, p1RightHeld, p1MoveTimerMs, leftLeftKey, leftRightKey, leftDownKey);
                         handleSide(CoopGame::PlayerSide::Right, p2LeftHeld, p2RightHeld, p2MoveTimerMs, rightLeftKey, rightRightKey, rightDownKey);
@@ -1357,8 +1616,10 @@ void TetrisApp::Impl::runLoop()
                         p2RightHeld = ks[rightRightKey];
                     }
 
-                    coopGame->tickGravity(frameMs);
-                    coopGame->updateVisualEffects(frameMs);
+                    if (!coopNetActive) {
+                        coopGame->tickGravity(frameMs);
+                        coopGame->updateVisualEffects(frameMs);
+                    }
                 }
 
                 if (coopGame->isGameOver()) {
@@ -1387,6 +1648,12 @@ void TetrisApp::Impl::runLoop()
                     }
                     state = AppState::GameOver;
                     stateMgr->setState(state);
+
+                    if (ctx.coopNetSession) {
+                        ctx.coopNetSession->shutdown();
+                        ctx.coopNetSession.reset();
+                    }
+                    ctx.coopNetEnabled = false;
                 }
 
             } else {
@@ -1587,7 +1854,15 @@ void TetrisApp::Impl::runLoop()
             if (totalTasks > 0) {
                 loadingProgress = std::min(1.0, double(doneTasks) / double(totalTasks));
                 if (loadingProgress >= 1.0 && musicLoaded) {
-                    state = AppState::Menu;
+                    startupFadeActive = false;
+                    startupFadeAlpha = 0.0f;
+                    startupFadeClockMs = 0.0;
+
+                    if (std::filesystem::exists(introVideoPath)) {
+                        state = AppState::Video;
+                    } else {
+                        state = AppState::Menu;
+                    }
                     stateMgr->setState(state);
                 }
             } else {
@@ -1615,7 +1890,15 @@ void TetrisApp::Impl::runLoop()
                 if (loadingProgress > 0.99) loadingProgress = 1.0;
                 if (!musicLoaded && timeProgress >= 0.1) loadingProgress = 1.0;
                 if (loadingProgress >= 1.0 && musicLoaded) {
-                    state = AppState::Menu;
+                    startupFadeActive = false;
+                    startupFadeAlpha = 0.0f;
+                    startupFadeClockMs = 0.0;
+
+                    if (std::filesystem::exists(introVideoPath)) {
+                        state = AppState::Video;
+                    } else {
+                        state = AppState::Menu;
+                    }
                     stateMgr->setState(state);
                 }
             }
@@ -1682,6 +1965,9 @@ void TetrisApp::Impl::runLoop()
             case AppState::Loading:
                 loadingState->update(frameMs);
                 break;
+            case AppState::Video:
+                if (videoState) videoState->update(frameMs);
+                break;
             case AppState::Menu:
                 menuState->update(frameMs);
                 break;
@@ -1984,6 +2270,11 @@ void TetrisApp::Impl::runLoop()
             }
         }
         break;
+        case AppState::Video:
+            if (videoState) {
+                videoState->render(renderer, logicalScale, logicalVP);
+            }
+            break;
         case AppState::Menu:
             if (!mainScreenTex) {
                 mainScreenTex = assetLoader.getTexture(Assets::MAIN_SCREEN);
@@ -2377,6 +2668,17 @@ void TetrisApp::Impl::runLoop()
             HelpOverlay::Render(renderer, pixelFont, LOGICAL_W, LOGICAL_H, contentOffsetX, contentOffsetY);
         }
 
+        if (startupFadeActive && startupFadeAlpha > 0.0f) {
+            SDL_SetRenderViewport(renderer, nullptr);
+            SDL_SetRenderScale(renderer, 1.0f, 1.0f);
+            SDL_SetRenderDrawBlendMode(renderer, SDL_BLENDMODE_BLEND);
+            const Uint8 a = (Uint8)std::clamp((int)std::lround(startupFadeAlpha * 255.0f), 0, 255);
+            SDL_SetRenderDrawColor(renderer, 0, 0, 0, a);
+            SDL_FRect full{0.f, 0.f, (float)winW, (float)winH};
+            SDL_RenderFillRect(renderer, &full);
+            SDL_SetRenderDrawBlendMode(renderer, SDL_BLENDMODE_NONE);
+        }
+
         SDL_RenderPresent(renderer);
         SDL_SetRenderScale(renderer, 1.f, 1.f);
     }

src/core/Settings.cpp

@@ -21,7 +21,11 @@ std::string Settings::getSettingsPath() {
 bool Settings::load() {
     std::ifstream file(getSettingsPath());
     if (!file.is_open()) {
-        SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "Settings file not found, using defaults");
+        SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "Settings file not found, using defaults. Creating settings file with defaults.");
+        // Persist defaults so next run has an explicit settings.ini
+        try {
+            save();
+        } catch (...) {}
         return false;
     }
     

src/core/Settings.h

@@ -48,7 +48,8 @@ private:
     Settings& operator=(const Settings&) = delete;
     
     // Settings values
-    bool m_fullscreen = false;
+    // Default to fullscreen on first run when no settings.ini exists
+    bool m_fullscreen = true;
     bool m_musicEnabled = true;
     bool m_soundEnabled = true;
     bool m_debugEnabled = false;

src/core/application/ApplicationManager.cpp

@@ -32,9 +32,19 @@
 #include <SDL3_ttf/SDL_ttf.h>
 #include "../../utils/ImagePathResolver.h"
 #include <iostream>
+#include "../../video/VideoPlayer.h"
 #include <cmath>
 #include <fstream>
 #include <algorithm>
+#ifdef _WIN32
+#define WIN32_LEAN_AND_MEAN
+#ifndef NOMINMAX
+#define NOMINMAX
+#endif
+#include <windows.h>
+#include <shellapi.h>
+#endif
+// (Intro video playback is now handled in-process via VideoPlayer)
 
 ApplicationManager::ApplicationManager() = default;
 
@@ -55,7 +65,15 @@ void ApplicationManager::renderLoading(ApplicationManager* app, RenderManager& r
         if (winW_actual > 0 && winH_actual > 0) app->m_starfield3D->resize(winW_actual, winH_actual);
         app->m_starfield3D->draw(renderer.getSDLRenderer());
     }
-
+    // If intro video is playing, render it instead of the loading UI
+    if (app->m_introStarted && app->m_videoPlayer) {
+        SDL_Renderer* sdlR = renderer.getSDLRenderer();
+        int winW=0, winH=0; renderer.getWindowSize(winW, winH);
+        app->m_videoPlayer->render(sdlR, winW, winH);
+        SDL_SetRenderViewport(renderer.getSDLRenderer(), nullptr);
+        SDL_SetRenderScale(renderer.getSDLRenderer(), 1.0f, 1.0f);
+        return;
+    }
     SDL_Rect logicalVP = {0,0,0,0};
     float logicalScale = 1.0f;
     if (app->m_renderManager) {
@@ -780,17 +798,44 @@ void ApplicationManager::setupStateHandlers() {
                 m_starfield3D->update(deltaTime / 1000.0f);
             }
 
-            // Check if loading is complete and transition to menu
+            // Check if loading is complete and transition to next stage
             if (m_assetManager->isLoadingComplete()) {
-                SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "Loading complete, transitioning to Menu");
+                SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "Loading complete, handling post-load flow");
 
                 // Update texture pointers now that assets are loaded
                 m_stateContext.backgroundTex = m_assetManager->getTexture("background");
                 m_stateContext.blocksTex = m_assetManager->getTexture("blocks");
 
-                bool ok = m_stateManager->setState(AppState::Menu);
-                SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "setState(AppState::Menu) returned %d", ok ? 1 : 0);
-                traceFile("- to Menu returned");
+                            // If an intro video exists and hasn't been started, attempt to play it in-process
+                            std::filesystem::path introPath = m_introPath;
+                            if (!m_introStarted && std::filesystem::exists(introPath)) {
+                                SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "Intro video found: %s", introPath.string().c_str());
+                                try {
+                                    if (!m_videoPlayer) m_videoPlayer = std::make_unique<VideoPlayer>();
+                                    SDL_Renderer* sdlRend = (m_renderManager) ? m_renderManager->getSDLRenderer() : nullptr;
+                                    if (m_videoPlayer->open(introPath.string(), sdlRend)) {
+                                        m_introStarted = true;
+                                        SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "Intro video started in-process");
+                                    } else {
+                                        SDL_LogWarn(SDL_LOG_CATEGORY_APPLICATION, "VideoPlayer failed to open intro; skipping");
+                                        m_stateManager->setState(AppState::Playing);
+                                    }
+                                } catch (const std::exception& ex) {
+                                    SDL_LogWarn(SDL_LOG_CATEGORY_APPLICATION, "Exception while starting VideoPlayer: %s", ex.what());
+                                    m_stateManager->setState(AppState::Playing);
+                                }
+                            } else if (m_introStarted) {
+                                // Let VideoPlayer decode frames; once finished, transition to playing
+                                if (m_videoPlayer) m_videoPlayer->update();
+                                if (!m_videoPlayer || m_videoPlayer->isFinished()) {
+                                    SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "Intro video finished (in-process), transitioning to Playing");
+                                    m_stateManager->setState(AppState::Playing);
+                                }
+                            } else {
+                                // No intro to play; transition directly to Playing
+                                SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "No intro video; transitioning to Playing");
+                                m_stateManager->setState(AppState::Playing);
+                            }
             }
         });
 

src/core/application/ApplicationManager.h

@@ -153,6 +153,11 @@ private:
     float m_logoAnimCounter = 0.0f;
     bool m_helpOverlayPausedGame = false;
 
+    // Intro video playback (in-process via FFmpeg)
+    bool m_introStarted = false;
+    std::string m_introPath = "assets/videos/spacetris_intro.mp4";
+    std::unique_ptr<class VideoPlayer> m_videoPlayer;
+
     // Gameplay background (per-level) with fade, mirroring main.cpp behavior
     SDL_Texture* m_levelBackgroundTex = nullptr;
     SDL_Texture* m_nextLevelBackgroundTex = nullptr; // used during fade transitions

src/core/state/StateManager.cpp

@@ -156,9 +156,19 @@ void StateManager::render(RenderManager& renderer) {
 }
 
 bool StateManager::isValidState(AppState state) const {
-    // All enum values are currently valid
-    return static_cast<int>(state) >= static_cast<int>(AppState::Loading) &&
-           static_cast<int>(state) <= static_cast<int>(AppState::GameOver);
+    switch (state) {
+        case AppState::Loading:
+        case AppState::Video:
+        case AppState::Menu:
+        case AppState::Options:
+        case AppState::LevelSelector:
+        case AppState::Playing:
+        case AppState::LevelSelect:
+        case AppState::GameOver:
+            return true;
+        default:
+            return false;
+    }
 }
 
 bool StateManager::canTransitionTo(AppState newState) const {
@@ -169,6 +179,7 @@ bool StateManager::canTransitionTo(AppState newState) const {
 const char* StateManager::getStateName(AppState state) const {
     switch (state) {
         case AppState::Loading: return "Loading";
+        case AppState::Video: return "Video";
         case AppState::Menu: return "Menu";
         case AppState::Options: return "Options";
         case AppState::LevelSelector: return "LevelSelector";

src/core/state/StateManager.h

@@ -12,6 +12,7 @@ class RenderManager;
 // Application states used across the app
 enum class AppState {
     Loading,
+    Video,
     Menu,
     Options,
     LevelSelector,

src/gameplay/coop/CoopGame.cpp

@@ -2,6 +2,7 @@
 
 #include <algorithm>
 #include <cmath>
+#include <cstring>
 
 namespace {
 // NES (NTSC) gravity table reused from single-player for level progression (ms per cell)
@@ -41,7 +42,23 @@ CoopGame::CoopGame(int startLevel_) {
     reset(startLevel_);
 }
 
-void CoopGame::reset(int startLevel_) {
+namespace {
+uint64_t fnv1a64(uint64_t h, const void* data, size_t size) {
+    const uint8_t* p = static_cast<const uint8_t*>(data);
+    for (size_t i = 0; i < size; ++i) {
+        h ^= static_cast<uint64_t>(p[i]);
+        h *= 1099511628211ull;
+    }
+    return h;
+}
+
+template <typename T>
+uint64_t hashPod(uint64_t h, const T& v) {
+    return fnv1a64(h, &v, sizeof(T));
+}
+}
+
+void CoopGame::resetInternal(int startLevel_, const std::optional<uint32_t>& seedOpt) {
     std::fill(board.begin(), board.end(), Cell{});
     rowStates.fill(RowHalfState{});
     completedLines.clear();
@@ -60,7 +77,7 @@ void CoopGame::reset(int startLevel_) {
     left = PlayerState{};
     right = PlayerState{ PlayerSide::Right };
 
-    auto initPlayer = [&](PlayerState& ps) {
+    auto initPlayer = [&](PlayerState& ps, uint32_t seed) {
         ps.canHold = true;
         ps.hold.type = PIECE_COUNT;
         ps.softDropping = false;
@@ -77,16 +94,34 @@ void CoopGame::reset(int startLevel_) {
         ps.comboCount = 0;
         ps.bag.clear();
         ps.next.type = PIECE_COUNT;
+        ps.rng.seed(seed);
         refillBag(ps);
     };
-    initPlayer(left);
-    initPlayer(right);
+
+    if (seedOpt.has_value()) {
+        const uint32_t seed = seedOpt.value();
+        initPlayer(left, seed);
+        initPlayer(right, seed ^ 0x9E3779B9u);
+    } else {
+        // Preserve existing behavior: random seed when not in deterministic mode.
+        std::random_device rd;
+        initPlayer(left, static_cast<uint32_t>(rd()));
+        initPlayer(right, static_cast<uint32_t>(rd()));
+    }
 
     spawn(left);
     spawn(right);
     updateRowStates();
 }
 
+void CoopGame::reset(int startLevel_) {
+    resetInternal(startLevel_, std::nullopt);
+}
+
+void CoopGame::resetDeterministic(int startLevel_, uint32_t seed) {
+    resetInternal(startLevel_, seed);
+}
+
 void CoopGame::setSoftDropping(PlayerSide side, bool on) {
     PlayerState& ps = player(side);
     auto stepFor = [&](bool soft)->double { return soft ? std::max(5.0, gravityMs / 5.0) : gravityMs; };
@@ -103,6 +138,74 @@ void CoopGame::setSoftDropping(PlayerSide side, bool on) {
     ps.softDropping = on;
 }
 
+uint64_t CoopGame::computeStateHash() const {
+    uint64_t h = 1469598103934665603ull;
+
+    // Board
+    for (const auto& c : board) {
+        const uint8_t occ = c.occupied ? 1u : 0u;
+        const uint8_t owner = (c.owner == PlayerSide::Left) ? 0u : 1u;
+        const uint8_t val = static_cast<uint8_t>(std::clamp(c.value, 0, 255));
+        h = hashPod(h, occ);
+        h = hashPod(h, owner);
+        h = hashPod(h, val);
+    }
+
+    auto hashPiece = [&](const Piece& p) {
+        const uint8_t type = static_cast<uint8_t>(p.type);
+        const int32_t rot = p.rot;
+        const int32_t x = p.x;
+        const int32_t y = p.y;
+        h = hashPod(h, type);
+        h = hashPod(h, rot);
+        h = hashPod(h, x);
+        h = hashPod(h, y);
+    };
+
+    auto hashPlayer = [&](const PlayerState& ps) {
+        const uint8_t side = (ps.side == PlayerSide::Left) ? 0u : 1u;
+        h = hashPod(h, side);
+        hashPiece(ps.cur);
+        hashPiece(ps.next);
+        hashPiece(ps.hold);
+        const uint8_t canHoldB = ps.canHold ? 1u : 0u;
+        const uint8_t toppedOutB = ps.toppedOut ? 1u : 0u;
+        h = hashPod(h, canHoldB);
+        h = hashPod(h, toppedOutB);
+        h = hashPod(h, ps.score);
+        h = hashPod(h, ps.lines);
+        h = hashPod(h, ps.level);
+        h = hashPod(h, ps.tetrisesMade);
+        h = hashPod(h, ps.currentCombo);
+        h = hashPod(h, ps.maxCombo);
+        h = hashPod(h, ps.comboCount);
+        h = hashPod(h, ps.pieceSeq);
+
+        const uint32_t bagSize = static_cast<uint32_t>(ps.bag.size());
+        h = hashPod(h, bagSize);
+        for (auto t : ps.bag) {
+            const uint8_t tt = static_cast<uint8_t>(t);
+            h = hashPod(h, tt);
+        }
+    };
+
+    hashPlayer(left);
+    hashPlayer(right);
+
+    // Session-wide counters/stats
+    h = hashPod(h, _score);
+    h = hashPod(h, _lines);
+    h = hashPod(h, _level);
+    h = hashPod(h, _tetrisesMade);
+    h = hashPod(h, _currentCombo);
+    h = hashPod(h, _maxCombo);
+    h = hashPod(h, _comboCount);
+    h = hashPod(h, startLevel);
+    h = hashPod(h, pieceSequence);
+
+    return h;
+}
+
 void CoopGame::move(PlayerSide side, int dx) {
     PlayerState& ps = player(side);
     if (gameOver || ps.toppedOut) return;

src/gameplay/coop/CoopGame.h

@@ -62,9 +62,13 @@ public:
     void setLevelUpCallback(LevelUpCallback cb) { levelUpCallback = cb; }
 
     void reset(int startLevel = 0);
+    void resetDeterministic(int startLevel, uint32_t seed);
     void tickGravity(double frameMs);
     void updateVisualEffects(double frameMs);
 
+    // Determinism / desync detection
+    uint64_t computeStateHash() const;
+
     // Per-player inputs -----------------------------------------------------
     void setSoftDropping(PlayerSide side, bool on);
     void move(PlayerSide side, int dx);
@@ -111,6 +115,8 @@ public:
 private:
     static constexpr double LOCK_DELAY_MS = 500.0;
 
+    void resetInternal(int startLevel_, const std::optional<uint32_t>& seedOpt);
+
     std::array<Cell, COLS * ROWS> board{};
     std::array<RowHalfState, ROWS> rowStates{};
     PlayerState left{};

src/network/CoopNetButtons.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <cstdint>
+
+namespace coopnet {
+// 8-bit input mask carried in NetSession::InputFrame.
+// Keep in sync across capture/apply on both peers.
+enum Buttons : uint8_t {
+    MoveLeft  = 1u << 0,
+    MoveRight = 1u << 1,
+    SoftDrop  = 1u << 2,
+    RotCW     = 1u << 3,
+    RotCCW    = 1u << 4,
+    HardDrop  = 1u << 5,
+    Hold      = 1u << 6,
+};
+
+inline bool has(uint8_t mask, Buttons b) {
+    return (mask & static_cast<uint8_t>(b)) != 0;
+}
+}

src/network/NetSession.cpp

@@ -0,0 +1,324 @@
+#include "NetSession.h"
+
+#include <enet/enet.h>
+
+#include <SDL3/SDL.h>
+
+#include <cstring>
+
+namespace {
+constexpr uint8_t kChannelReliable = 0;
+
+static bool netLogVerboseEnabled() {
+    // Set environment variable / hint: SPACETRIS_NET_LOG=1
+    const char* v = SDL_GetHint("SPACETRIS_NET_LOG");
+    return v && v[0] == '1';
+}
+
+template <typename T>
+static void append(std::vector<uint8_t>& out, const T& value) {
+    const uint8_t* p = reinterpret_cast<const uint8_t*>(&value);
+    out.insert(out.end(), p, p + sizeof(T));
+}
+
+template <typename T>
+static bool read(const uint8_t* data, size_t size, size_t& off, T& out) {
+    if (off + sizeof(T) > size) return false;
+    std::memcpy(&out, data + off, sizeof(T));
+    off += sizeof(T);
+    return true;
+}
+}
+
+NetSession::NetSession() = default;
+
+NetSession::~NetSession() {
+    shutdown();
+}
+
+bool NetSession::ensureEnetInitialized() {
+    static bool s_inited = false;
+    if (s_inited) return true;
+    if (enet_initialize() != 0) {
+        setError("enet_initialize failed");
+        m_state = ConnState::Error;
+        return false;
+    }
+    s_inited = true;
+    return true;
+}
+
+void NetSession::setError(const std::string& msg) {
+    m_lastError = msg;
+}
+
+bool NetSession::host(const std::string& bindHost, uint16_t port) {
+    shutdown();
+    if (!ensureEnetInitialized()) return false;
+
+    SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "[NET] host(bind='%s', port=%u)", bindHost.c_str(), (unsigned)port);
+
+    ENetAddress address{};
+    address.host = ENET_HOST_ANY;
+    address.port = port;
+
+    if (!bindHost.empty() && bindHost != "0.0.0.0") {
+        if (enet_address_set_host(&address, bindHost.c_str()) != 0) {
+            setError("enet_address_set_host (bind) failed");
+            m_state = ConnState::Error;
+            return false;
+        }
+    }
+
+    // 1 peer, 2 channels (reserve extra)
+    m_host = enet_host_create(&address, 1, 2, 0, 0);
+    if (!m_host) {
+        setError("enet_host_create (host) failed");
+        m_state = ConnState::Error;
+        return false;
+    }
+
+    m_mode = Mode::Host;
+    m_state = ConnState::Connecting;
+    return true;
+}
+
+bool NetSession::join(const std::string& hostNameOrIp, uint16_t port) {
+    shutdown();
+    if (!ensureEnetInitialized()) return false;
+
+    SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "[NET] join(remote='%s', port=%u)", hostNameOrIp.c_str(), (unsigned)port);
+
+    m_host = enet_host_create(nullptr, 1, 2, 0, 0);
+    if (!m_host) {
+        setError("enet_host_create (client) failed");
+        m_state = ConnState::Error;
+        return false;
+    }
+
+    ENetAddress address{};
+    if (enet_address_set_host(&address, hostNameOrIp.c_str()) != 0) {
+        setError("enet_address_set_host failed");
+        m_state = ConnState::Error;
+        return false;
+    }
+    address.port = port;
+
+    m_peer = enet_host_connect(m_host, &address, 2, 0);
+    if (!m_peer) {
+        setError("enet_host_connect failed");
+        m_state = ConnState::Error;
+        return false;
+    }
+
+    m_mode = Mode::Client;
+    m_state = ConnState::Connecting;
+    return true;
+}
+
+void NetSession::shutdown() {
+    if (m_host || m_peer) {
+        SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "[NET] shutdown(mode=%d state=%d)", (int)m_mode, (int)m_state);
+    }
+
+    m_remoteInputs.clear();
+    m_remoteHashes.clear();
+    m_receivedHandshake.reset();
+
+    m_inputsSent = 0;
+    m_inputsReceived = 0;
+    m_hashesSent = 0;
+    m_hashesReceived = 0;
+    m_handshakesSent = 0;
+    m_handshakesReceived = 0;
+    m_lastRecvInputTick = 0xFFFFFFFFu;
+    m_lastRecvHashTick = 0xFFFFFFFFu;
+    m_lastStatsLogMs = 0;
+
+    if (m_peer) {
+        enet_peer_disconnect(m_peer, 0);
+        m_peer = nullptr;
+    }
+
+    if (m_host) {
+        enet_host_destroy(m_host);
+        m_host = nullptr;
+    }
+
+    m_mode = Mode::None;
+    m_state = ConnState::Disconnected;
+    m_lastError.clear();
+}
+
+void NetSession::poll(uint32_t timeoutMs) {
+    if (!m_host) return;
+
+    ENetEvent event{};
+    while (enet_host_service(m_host, &event, static_cast<enet_uint32>(timeoutMs)) > 0) {
+        switch (event.type) {
+            case ENET_EVENT_TYPE_CONNECT:
+                m_peer = event.peer;
+                m_state = ConnState::Connected;
+                SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "[NET] connected (mode=%d)", (int)m_mode);
+                break;
+            case ENET_EVENT_TYPE_RECEIVE:
+                if (event.packet) {
+                    handlePacket(event.packet->data, event.packet->dataLength);
+                    enet_packet_destroy(event.packet);
+                }
+                break;
+            case ENET_EVENT_TYPE_DISCONNECT:
+                m_peer = nullptr;
+                m_state = ConnState::Disconnected;
+                SDL_LogWarn(SDL_LOG_CATEGORY_APPLICATION, "[NET] disconnected");
+                break;
+            case ENET_EVENT_TYPE_NONE:
+            default:
+                break;
+        }
+
+        // After first event, do non-blocking passes.
+        timeoutMs = 0;
+    }
+
+    // Rate-limited stats log (opt-in)
+    if (netLogVerboseEnabled()) {
+        const uint32_t nowMs = SDL_GetTicks();
+        if (m_lastStatsLogMs == 0) m_lastStatsLogMs = nowMs;
+        if (nowMs - m_lastStatsLogMs >= 1000u) {
+            m_lastStatsLogMs = nowMs;
+            SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION,
+                        "[NET] stats: sent(in=%u hash=%u hs=%u) recv(in=%u hash=%u hs=%u) lastRecv(inTick=%u hashTick=%u) state=%d",
+                        m_inputsSent,
+                        m_hashesSent,
+                        m_handshakesSent,
+                        m_inputsReceived,
+                        m_hashesReceived,
+                        m_handshakesReceived,
+                        m_lastRecvInputTick,
+                        m_lastRecvHashTick,
+                        (int)m_state);
+        }
+    }
+}
+
+bool NetSession::sendBytesReliable(const void* data, size_t size) {
+    if (!m_peer) return false;
+    ENetPacket* packet = enet_packet_create(data, size, ENET_PACKET_FLAG_RELIABLE);
+    if (!packet) return false;
+    if (enet_peer_send(m_peer, kChannelReliable, packet) != 0) {
+        enet_packet_destroy(packet);
+        return false;
+    }
+    // Let the caller decide flush cadence; but for tiny control packets, flushing is cheap.
+    enet_host_flush(m_host);
+    return true;
+}
+
+bool NetSession::sendHandshake(const Handshake& hs) {
+    if (m_mode != Mode::Host) return false;
+    m_handshakesSent++;
+    SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "[NET] sendHandshake(seed=%u startTick=%u startLevel=%u)", hs.rngSeed, hs.startTick, (unsigned)hs.startLevel);
+    std::vector<uint8_t> buf;
+    buf.reserve(1 + sizeof(uint32_t) * 2 + sizeof(uint8_t));
+    buf.push_back(static_cast<uint8_t>(MsgType::Handshake));
+    append(buf, hs.rngSeed);
+    append(buf, hs.startTick);
+    append(buf, hs.startLevel);
+    return sendBytesReliable(buf.data(), buf.size());
+}
+
+std::optional<NetSession::Handshake> NetSession::takeReceivedHandshake() {
+    auto out = m_receivedHandshake;
+    m_receivedHandshake.reset();
+    return out;
+}
+
+bool NetSession::sendLocalInput(uint32_t tick, uint8_t buttons) {
+    m_inputsSent++;
+    if (netLogVerboseEnabled() && (tick % 60u) == 0u) {
+        SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "[NET] sendInput(tick=%u buttons=0x%02X)", tick, (unsigned)buttons);
+    }
+    std::vector<uint8_t> buf;
+    buf.reserve(1 + sizeof(uint32_t) + sizeof(uint8_t));
+    buf.push_back(static_cast<uint8_t>(MsgType::Input));
+    append(buf, tick);
+    append(buf, buttons);
+    return sendBytesReliable(buf.data(), buf.size());
+}
+
+std::optional<uint8_t> NetSession::getRemoteButtons(uint32_t tick) const {
+    auto it = m_remoteInputs.find(tick);
+    if (it == m_remoteInputs.end()) return std::nullopt;
+    return it->second;
+}
+
+bool NetSession::sendStateHash(uint32_t tick, uint64_t hash) {
+    m_hashesSent++;
+    if (netLogVerboseEnabled()) {
+        SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "[NET] sendHash(tick=%u hash=%llu)", tick, (unsigned long long)hash);
+    }
+    std::vector<uint8_t> buf;
+    buf.reserve(1 + sizeof(uint32_t) + sizeof(uint64_t));
+    buf.push_back(static_cast<uint8_t>(MsgType::Hash));
+    append(buf, tick);
+    append(buf, hash);
+    return sendBytesReliable(buf.data(), buf.size());
+}
+
+std::optional<uint64_t> NetSession::takeRemoteHash(uint32_t tick) {
+    auto it = m_remoteHashes.find(tick);
+    if (it == m_remoteHashes.end()) return std::nullopt;
+    uint64_t v = it->second;
+    m_remoteHashes.erase(it);
+    return v;
+}
+
+void NetSession::handlePacket(const uint8_t* data, size_t size) {
+    if (!data || size < 1) return;
+    size_t off = 0;
+    uint8_t typeByte = 0;
+    if (!read(data, size, off, typeByte)) return;
+
+    MsgType t = static_cast<MsgType>(typeByte);
+    switch (t) {
+        case MsgType::Handshake: {
+            Handshake hs{};
+            if (!read(data, size, off, hs.rngSeed)) return;
+            if (!read(data, size, off, hs.startTick)) return;
+            if (!read(data, size, off, hs.startLevel)) return;
+            m_receivedHandshake = hs;
+            m_handshakesReceived++;
+            SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "[NET] recvHandshake(seed=%u startTick=%u startLevel=%u)", hs.rngSeed, hs.startTick, (unsigned)hs.startLevel);
+            break;
+        }
+        case MsgType::Input: {
+            uint32_t tick = 0;
+            uint8_t buttons = 0;
+            if (!read(data, size, off, tick)) return;
+            if (!read(data, size, off, buttons)) return;
+            m_remoteInputs[tick] = buttons;
+            m_inputsReceived++;
+            m_lastRecvInputTick = tick;
+            if (netLogVerboseEnabled() && (tick % 60u) == 0u) {
+                SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "[NET] recvInput(tick=%u buttons=0x%02X)", tick, (unsigned)buttons);
+            }
+            break;
+        }
+        case MsgType::Hash: {
+            uint32_t tick = 0;
+            uint64_t hash = 0;
+            if (!read(data, size, off, tick)) return;
+            if (!read(data, size, off, hash)) return;
+            m_remoteHashes[tick] = hash;
+            m_hashesReceived++;
+            m_lastRecvHashTick = tick;
+            if (netLogVerboseEnabled()) {
+                SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "[NET] recvHash(tick=%u hash=%llu)", tick, (unsigned long long)hash);
+            }
+            break;
+        }
+        default:
+            break;
+    }
+}

src/network/NetSession.h

@@ -0,0 +1,118 @@
+#pragma once
+
+#include <cstdint>
+#include <optional>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+struct _ENetHost;
+struct _ENetPeer;
+
+// Lockstep networking session for COOPERATE (network) mode.
+//
+// Design goals:
+// - Non-blocking polling (caller drives poll from the main loop)
+// - Reliable, ordered delivery for inputs and control messages
+// - Host provides seed + start tick (handshake)
+// - Only inputs/state hashes are exchanged (no board sync)
+class NetSession {
+public:
+    enum class Mode {
+        None,
+        Host,
+        Client,
+    };
+
+    enum class ConnState {
+        Disconnected,
+        Connecting,
+        Connected,
+        Error,
+    };
+
+    struct Handshake {
+        uint32_t rngSeed = 0;
+        uint32_t startTick = 0;
+        uint8_t startLevel = 0;
+    };
+
+    struct InputFrame {
+        uint32_t tick = 0;
+        uint8_t buttons = 0;
+    };
+
+    NetSession();
+    ~NetSession();
+
+    NetSession(const NetSession&) = delete;
+    NetSession& operator=(const NetSession&) = delete;
+
+    // If bindHost is empty or "0.0.0.0", binds to ENET_HOST_ANY.
+    bool host(const std::string& bindHost, uint16_t port);
+    bool join(const std::string& hostNameOrIp, uint16_t port);
+    void shutdown();
+
+    void poll(uint32_t timeoutMs = 0);
+
+    Mode mode() const { return m_mode; }
+    ConnState state() const { return m_state; }
+    bool isConnected() const { return m_state == ConnState::Connected; }
+
+    // Host-only: send handshake once the peer connects.
+    bool sendHandshake(const Handshake& hs);
+
+    // Client-only: becomes available once received from host.
+    std::optional<Handshake> takeReceivedHandshake();
+
+    // Input exchange --------------------------------------------------------
+    // Send local input for a given simulation tick.
+    bool sendLocalInput(uint32_t tick, uint8_t buttons);
+
+    // Returns the last received remote input for a tick (if any).
+    std::optional<uint8_t> getRemoteButtons(uint32_t tick) const;
+
+    // Hash exchange (for desync detection) ---------------------------------
+    bool sendStateHash(uint32_t tick, uint64_t hash);
+    std::optional<uint64_t> takeRemoteHash(uint32_t tick);
+
+    // Diagnostics
+    std::string lastError() const { return m_lastError; }
+
+private:
+    enum class MsgType : uint8_t {
+        Handshake = 1,
+        Input = 2,
+        Hash = 3,
+    };
+
+    bool ensureEnetInitialized();
+    void setError(const std::string& msg);
+
+    bool sendBytesReliable(const void* data, size_t size);
+    void handlePacket(const uint8_t* data, size_t size);
+
+    Mode m_mode = Mode::None;
+    ConnState m_state = ConnState::Disconnected;
+
+    _ENetHost* m_host = nullptr;
+    _ENetPeer* m_peer = nullptr;
+
+    std::string m_lastError;
+
+    std::optional<Handshake> m_receivedHandshake;
+
+    std::unordered_map<uint32_t, uint8_t> m_remoteInputs;
+    std::unordered_map<uint32_t, uint64_t> m_remoteHashes;
+
+    // Debug logging (rate-limited)
+    uint32_t m_inputsSent = 0;
+    uint32_t m_inputsReceived = 0;
+    uint32_t m_hashesSent = 0;
+    uint32_t m_hashesReceived = 0;
+    uint32_t m_handshakesSent = 0;
+    uint32_t m_handshakesReceived = 0;
+    uint32_t m_lastRecvInputTick = 0xFFFFFFFFu;
+    uint32_t m_lastRecvHashTick = 0xFFFFFFFFu;
+    uint32_t m_lastStatsLogMs = 0;
+};

src/states/MenuState.cpp

@@ -1,6 +1,7 @@
 #include "MenuState.h"
 #include "persistence/Scores.h"
 #include "../network/supabase_client.h"
+#include "../network/NetSession.h"
 #include "graphics/Font.h"
 #include "../graphics/ui/HelpOverlay.h"
 #include "../core/GlobalState.h"
@@ -16,6 +17,7 @@
 #include <array>
 #include <cmath>
 #include <vector>
+#include <random>
 
 // Use dynamic logical dimensions from GlobalState instead of hardcoded values
 // This allows the UI to adapt when the window is resized or goes fullscreen
@@ -141,6 +143,17 @@ void MenuState::showCoopSetupPanel(bool show, bool resumeMusic) {
             coopSetupAnimating = true;
             coopSetupDirection = 1;
             coopSetupSelected = (ctx.coopVsAI && *ctx.coopVsAI) ? 1 : 0;
+            coopSetupStep = CoopSetupStep::ChoosePartner;
+            coopNetworkRoleSelected = 0;
+            coopNetworkHandshakeSent = false;
+            coopNetworkStatusText.clear();
+            if (SDL_Window* focusWin = SDL_GetKeyboardFocus()) {
+                SDL_StopTextInput(focusWin);
+            }
+            if (coopNetworkSession) {
+                coopNetworkSession->shutdown();
+                coopNetworkSession.reset();
+            }
             coopSetupRectsValid = false;
             selectedButton = static_cast<int>(ui::BottomMenuItem::Cooperate);
             // Ensure the transition value is non-zero so render code can show
@@ -152,6 +165,19 @@ void MenuState::showCoopSetupPanel(bool show, bool resumeMusic) {
             coopSetupAnimating = true;
             coopSetupDirection = -1;
             coopSetupRectsValid = false;
+
+            if (SDL_Window* focusWin = SDL_GetKeyboardFocus()) {
+                SDL_StopTextInput(focusWin);
+            }
+
+            // Cancel any pending network session if the coop setup is being closed.
+            if (coopNetworkSession) {
+                coopNetworkSession->shutdown();
+                coopNetworkSession.reset();
+            }
+            coopNetworkHandshakeSent = false;
+            coopNetworkStatusText.clear();
+            coopSetupStep = CoopSetupStep::ChoosePartner;
             // Resume menu music only when requested (ESC should pass resumeMusic=false)
             if (resumeMusic && ctx.musicEnabled && *ctx.musicEnabled) {
                 Audio::instance().playMenuMusic();
@@ -280,58 +306,196 @@ void MenuState::onExit() {
 }
 
 void MenuState::handleEvent(const SDL_Event& e) {
+    // Text input for network IP entry (only while coop setup panel is active).
+    if ((coopSetupVisible || coopSetupAnimating) && coopSetupTransition > 0.0 && e.type == SDL_EVENT_TEXT_INPUT) {
+        if (coopSetupStep == CoopSetupStep::NetworkEnterAddress) {
+            std::string& target = (coopNetworkRoleSelected == 0) ? coopNetworkBindAddress : coopNetworkJoinAddress;
+            if (target.size() < 64) {
+                target += e.text.text;
+            }
+            return;
+        }
+    }
+
     // Coop setup panel navigation (modal within the menu)
     // Handle this FIRST and consume key events so the main menu navigation doesn't interfere.
     // Note: Do not require !repeat here; some keyboards/OS configs may emit Enter with repeat.
     if ((coopSetupVisible || coopSetupAnimating) && coopSetupTransition > 0.0 && e.type == SDL_EVENT_KEY_DOWN) {
+        // Coop setup panel navigation (modal within the menu)
         switch (e.key.scancode) {
+            case SDL_SCANCODE_UP:
+            case SDL_SCANCODE_DOWN:
+                // Do NOT allow up/down to change anything while this panel is active
+                return;
+            case SDL_SCANCODE_ESCAPE:
+                // When in a nested network step, go back one step; otherwise close the panel.
+                if (coopSetupStep == CoopSetupStep::NetworkChooseRole) {
+                    coopSetupStep = CoopSetupStep::ChoosePartner;
+                    coopNetworkHandshakeSent = false;
+                    coopNetworkStatusText.clear();
+                    return;
+                }
+                if (coopSetupStep == CoopSetupStep::NetworkEnterAddress) {
+                    if (SDL_Window* focusWin = SDL_GetKeyboardFocus()) {
+                        SDL_StopTextInput(focusWin);
+                    }
+                    coopSetupStep = CoopSetupStep::NetworkChooseRole;
+                    coopNetworkHandshakeSent = false;
+                    coopNetworkStatusText.clear();
+                    return;
+                }
+                if (coopSetupStep == CoopSetupStep::NetworkWaiting) {
+                    if (coopNetworkSession) {
+                        coopNetworkSession->shutdown();
+                        coopNetworkSession.reset();
+                    }
+                    if (SDL_Window* focusWin = SDL_GetKeyboardFocus()) {
+                        SDL_StopTextInput(focusWin);
+                    }
+                    coopNetworkHandshakeSent = false;
+                    coopNetworkStatusText.clear();
+                    coopSetupStep = CoopSetupStep::NetworkChooseRole;
+                    return;
+                }
+                showCoopSetupPanel(false, false);
+                return;
             case SDL_SCANCODE_LEFT:
             case SDL_SCANCODE_A:
-                coopSetupSelected = 0;
-                buttonFlash = 1.0;
+                if (coopSetupStep == CoopSetupStep::ChoosePartner) {
+                    // 3-way selection: LOCAL / AI / NETWORK
+                    coopSetupSelected = (coopSetupSelected + 3 - 1) % 3;
+                    buttonFlash = 1.0;
+                    return;
+                }
+                if (coopSetupStep == CoopSetupStep::NetworkChooseRole) {
+                    coopNetworkRoleSelected = (coopNetworkRoleSelected + 2 - 1) % 2;
+                    buttonFlash = 1.0;
+                    return;
+                }
+                if (coopSetupStep == CoopSetupStep::NetworkEnterAddress) {
+                    return;
+                }
                 return;
             case SDL_SCANCODE_RIGHT:
             case SDL_SCANCODE_D:
-                coopSetupSelected = 1;
-                buttonFlash = 1.0;
-                return;
-            // Do NOT allow up/down to change anything
-            case SDL_SCANCODE_UP:
-            case SDL_SCANCODE_DOWN:
-                return;
-            case SDL_SCANCODE_ESCAPE:
-                showCoopSetupPanel(false, false);
+                if (coopSetupStep == CoopSetupStep::ChoosePartner) {
+                    coopSetupSelected = (coopSetupSelected + 1) % 3;
+                    buttonFlash = 1.0;
+                    return;
+                }
+                if (coopSetupStep == CoopSetupStep::NetworkChooseRole) {
+                    coopNetworkRoleSelected = (coopNetworkRoleSelected + 1) % 2;
+                    buttonFlash = 1.0;
+                    return;
+                }
+                if (coopSetupStep == CoopSetupStep::NetworkEnterAddress) {
+                    return;
+                }
                 return;
+            case SDL_SCANCODE_BACKSPACE:
+                if (coopSetupStep == CoopSetupStep::NetworkEnterAddress) {
+                    std::string& target = (coopNetworkRoleSelected == 0) ? coopNetworkBindAddress : coopNetworkJoinAddress;
+                    if (!target.empty()) target.pop_back();
+                    return;
+                }
+                break;
             case SDL_SCANCODE_RETURN:
             case SDL_SCANCODE_KP_ENTER:
             case SDL_SCANCODE_SPACE:
             {
-                const bool useAI = (coopSetupSelected == 1);
-                if (ctx.coopVsAI) {
-                    *ctx.coopVsAI = useAI;
-                }
-                if (ctx.game) {
-                    ctx.game->setMode(GameMode::Cooperate);
-                    ctx.game->reset(ctx.startLevelSelection ? *ctx.startLevelSelection : 0);
-                }
-                if (ctx.coopGame) {
-                    ctx.coopGame->reset(ctx.startLevelSelection ? *ctx.startLevelSelection : 0);
+                // Existing flows (Local 2P / AI) are preserved exactly.
+                if (coopSetupStep == CoopSetupStep::ChoosePartner && (coopSetupSelected == 0 || coopSetupSelected == 1)) {
+                    const bool useAI = (coopSetupSelected == 1);
+                    if (ctx.coopVsAI) {
+                        *ctx.coopVsAI = useAI;
+                    }
+                    if (ctx.game) {
+                        ctx.game->setMode(GameMode::Cooperate);
+                        ctx.game->reset(ctx.startLevelSelection ? *ctx.startLevelSelection : 0);
+                    }
+                    if (ctx.coopGame) {
+                        ctx.coopGame->reset(ctx.startLevelSelection ? *ctx.startLevelSelection : 0);
+                    }
+
+                    // Close the panel without restarting menu music; gameplay will take over.
+                    showCoopSetupPanel(false, false);
+
+                    SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION,
+                                "MenuState: coop start via key, selected=%d, startPlayTransition_present=%d, stateManager=%p",
+                                coopSetupSelected,
+                                ctx.startPlayTransition ? 1 : 0,
+                                (void*)ctx.stateManager);
+
+                    if (ctx.startPlayTransition) {
+                        ctx.startPlayTransition();
+                    } else if (ctx.stateManager) {
+                        ctx.stateManager->setState(AppState::Playing);
+                    }
+                    return;
                 }
 
-                // Close the panel without restarting menu music; gameplay will take over.
-                showCoopSetupPanel(false, false);
-
-                SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "MenuState: coop start via key, selected=%d, startPlayTransition_present=%d, stateManager=%p", coopSetupSelected, ctx.startPlayTransition ? 1 : 0, (void*)ctx.stateManager);
-
-                if (ctx.startPlayTransition) {
-                    ctx.startPlayTransition();
-                } else if (ctx.stateManager) {
-                    ctx.stateManager->setState(AppState::Playing);
+                // Network flow (new): choose host/join, confirm connection before starting.
+                if (coopSetupStep == CoopSetupStep::ChoosePartner && coopSetupSelected == 2) {
+                    coopSetupStep = CoopSetupStep::NetworkChooseRole;
+                    coopNetworkRoleSelected = 0;
+                    coopNetworkHandshakeSent = false;
+                    coopNetworkStatusText.clear();
+                    if (coopNetworkSession) {
+                        coopNetworkSession->shutdown();
+                        coopNetworkSession.reset();
+                    }
+                    buttonFlash = 1.0;
+                    return;
                 }
+
+                if (coopSetupStep == CoopSetupStep::NetworkChooseRole) {
+                    // First, let the user enter the address (bind for host, remote for join).
+                    coopSetupStep = CoopSetupStep::NetworkEnterAddress;
+                    coopNetworkStatusText.clear();
+                    if (SDL_Window* focusWin = SDL_GetKeyboardFocus()) {
+                        SDL_StartTextInput(focusWin);
+                    }
+                    return;
+                }
+
+                if (coopSetupStep == CoopSetupStep::NetworkEnterAddress) {
+                    coopNetworkHandshakeSent = false;
+                    coopNetworkStatusText.clear();
+                    coopNetworkSession = std::make_unique<NetSession>();
+
+                    const uint16_t port = coopNetworkPort;
+                    bool ok = false;
+                    if (coopNetworkRoleSelected == 0) {
+                        const std::string bindIp = coopNetworkBindAddress;
+                        SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "[NET COOP] HOST start bind=%s port=%u", bindIp.c_str(), (unsigned)port);
+                        ok = coopNetworkSession->host(bindIp, port);
+                        coopNetworkStatusText = ok ? "WAITING FOR PLAYER..." : ("HOST FAILED: " + coopNetworkSession->lastError());
+                    } else {
+                        const std::string joinIp = coopNetworkJoinAddress.empty() ? std::string("127.0.0.1") : coopNetworkJoinAddress;
+                        SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "[NET COOP] JOIN start remote=%s port=%u", joinIp.c_str(), (unsigned)port);
+                        ok = coopNetworkSession->join(joinIp, port);
+                        coopNetworkStatusText = ok ? "CONNECTING..." : ("JOIN FAILED: " + coopNetworkSession->lastError());
+                    }
+
+                    if (SDL_Window* focusWin = SDL_GetKeyboardFocus()) {
+                        SDL_StopTextInput(focusWin);
+                    }
+
+                    if (ok) {
+                        coopSetupStep = CoopSetupStep::NetworkWaiting;
+                    } else {
+                        // Stay on role choice screen so user can back out.
+                        coopNetworkSession.reset();
+                        coopSetupStep = CoopSetupStep::NetworkChooseRole;
+                    }
+                    return;
+                }
+
+                // While waiting for connection, Enter does nothing.
                 return;
             }
             default:
-                // Allow all other keys to be pressed, but don't let them affect the main menu while coop is open.
+                // Allow other keys, but don't let them affect the main menu while coop is open.
                 return;
         }
     }
@@ -796,6 +960,15 @@ void MenuState::handleEvent(const SDL_Event& e) {
 }
 
 void MenuState::update(double frameMs) {
+    // Transient network status message (e.g., disconnect) shown on return to menu.
+    if (ctx.coopNetUiStatusRemainingMs > 0.0) {
+        ctx.coopNetUiStatusRemainingMs -= frameMs;
+        if (ctx.coopNetUiStatusRemainingMs <= 0.0) {
+            ctx.coopNetUiStatusRemainingMs = 0.0;
+            ctx.coopNetUiStatusText.clear();
+        }
+    }
+
     // Update logo animation counter
     GlobalState::instance().logoAnimCounter += frameMs;
     // Advance options panel animation if active
@@ -1056,14 +1229,27 @@ void MenuState::render(SDL_Renderer* renderer, float logicalScale, SDL_Rect logi
         float titleX = (LOGICAL_W - (float)tW) * 0.5f + contentOffsetX;
                 useFont->draw(renderer, titleX, scoresStartY, smallTitle, titleScale, SDL_Color{200,220,230,255});
         scoresStartY += (float)tH + 12.0f;
+
+        if (!ctx.coopNetUiStatusText.empty() && ctx.coopNetUiStatusRemainingMs > 0.0) {
+            float msgScale = 0.75f;
+            int mW = 0, mH = 0;
+            useFont->measure(ctx.coopNetUiStatusText, msgScale, mW, mH);
+            float msgX = (LOGICAL_W - (float)mW) * 0.5f + contentOffsetX;
+            useFont->draw(renderer, msgX, scoresStartY, ctx.coopNetUiStatusText, msgScale, SDL_Color{255, 224, 130, 255});
+            scoresStartY += (float)mH + 10.0f;
+        }
     }
     static const std::vector<ScoreEntry> EMPTY_SCORES;
     const auto& hs = ctx.scores ? ctx.scores->all() : EMPTY_SCORES;
-    // Choose which game_type to show based on current menu selection
+    // Choose which game_type to show based on current menu selection or mouse hover.
+    // Prefer `hoveredButton` (mouse-over) when available so the TOP PLAYER panel
+    // updates responsively while the user moves the pointer over the bottom menu.
+    int activeBtn = (ctx.hoveredButton ? *ctx.hoveredButton : -1);
+    if (activeBtn < 0) activeBtn = selectedButton;
     std::string wantedType = "classic";
-    if (selectedButton == 0) wantedType = "classic"; // Play / Endless
-    else if (selectedButton == 1) wantedType = "cooperate"; // Coop
-    else if (selectedButton == 2) wantedType = "challenge"; // Challenge
+    if (activeBtn == 0) wantedType = "classic"; // Play / Endless
+    else if (activeBtn == 1) wantedType = "cooperate"; // Coop
+    else if (activeBtn == 2) wantedType = "challenge"; // Challenge
     // Filter highscores to the desired game type
     std::vector<ScoreEntry> filtered;
     filtered.reserve(hs.size());
@@ -1358,18 +1544,20 @@ void MenuState::render(SDL_Renderer* renderer, float logicalScale, SDL_Rect logi
         // highscores area (not sliding offscreen with the scores).
         const float panelBaseY = scoresStartY - 20.0f;
 
-        // Make the choice buttons smaller, add more spacing, and raise them higher
-        const float btnW2 = std::min(300.0f, panelW * 0.30f);
+        // Choice buttons (partner selection) and nested network host/join UI
         const float btnH2 = 60.0f;
-        const float gap = 96.0f;
-        // Shift the image and buttons to the right for layout balance (reduced)
-        const float shiftX = 20.0f; // move right by 30px (moved 20px left from previous)
-        const float bx = panelBaseX + (panelW - (btnW2 * 2.0f + gap)) * 0.5f + shiftX;
+        const float gap = 34.0f;
+        const float btnW2 = std::min(280.0f, (panelW - gap * 2.0f) / 3.0f);
+        const float totalChoiceW = btnW2 * 3.0f + gap * 2.0f;
+        // Shift the image and buttons slightly for layout balance
+        const float shiftX = 20.0f;
+        const float bx = panelBaseX + (panelW - totalChoiceW) * 0.5f + shiftX;
         // Move the buttons up by ~80px to sit closer under the logo
         const float by = panelBaseY + (panelH - btnH2) * 0.5f - 80.0f;
 
         coopSetupBtnRects[0] = SDL_FRect{ bx, by, btnW2, btnH2 };
-        coopSetupBtnRects[1] = SDL_FRect{ bx + btnW2 + gap, by, btnW2, btnH2 };
+        coopSetupBtnRects[1] = SDL_FRect{ bx + (btnW2 + gap), by, btnW2, btnH2 };
+        coopSetupBtnRects[2] = SDL_FRect{ bx + (btnW2 + gap) * 2.0f, by, btnW2, btnH2 };
         coopSetupRectsValid = true;
 
         SDL_Color bg{ 24, 36, 52, 220 };
@@ -1392,21 +1580,24 @@ void MenuState::render(SDL_Renderer* renderer, float logicalScale, SDL_Rect logi
             }
         }
 
-        // If the image loaded, render it centered above the two choice buttons
+        // If the image loaded, render it centered above the choice buttons
         // Compute fade alpha from the coop transition so it can be used for image, text and buttons
         float alphaFactor = static_cast<float>(coopSetupTransition);
         if (alphaFactor < 0.0f) alphaFactor = 0.0f;
         if (alphaFactor > 1.0f) alphaFactor = 1.0f;
+        // Compute coop info image placement (draw as background for both ChoosePartner and Network steps)
+        float imgX = 0.0f, imgY = 0.0f, targetW = 0.0f, targetH = 0.0f;
+        bool hasCoopImg = false;
         if (coopInfoTexture && coopInfoTexW > 0 && coopInfoTexH > 0) {
-            float totalW = btnW2 * 2.0f + gap;
-            // Increase allowed image width by ~15% (was 0.75 of totalW)
-            const float scaleFactor = 0.75f * 1.25f; // ~0.8625
-            float maxImgW = totalW * scaleFactor;
-            float targetW = std::min(maxImgW, static_cast<float>(coopInfoTexW));
+            float totalW = totalChoiceW;
+            // Keep coop info image slightly smaller than the button row.
+            // Use a modest scale so it doesn't dominate the UI.
+            float maxImgW = totalW * 0.65f;
+            targetW = std::min(maxImgW, static_cast<float>(coopInfoTexW));
             float scale = targetW / static_cast<float>(coopInfoTexW);
-            float targetH = static_cast<float>(coopInfoTexH) * scale;
-            float imgX = bx + (totalW - targetW) * 0.5f;
-            float imgY = by - targetH - 8.0f; // keep the small gap above buttons
+            targetH = static_cast<float>(coopInfoTexH) * scale;
+            imgX = bx + (totalW - targetW) * 0.5f;
+            imgY = by - targetH - 8.0f; // keep the small gap above buttons
             float minY = panelBaseY + 6.0f;
             if (imgY < minY) imgY = minY;
             SDL_FRect dst{ imgX, imgY, targetW, targetH };
@@ -1414,28 +1605,30 @@ void MenuState::render(SDL_Renderer* renderer, float logicalScale, SDL_Rect logi
             // Make the coop info image slightly transparent scaled by transition
             SDL_SetTextureAlphaMod(coopInfoTexture, static_cast<Uint8>(std::round(200.0f * alphaFactor)));
             SDL_RenderTexture(renderer, coopInfoTexture, nullptr, &dst);
+            hasCoopImg = true;
 
-            // Draw cooperative instructions inside the panel area (overlayed on the panel background)
-            FontAtlas* f = ctx.font ? ctx.font : ctx.pixelFont;
-            if (f) {
-                const float pad = 38.0f;
-                float textX = panelBaseX + pad;
-                // Position the text over the lower portion of the image (overlay)
-                // Move the block upward by ~150px to match UI request
-                float textY = imgY + targetH - std::min(80.0f, targetH * 0.35f) - 150.0f;
+            // Only draw the instructional overlay text when choosing partner.
+            if (coopSetupStep == CoopSetupStep::ChoosePartner) {
+                FontAtlas* f = ctx.font ? ctx.font : ctx.pixelFont;
+                if (f) {
+                    const float pad = 38.0f;
+                    float textX = panelBaseX + pad;
+                    // Position the text over the lower portion of the image (overlay)
+                    // Move the block upward by ~150px to match UI request
+                    float textY = imgY + targetH - std::min(80.0f, targetH * 0.35f) - 150.0f;
 
-                // Bulleted list (measure sample line height first)
-                const std::vector<std::string> bullets = {
-                    "The playfield is shared between two players",
-                    "Each player controls one half of the grid",
-                    "A line clears only when both halves are filled",
-                    "Timing and coordination are essential"
-                };
-                float bulletScale = 0.78f;
-                SDL_Color bulletCol{200,220,230,220};
-                bulletCol.a = static_cast<Uint8>(std::round(bulletCol.a * alphaFactor));
-                int sampleLW = 0, sampleLH = 0;
-                f->measure(bullets[0], bulletScale, sampleLW, sampleLH);
+                    // Bulleted list (measure sample line height first)
+                    const std::vector<std::string> bullets = {
+                        "The playfield is shared between two players",
+                        "Each player controls one half of the grid",
+                        "A line clears only when both halves are filled",
+                        "Timing and coordination are essential"
+                    };
+                    float bulletScale = 0.78f;
+                    SDL_Color bulletCol{200,220,230,220};
+                    bulletCol.a = static_cast<Uint8>(std::round(bulletCol.a * alphaFactor));
+                    int sampleLW = 0, sampleLH = 0;
+                    f->measure(bullets[0], bulletScale, sampleLW, sampleLH);
 
                 // Header: move it up by one sample row so it sits higher
                 const std::string header = "* HOW TO PLAY – COOPERATE MODE *";
@@ -1470,6 +1663,7 @@ void MenuState::render(SDL_Renderer* renderer, float logicalScale, SDL_Rect logi
                 f->draw(renderer, goalX, textY, goalText, 0.86f, goalTextCol);
             }
         }
+        }
 
         // Delay + eased fade specifically for the two coop buttons so they appear after the image/text.
         const float btnDelay = 0.25f; // fraction of transition to wait before buttons start fading
@@ -1479,10 +1673,134 @@ void MenuState::render(SDL_Renderer* renderer, float logicalScale, SDL_Rect logi
         float buttonFade = rawBtn * rawBtn;
         SDL_Color bgA = bg; bgA.a = static_cast<Uint8>(std::round(bgA.a * buttonFade));
         SDL_Color borderA = border; borderA.a = static_cast<Uint8>(std::round(borderA.a * buttonFade));
-        UIRenderer::drawButton(renderer, ctx.pixelFont, coopSetupBtnRects[0].x + btnW2 * 0.5f, coopSetupBtnRects[0].y + btnH2 * 0.5f,
-              btnW2, btnH2, "2 PLAYERS", false, coopSetupSelected == 0, bgA, borderA, false, nullptr);
-        UIRenderer::drawButton(renderer, ctx.pixelFont, coopSetupBtnRects[1].x + btnW2 * 0.5f, coopSetupBtnRects[1].y + btnH2 * 0.5f,
-              btnW2, btnH2, "COMPUTER (AI)", false, coopSetupSelected == 1, bgA, borderA, false, nullptr);
+
+        // Step 1: choose partner mode
+        if (coopSetupStep == CoopSetupStep::ChoosePartner) {
+            UIRenderer::drawButton(renderer, ctx.pixelFont,
+                coopSetupBtnRects[0].x + btnW2 * 0.5f,
+                coopSetupBtnRects[0].y + btnH2 * 0.5f,
+                btnW2, btnH2,
+                "LOCAL CO-OP",
+                false,
+                coopSetupSelected == 0,
+                bgA,
+                borderA,
+                false,
+                nullptr);
+
+            UIRenderer::drawButton(renderer, ctx.pixelFont,
+                coopSetupBtnRects[1].x + btnW2 * 0.5f,
+                coopSetupBtnRects[1].y + btnH2 * 0.5f,
+                btnW2, btnH2,
+                "AI PARTNER",
+                false,
+                coopSetupSelected == 1,
+                bgA,
+                borderA,
+                false,
+                nullptr);
+
+            UIRenderer::drawButton(renderer, ctx.pixelFont,
+                coopSetupBtnRects[2].x + btnW2 * 0.5f,
+                coopSetupBtnRects[2].y + btnH2 * 0.5f,
+                btnW2, btnH2,
+                "2 PLAYER (NET)",
+                false,
+                coopSetupSelected == 2,
+                bgA,
+                borderA,
+                false,
+                nullptr);
+        }
+
+        // Step 2: network host/join selection and address entry
+        if (coopSetupStep == CoopSetupStep::NetworkChooseRole || coopSetupStep == CoopSetupStep::NetworkEnterAddress || coopSetupStep == CoopSetupStep::NetworkWaiting) {
+            // Draw two buttons centered under the main row.
+            const float roleBtnW = std::min(280.0f, panelW * 0.30f);
+            const float roleGap = 48.0f;
+            const float roleTotalW = roleBtnW * 2.0f + roleGap;
+            const float roleX = panelBaseX + (panelW - roleTotalW) * 0.5f + shiftX;
+            // Move the host/join buttons down from the previous higher position.
+            // Shift down by one button height plus half a button (effectively lower them):
+            const float roleY = by + (btnH2 * 0.5f) - 18.0f;
+
+            SDL_FRect hostRect{ roleX, roleY, roleBtnW, btnH2 };
+            SDL_FRect joinRect{ roleX + roleBtnW + roleGap, roleY, roleBtnW, btnH2 };
+
+            UIRenderer::drawButton(renderer, ctx.pixelFont,
+                hostRect.x + roleBtnW * 0.5f,
+                hostRect.y + btnH2 * 0.5f,
+                roleBtnW,
+                btnH2,
+                "HOST GAME",
+                false,
+                coopNetworkRoleSelected == 0,
+                bgA,
+                borderA,
+                false,
+                nullptr);
+
+            UIRenderer::drawButton(renderer, ctx.pixelFont,
+                joinRect.x + roleBtnW * 0.5f,
+                joinRect.y + btnH2 * 0.5f,
+                roleBtnW,
+                btnH2,
+                "JOIN GAME",
+                false,
+                coopNetworkRoleSelected == 1,
+                bgA,
+                borderA,
+                false,
+                nullptr);
+
+            FontAtlas* f = ctx.font ? ctx.font : ctx.pixelFont;
+            if (f) {
+                SDL_Color infoCol{200, 220, 230, static_cast<Uint8>(std::round(220.0f * buttonFade))};
+                // Draw connection info on separate lines and shift right by ~200px
+                char portLine[64];
+                std::snprintf(portLine, sizeof(portLine), "PORT %u", (unsigned)coopNetworkPort);
+                char hostLine[128];
+                std::snprintf(hostLine, sizeof(hostLine), "HOST IP %s", coopNetworkBindAddress.c_str());
+                char joinLine[128];
+                std::snprintf(joinLine, sizeof(joinLine), "JOIN IP %s", coopNetworkJoinAddress.c_str());
+
+                const float textShiftX = 200.0f;
+                const float textX = panelBaseX + 60.0f + textShiftX;
+                const float endpointY = (hasCoopImg ? (imgY + targetH * 0.62f) : (roleY + btnH2 + 12.0f));
+                const float lineSpacing = 28.0f;
+                // Show only the minimal info needed for the selected role.
+                f->draw(renderer, textX, endpointY, portLine, 0.90f, infoCol);
+                if (coopNetworkRoleSelected == 0) {
+                    // Host: show bind address only
+                    f->draw(renderer, textX, endpointY + lineSpacing, hostLine, 0.90f, infoCol);
+                } else {
+                    // Client: show join target only
+                    f->draw(renderer, textX, endpointY + lineSpacing, joinLine, 0.90f, infoCol);
+                }
+
+                float hintY = endpointY + lineSpacing * 2.0f + 6.0f;
+
+                // Bottom helper prompt: show a compact instruction under the image window
+                float bottomY = hasCoopImg ? (imgY + targetH + 18.0f) : (hintY + 36.0f);
+                SDL_Color bottomCol{180,200,210,200};
+                if (coopNetworkRoleSelected == 0) {
+                    f->draw(renderer, textX, bottomY, "HOST: press ENTER to edit bind IP, then press ENTER to confirm", 0.82f, bottomCol);
+                } else {
+                    f->draw(renderer, textX, bottomY, "JOIN: press ENTER to type server IP, then press ENTER to connect", 0.82f, bottomCol);
+                }
+                if (coopSetupStep == CoopSetupStep::NetworkWaiting && !coopNetworkStatusText.empty()) {
+                    SDL_Color statusCol{255, 215, 80, static_cast<Uint8>(std::round(240.0f * buttonFade))};
+                    f->draw(renderer, textX, hintY, coopNetworkStatusText, 1.00f, statusCol);
+                } else if (coopSetupStep == CoopSetupStep::NetworkEnterAddress) {
+                    SDL_Color hintCol{160, 190, 210, static_cast<Uint8>(std::round(200.0f * buttonFade))};
+                    const char* label = (coopNetworkRoleSelected == 0) ? "TYPE HOST IP (BIND) THEN ENTER" : "TYPE JOIN IP THEN ENTER";
+                    f->draw(renderer, textX, hintY, label, 0.82f, hintCol);
+                } else {
+                    SDL_Color hintCol{160, 190, 210, static_cast<Uint8>(std::round(200.0f * buttonFade))};
+                    f->draw(renderer, textX, hintY, "PRESS ENTER TO EDIT/CONFIRM   ESC TO GO BACK", 0.82f, hintCol);
+                }
+            }
+        }
     }
     // NOTE: slide-up COOP panel intentionally removed. Only the inline
     // highscores-area choice buttons are shown when coop setup is active.
@@ -1840,6 +2158,108 @@ void MenuState::render(SDL_Renderer* renderer, float logicalScale, SDL_Rect logi
     {
         FILE* f = fopen("spacetris_trace.log", "a"); if (f) { fprintf(f, "MenuState::render exit\n"); fclose(f); }
     }
+
+    // Network coop flow polling (non-blocking)
+    if (coopSetupAnimating || coopSetupVisible) {
+        if (coopSetupStep == CoopSetupStep::NetworkWaiting && coopNetworkSession) {
+            coopNetworkSession->poll(0);
+
+            // Update status depending on connection and role.
+            if (!coopNetworkSession->isConnected()) {
+                // Keep existing text (WAITING/CONNECTING) unless an error occurs.
+            } else {
+                // Host sends handshake after peer connects.
+                if (coopNetworkRoleSelected == 0 && !coopNetworkHandshakeSent) {
+                    std::random_device rd;
+                    uint32_t seed = static_cast<uint32_t>(rd());
+                    if (seed == 0u) seed = 1u;
+                    const uint8_t startLevel = static_cast<uint8_t>(ctx.startLevelSelection ? *ctx.startLevelSelection : 0);
+                    NetSession::Handshake hs{ seed, 0u, startLevel };
+                    if (coopNetworkSession->sendHandshake(hs)) {
+                        coopNetworkHandshakeSent = true;
+                        ctx.coopNetRngSeed = seed;
+                        coopNetworkStatusText = "CONNECTED";
+                        SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "[NET COOP] HOST handshake sent seed=%u level=%u", seed, (unsigned)startLevel);
+                    } else {
+                        coopNetworkStatusText = "HANDSHAKE FAILED";
+                    }
+                }
+
+                // Client waits for handshake.
+                if (coopNetworkRoleSelected == 1) {
+                    auto hs = coopNetworkSession->takeReceivedHandshake();
+                    if (hs.has_value()) {
+                        coopNetworkStatusText = "CONNECTED";
+                        coopNetworkHandshakeSent = true;
+                        ctx.coopNetRngSeed = hs->rngSeed;
+                        if (ctx.startLevelSelection) {
+                            *ctx.startLevelSelection = static_cast<int>(hs->startLevel);
+                        }
+                        SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "[NET COOP] CLIENT handshake recv seed=%u level=%u", hs->rngSeed, (unsigned)hs->startLevel);
+                    } else {
+                        coopNetworkStatusText = "CONNECTED - WAITING FOR HOST...";
+                    }
+                }
+
+                // Confirmed connection => start COOPERATE (network) session.
+                // Note: gameplay/network input injection is implemented separately.
+                if (coopNetworkHandshakeSent) {
+                    SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION,
+                                "[NET COOP] START gameplay (role=%s localIsLeft=%d seed=%u level=%u)",
+                                (coopNetworkRoleSelected == 0 ? "HOST" : "CLIENT"),
+                                (coopNetworkRoleSelected == 0 ? 1 : 0),
+                                (unsigned)ctx.coopNetRngSeed,
+                                (unsigned)(ctx.startLevelSelection ? *ctx.startLevelSelection : 0));
+                    // Hand off the session to gameplay.
+                    if (ctx.coopNetSession) {
+                        ctx.coopNetSession->shutdown();
+                        ctx.coopNetSession.reset();
+                    }
+
+                    ctx.coopNetEnabled = true;
+                    ctx.coopNetIsHost = (coopNetworkRoleSelected == 0);
+                    ctx.coopNetLocalIsLeft = (coopNetworkRoleSelected == 0);
+                    ctx.coopNetTick = 0;
+                    ctx.coopNetPendingButtons = 0;
+                    ctx.coopNetDesyncDetected = false;
+
+                    const uint32_t seed = (ctx.coopNetRngSeed == 0u) ? 1u : ctx.coopNetRngSeed;
+                    const uint8_t startLevel = static_cast<uint8_t>(ctx.startLevelSelection ? *ctx.startLevelSelection : 0);
+
+                    if (ctx.coopVsAI) {
+                        *ctx.coopVsAI = false;
+                    }
+                    if (ctx.game) {
+                        ctx.game->setMode(GameMode::Cooperate);
+                        ctx.game->reset(ctx.startLevelSelection ? *ctx.startLevelSelection : 0);
+                    }
+                    if (ctx.coopGame) {
+                        // Deterministic reset for network coop.
+                        ctx.coopGame->resetDeterministic(startLevel, seed);
+                    }
+
+                    // Transfer ownership of the active session.
+                    ctx.coopNetSession = std::move(coopNetworkSession);
+
+                    // Close the panel without restarting menu music; gameplay will take over.
+                    showCoopSetupPanel(false, false);
+
+                    // For network lockstep, do NOT run the menu->play countdown/fade.
+                    // Any local countdown introduces drift and stalls.
+                    if (ctx.gameplayCountdownActive) *ctx.gameplayCountdownActive = false;
+                    if (ctx.menuPlayCountdownArmed) *ctx.menuPlayCountdownArmed = false;
+                    if (ctx.game) ctx.game->setPaused(false);
+
+                    if (ctx.stateManager) {
+                        ctx.stateManager->setState(AppState::Playing);
+                    } else if (ctx.startPlayTransition) {
+                        // Fallback if state manager is unavailable.
+                        ctx.startPlayTransition();
+                    }
+                }
+            }
+        }
+    }
 }
 
 

src/states/MenuState.h

@@ -2,6 +2,12 @@
 #pragma once
 #include "State.h"
 
+#include <cstdint>
+#include <memory>
+#include <string>
+
+class NetSession;
+
 class MenuState : public State {
 public:
     MenuState(StateContext& ctx);
@@ -105,8 +111,27 @@ private:
     double coopSetupTransition = 0.0; // 0..1
     double coopSetupTransitionDurationMs = 320.0;
     int coopSetupDirection = 1; // 1 show, -1 hide
-    int coopSetupSelected = 0; // 0 = 2 players, 1 = AI
-    SDL_FRect coopSetupBtnRects[2]{};
+    // 0 = Local co-op (2 players), 1 = AI partner, 2 = 2 player (network)
+    int coopSetupSelected = 0;
+
+    enum class CoopSetupStep {
+        ChoosePartner,
+        NetworkChooseRole,
+        NetworkEnterAddress,
+        NetworkWaiting,
+    };
+    CoopSetupStep coopSetupStep = CoopSetupStep::ChoosePartner;
+
+    // Network sub-flow state (only used when coopSetupSelected == 2)
+    int coopNetworkRoleSelected = 0; // 0 = host, 1 = join
+    std::string coopNetworkBindAddress = "0.0.0.0";
+    std::string coopNetworkJoinAddress = "127.0.0.1";
+    uint16_t coopNetworkPort = 7777;
+    bool coopNetworkHandshakeSent = false;
+    std::string coopNetworkStatusText;
+    std::unique_ptr<NetSession> coopNetworkSession;
+
+    SDL_FRect coopSetupBtnRects[3]{};
     bool coopSetupRectsValid = false;
     // Optional cooperative info image shown when coop setup panel is active
     SDL_Texture* coopInfoTexture = nullptr;

src/states/PlayingState.cpp

@@ -6,9 +6,11 @@
 #include "../persistence/Scores.h"
 #include "../audio/Audio.h"
 #include "../audio/SoundEffect.h"
+#include "../graphics/Font.h"
 #include "../graphics/renderers/GameRenderer.h"
 #include "../core/Settings.h"
 #include "../core/Config.h"
+#include "../network/CoopNetButtons.h"
 #include <SDL3/SDL.h>
 
 // File-scope transport/spawn detection state
@@ -24,9 +26,17 @@ void PlayingState::onEnter() {
         if (ctx.game->getMode() == GameMode::Endless || ctx.game->getMode() == GameMode::Cooperate) {
             if (ctx.startLevelSelection) {
                 SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION, "[PLAYING] Resetting game with level %d", *ctx.startLevelSelection);
-                ctx.game->reset(*ctx.startLevelSelection);
-                if (ctx.game->getMode() == GameMode::Cooperate && ctx.coopGame) {
-                    ctx.coopGame->reset(*ctx.startLevelSelection);
+                const bool coopNetActive = (ctx.game->getMode() == GameMode::Cooperate) && ctx.coopNetEnabled && ctx.coopNetSession;
+
+                // For network co-op, MenuState already performed a deterministic reset using the negotiated seed.
+                // Re-resetting here would overwrite it (and will desync).
+                if (!coopNetActive) {
+                    ctx.game->reset(*ctx.startLevelSelection);
+                    if (ctx.game->getMode() == GameMode::Cooperate && ctx.coopGame) {
+                        ctx.coopGame->reset(*ctx.startLevelSelection);
+                    }
+                } else {
+                    ctx.game->setPaused(false);
                 }
             }
         } else {
@@ -46,6 +56,18 @@ void PlayingState::onExit() {
         SDL_DestroyTexture(m_renderTarget);
         m_renderTarget = nullptr;
     }
+
+    // If we are leaving gameplay during network co-op, tear down the session so
+    // hosting/joining again works without restarting the app.
+    if (ctx.coopNetSession) {
+        ctx.coopNetSession->shutdown();
+        ctx.coopNetSession.reset();
+    }
+    ctx.coopNetEnabled = false;
+    ctx.coopNetStalled = false;
+    ctx.coopNetDesyncDetected = false;
+    ctx.coopNetTick = 0;
+    ctx.coopNetPendingButtons = 0;
 }
 
 void PlayingState::handleEvent(const SDL_Event& e) {
@@ -135,6 +157,10 @@ void PlayingState::handleEvent(const SDL_Event& e) {
 
     // Pause toggle (P) - matches classic behavior; disabled during countdown
     if (e.key.scancode == SDL_SCANCODE_P) {
+        // Network co-op uses lockstep; local pause would desync/stall the peer.
+        if (ctx.coopNetEnabled && ctx.coopNetSession) {
+            return;
+        }
         const bool countdown = (ctx.gameplayCountdownActive && *ctx.gameplayCountdownActive) ||
                                (ctx.menuPlayCountdownArmed && *ctx.menuPlayCountdownArmed);
         if (!countdown) {
@@ -149,6 +175,49 @@ void PlayingState::handleEvent(const SDL_Event& e) {
     }
 
     if (coopActive && ctx.coopGame) {
+        // Network co-op: route one-shot actions into a pending bitmask for lockstep.
+        if (ctx.coopNetEnabled && ctx.coopNetSession) {
+            const bool localIsLeft = ctx.coopNetLocalIsLeft;
+            const SDL_Scancode sc = e.key.scancode;
+            if (localIsLeft) {
+                if (sc == SDL_SCANCODE_W) {
+                    ctx.coopNetPendingButtons |= coopnet::RotCW;
+                    return;
+                }
+                if (sc == SDL_SCANCODE_Q) {
+                    ctx.coopNetPendingButtons |= coopnet::RotCCW;
+                    return;
+                }
+                if (sc == SDL_SCANCODE_LSHIFT || sc == SDL_SCANCODE_E) {
+                    ctx.coopNetPendingButtons |= coopnet::HardDrop;
+                    return;
+                }
+                if (sc == SDL_SCANCODE_LCTRL) {
+                    ctx.coopNetPendingButtons |= coopnet::Hold;
+                    return;
+                }
+            } else {
+                if (sc == SDL_SCANCODE_UP) {
+                    const bool upIsCW = Settings::instance().isUpRotateClockwise();
+                    ctx.coopNetPendingButtons |= upIsCW ? coopnet::RotCW : coopnet::RotCCW;
+                    return;
+                }
+                if (sc == SDL_SCANCODE_RALT) {
+                    ctx.coopNetPendingButtons |= coopnet::RotCCW;
+                    return;
+                }
+                if (sc == SDL_SCANCODE_SPACE || sc == SDL_SCANCODE_RSHIFT) {
+                    ctx.coopNetPendingButtons |= coopnet::HardDrop;
+                    return;
+                }
+                if (sc == SDL_SCANCODE_RCTRL) {
+                    ctx.coopNetPendingButtons |= coopnet::Hold;
+                    return;
+                }
+            }
+            // If coopNet is active, suppress local co-op direct action keys.
+        }
+
         const bool coopAIEnabled = (ctx.coopVsAI && *ctx.coopVsAI);
 
         // Player 1 (left): when AI is enabled it controls the left side so
@@ -313,6 +382,31 @@ void PlayingState::render(SDL_Renderer* renderer, float logicalScale, SDL_Rect l
     // But countdown should definitely NOT show the "PAUSED" overlay.
     bool shouldBlur = paused && !countdown && !challengeClearFx;
 
+    auto renderNetOverlay = [&]() {
+        if (!coopActive || !ctx.coopNetEnabled || !ctx.pixelFont) return;
+        if (!ctx.coopNetDesyncDetected && !ctx.coopNetStalled) return;
+
+        const char* text = ctx.coopNetDesyncDetected ? "NET: DESYNC" : "NET: STALLED";
+        SDL_Color textColor = ctx.coopNetDesyncDetected ? SDL_Color{255, 230, 180, 255} : SDL_Color{255, 224, 130, 255};
+        float scale = 0.75f;
+        int tw = 0, th = 0;
+        ctx.pixelFont->measure(text, scale, tw, th);
+
+        SDL_BlendMode prevBlend = SDL_BLENDMODE_NONE;
+        SDL_GetRenderDrawBlendMode(renderer, &prevBlend);
+        SDL_SetRenderDrawBlendMode(renderer, SDL_BLENDMODE_BLEND);
+
+        const float pad = 8.0f;
+        const float x = 18.0f;
+        const float y = 14.0f;
+        SDL_FRect bg{ x - pad, y - pad, (float)tw + pad * 2.0f, (float)th + pad * 2.0f };
+        SDL_SetRenderDrawColor(renderer, 0, 0, 0, 160);
+        SDL_RenderFillRect(renderer, &bg);
+        ctx.pixelFont->draw(renderer, x, y, text, scale, textColor);
+
+        SDL_SetRenderDrawBlendMode(renderer, prevBlend);
+    };
+
     if (shouldBlur && m_renderTarget) {
         // Render game to texture
         SDL_SetRenderTarget(renderer, m_renderTarget);
@@ -421,6 +515,9 @@ void PlayingState::render(SDL_Renderer* renderer, float logicalScale, SDL_Rect l
         SDL_SetRenderViewport(renderer, &oldVP);
         SDL_SetRenderScale(renderer, oldSX, oldSY);
 
+        // Net overlay (on top of blurred game, under pause/exit overlays)
+        renderNetOverlay();
+
         // Draw overlays
         if (exitPopup) {
             GameRenderer::renderExitPopup(
@@ -466,6 +563,9 @@ void PlayingState::render(SDL_Renderer* renderer, float logicalScale, SDL_Rect l
                 (float)winW,
                 (float)winH
             );
+
+            // Net overlay (on top of coop HUD)
+            renderNetOverlay();
         } else {
             GameRenderer::renderPlayingState(
                 renderer,

src/states/State.h

@@ -6,6 +6,9 @@
 #include <functional>
 #include <string>
 #include <array>
+#include <cstdint>
+
+#include "../network/NetSession.h"
 
 // Forward declarations for frequently used types
 class Game;
@@ -81,12 +84,31 @@ struct StateContext {
     std::string* playerName = nullptr; // Shared player name buffer for highscores/options
     // Coop setting: when true, COOPERATE runs with a computer-controlled right player.
     bool* coopVsAI = nullptr;
+
+    // COOPERATE (network) --------------------------------------------------
+    // These fields are only meaningful when `coopNetEnabled` is true.
+    bool coopNetEnabled = false;
+    bool coopNetIsHost = false;
+    bool coopNetLocalIsLeft = true; // host = left (WASD), client = right (arrows)
+    uint32_t coopNetRngSeed = 0;
+    uint32_t coopNetTick = 0;
+    uint8_t coopNetPendingButtons = 0; // one-shot actions captured from keydown (rotate/hold/harddrop)
+    bool coopNetStalled = false; // true when waiting for remote input for current tick
+    bool coopNetDesyncDetected = false;
+    std::string coopNetUiStatusText; // transient status shown in menu after net abort
+    double coopNetUiStatusRemainingMs = 0.0;
+    std::unique_ptr<NetSession> coopNetSession;
     bool* fullscreenFlag = nullptr; // Tracks current fullscreen state when available
     std::function<void(bool)> applyFullscreen; // Allows states to request fullscreen changes
     std::function<bool()> queryFullscreen; // Optional callback if fullscreenFlag is not reliable
     std::function<void()> requestQuit; // Allows menu/option states to close the app gracefully
     std::function<void()> startPlayTransition; // Optional fade hook when transitioning from menu to gameplay
     std::function<void(AppState)> requestFadeTransition; // Generic state fade requests (menu/options/level)
+
+    // Startup transition fade (used for intro video -> main).
+    // When active, the app should render a black overlay with alpha = startupFadeAlpha*255.
+    bool* startupFadeActive = nullptr;
+    float* startupFadeAlpha = nullptr;
     // Pointer to the application's StateManager so states can request transitions
     StateManager* stateManager = nullptr;
     // Optional explicit per-button coordinates (logical coordinates). When

src/states/VideoState.cpp

@@ -0,0 +1,389 @@
+// VideoState.cpp
+#include "VideoState.h"
+
+#include "../video/VideoPlayer.h"
+#include "../audio/Audio.h"
+#include "../core/state/StateManager.h"
+
+#include <SDL3/SDL.h>
+
+#include <algorithm>
+#include <cmath>
+#include <cstring>
+#include <cstdint>
+
+extern "C" {
+#include <libavformat/avformat.h>
+#include <libavcodec/avcodec.h>
+#include <libavutil/avutil.h>
+#include <libavutil/channel_layout.h>
+#include <libswresample/swresample.h>
+}
+
+VideoState::VideoState(StateContext& ctx)
+    : State(ctx)
+    , m_player(std::make_unique<VideoPlayer>())
+{
+}
+
+VideoState::~VideoState() {
+    onExit();
+}
+
+bool VideoState::begin(SDL_Renderer* renderer, const std::string& path) {
+    m_path = path;
+
+    if (!m_player) {
+        m_player = std::make_unique<VideoPlayer>();
+    }
+
+    if (!m_player->open(m_path, renderer)) {
+        SDL_LogWarn(SDL_LOG_CATEGORY_APPLICATION, "[VideoState] Failed to open intro video: %s", m_path.c_str());
+        return false;
+    }
+
+    if (!m_player->decodeFirstFrame()) {
+        SDL_LogWarn(SDL_LOG_CATEGORY_APPLICATION, "[VideoState] Failed to decode first frame: %s", m_path.c_str());
+        // Still allow entering; we will likely render black.
+    }
+
+    return true;
+}
+
+void VideoState::onEnter() {
+    m_phase = Phase::FadeInFirstFrame;
+    m_phaseClockMs = 0.0;
+    m_blackOverlayAlpha = 1.0f;
+
+    m_audioDecoded.store(false);
+    m_audioDecodeFailed.store(false);
+    m_audioStarted = false;
+    m_audioPcm.clear();
+    m_audioRate = 44100;
+    m_audioChannels = 2;
+
+    // Decode audio in the background during fade-in.
+    m_audioThread = std::make_unique<std::jthread>([this](std::stop_token st) {
+        (void)st;
+        std::vector<int16_t> pcm;
+        int rate = 44100;
+        int channels = 2;
+
+        const bool ok = decodeAudioPcm16Stereo44100(m_path, pcm, rate, channels);
+        if (!ok) {
+            m_audioDecodeFailed.store(true);
+            m_audioDecoded.store(true, std::memory_order_release);
+            return;
+        }
+
+        // Transfer results.
+        m_audioRate = rate;
+        m_audioChannels = channels;
+        m_audioPcm = std::move(pcm);
+        m_audioDecoded.store(true, std::memory_order_release);
+    });
+}
+
+void VideoState::onExit() {
+    stopAudio();
+
+    if (m_audioThread) {
+        // Request stop and join.
+        m_audioThread.reset();
+    }
+}
+
+void VideoState::handleEvent(const SDL_Event& e) {
+    (void)e;
+}
+
+void VideoState::startAudioIfReady() {
+    if (m_audioStarted) return;
+    if (!m_audioDecoded.load(std::memory_order_acquire)) return;
+    if (m_audioDecodeFailed.load()) return;
+    if (m_audioPcm.empty()) return;
+
+    // Use the existing audio output path (same device as music/SFX).
+    Audio::instance().playSfx(m_audioPcm, m_audioChannels, m_audioRate, 1.0f);
+    m_audioStarted = true;
+}
+
+void VideoState::stopAudio() {
+    // We currently feed intro audio as an SFX buffer into the mixer.
+    // It will naturally end; no explicit stop is required.
+}
+
+void VideoState::update(double frameMs) {
+    switch (m_phase) {
+        case Phase::FadeInFirstFrame: {
+            m_phaseClockMs += frameMs;
+            const float t = (FADE_IN_MS > 0.0) ? float(std::clamp(m_phaseClockMs / FADE_IN_MS, 0.0, 1.0)) : 1.0f;
+            m_blackOverlayAlpha = 1.0f - t;
+
+            if (t >= 1.0f) {
+                m_phase = Phase::Playing;
+                m_phaseClockMs = 0.0;
+                if (m_player) {
+                    m_player->start();
+                }
+                startAudioIfReady();
+            }
+            break;
+        }
+        case Phase::Playing: {
+            startAudioIfReady();
+            if (m_player) {
+                m_player->update(frameMs);
+                if (m_player->isFinished()) {
+                    m_phase = Phase::FadeOutToBlack;
+                    m_phaseClockMs = 0.0;
+                    m_blackOverlayAlpha = 0.0f;
+                }
+            } else {
+                m_phase = Phase::FadeOutToBlack;
+                m_phaseClockMs = 0.0;
+                m_blackOverlayAlpha = 0.0f;
+            }
+            break;
+        }
+        case Phase::FadeOutToBlack: {
+            m_phaseClockMs += frameMs;
+            const float t = (FADE_OUT_MS > 0.0) ? float(std::clamp(m_phaseClockMs / FADE_OUT_MS, 0.0, 1.0)) : 1.0f;
+            m_blackOverlayAlpha = t;
+            if (t >= 1.0f) {
+                // Switch to MAIN (Menu) with a fade-in from black.
+                if (ctx.startupFadeAlpha) {
+                    *ctx.startupFadeAlpha = 1.0f;
+                }
+                if (ctx.startupFadeActive) {
+                    *ctx.startupFadeActive = true;
+                }
+                if (ctx.stateManager) {
+                    ctx.stateManager->setState(AppState::Menu);
+                }
+                m_phase = Phase::Done;
+            }
+            break;
+        }
+        case Phase::Done:
+        default:
+            break;
+     }
+ }
+
+ void VideoState::render(SDL_Renderer* renderer, float logicalScale, SDL_Rect logicalVP) {
+    (void)logicalScale;
+    (void)logicalVP;
+
+    if (!renderer) return;
+
+    int winW = 0, winH = 0;
+    SDL_GetRenderOutputSize(renderer, &winW, &winH);
+
+    // Draw video fullscreen if available.
+    if (m_player && m_player->isTextureReady()) {
+        SDL_SetRenderViewport(renderer, nullptr);
+        SDL_SetRenderScale(renderer, 1.0f, 1.0f);
+        m_player->render(renderer, winW, winH);
+    } else {
+        SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
+        SDL_FRect r{0.f, 0.f, (float)winW, (float)winH};
+        SDL_RenderFillRect(renderer, &r);
+    }
+
+    // Apply fade overlay (black).
+    if (m_blackOverlayAlpha > 0.0f) {
+        const Uint8 a = (Uint8)std::clamp((int)std::lround(m_blackOverlayAlpha * 255.0f), 0, 255);
+        SDL_SetRenderDrawBlendMode(renderer, SDL_BLENDMODE_BLEND);
+        SDL_SetRenderDrawColor(renderer, 0, 0, 0, a);
+        SDL_FRect full{0.f, 0.f, (float)winW, (float)winH};
+        SDL_RenderFillRect(renderer, &full);
+    }
+ }
+
+bool VideoState::decodeAudioPcm16Stereo44100(
+    const std::string& path,
+    std::vector<int16_t>& outPcm,
+    int& outRate,
+    int& outChannels
+) {
+    outPcm.clear();
+    outRate = 44100;
+    outChannels = 2;
+
+    AVFormatContext* fmt = nullptr;
+    if (avformat_open_input(&fmt, path.c_str(), nullptr, nullptr) != 0) {
+        return false;
+    }
+
+    if (avformat_find_stream_info(fmt, nullptr) < 0) {
+        avformat_close_input(&fmt);
+        return false;
+    }
+
+    int audioStream = -1;
+    for (unsigned i = 0; i < fmt->nb_streams; ++i) {
+        if (fmt->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_AUDIO) {
+            audioStream = (int)i;
+            break;
+        }
+    }
+    if (audioStream < 0) {
+        avformat_close_input(&fmt);
+        return false;
+    }
+
+    AVCodecParameters* codecpar = fmt->streams[audioStream]->codecpar;
+    const AVCodec* codec = avcodec_find_decoder(codecpar->codec_id);
+    if (!codec) {
+        avformat_close_input(&fmt);
+        return false;
+    }
+
+    AVCodecContext* dec = avcodec_alloc_context3(codec);
+    if (!dec) {
+        avformat_close_input(&fmt);
+        return false;
+    }
+
+    if (avcodec_parameters_to_context(dec, codecpar) < 0) {
+        avcodec_free_context(&dec);
+        avformat_close_input(&fmt);
+        return false;
+    }
+
+    if (avcodec_open2(dec, codec, nullptr) < 0) {
+        avcodec_free_context(&dec);
+        avformat_close_input(&fmt);
+        return false;
+    }
+
+    AVChannelLayout outLayout{};
+    av_channel_layout_default(&outLayout, 2);
+
+    AVChannelLayout inLayout{};
+    if (av_channel_layout_copy(&inLayout, &dec->ch_layout) < 0 || inLayout.nb_channels <= 0) {
+        av_channel_layout_uninit(&inLayout);
+        av_channel_layout_default(&inLayout, 2);
+    }
+
+    SwrContext* swr = nullptr;
+    if (swr_alloc_set_opts2(
+            &swr,
+            &outLayout,
+            AV_SAMPLE_FMT_S16,
+            44100,
+            &inLayout,
+            dec->sample_fmt,
+            dec->sample_rate,
+            0,
+            nullptr
+        ) < 0) {
+        av_channel_layout_uninit(&inLayout);
+        av_channel_layout_uninit(&outLayout);
+        avcodec_free_context(&dec);
+        avformat_close_input(&fmt);
+        return false;
+    }
+
+    if (swr_init(swr) < 0) {
+        swr_free(&swr);
+        av_channel_layout_uninit(&inLayout);
+        av_channel_layout_uninit(&outLayout);
+        avcodec_free_context(&dec);
+        avformat_close_input(&fmt);
+        return false;
+    }
+
+    AVPacket* pkt = av_packet_alloc();
+    AVFrame* frame = av_frame_alloc();
+    if (!pkt || !frame) {
+        if (pkt) av_packet_free(&pkt);
+        if (frame) av_frame_free(&frame);
+        swr_free(&swr);
+        av_channel_layout_uninit(&inLayout);
+        av_channel_layout_uninit(&outLayout);
+        avcodec_free_context(&dec);
+        avformat_close_input(&fmt);
+        return false;
+    }
+
+    const int outRateConst = 44100;
+    const int outCh = 2;
+
+    while (av_read_frame(fmt, pkt) >= 0) {
+        if (pkt->stream_index != audioStream) {
+            av_packet_unref(pkt);
+            continue;
+        }
+
+        if (avcodec_send_packet(dec, pkt) < 0) {
+            av_packet_unref(pkt);
+            continue;
+        }
+        av_packet_unref(pkt);
+
+        while (true) {
+            const int rr = avcodec_receive_frame(dec, frame);
+            if (rr == AVERROR(EAGAIN) || rr == AVERROR_EOF) {
+                break;
+            }
+            if (rr < 0) {
+                break;
+            }
+
+            const int64_t delay = swr_get_delay(swr, dec->sample_rate);
+            const int dstNbSamples = (int)av_rescale_rnd(delay + frame->nb_samples, outRateConst, dec->sample_rate, AV_ROUND_UP);
+
+            std::vector<uint8_t> outBytes;
+            outBytes.resize((size_t)dstNbSamples * (size_t)outCh * sizeof(int16_t));
+
+            uint8_t* outData[1] = { outBytes.data() };
+            const uint8_t** inData = (const uint8_t**)frame->data;
+
+            const int converted = swr_convert(swr, outData, dstNbSamples, inData, frame->nb_samples);
+            if (converted > 0) {
+                const size_t samplesOut = (size_t)converted * (size_t)outCh;
+                const int16_t* asS16 = (const int16_t*)outBytes.data();
+                const size_t oldSize = outPcm.size();
+                outPcm.resize(oldSize + samplesOut);
+                std::memcpy(outPcm.data() + oldSize, asS16, samplesOut * sizeof(int16_t));
+            }
+
+            av_frame_unref(frame);
+        }
+    }
+
+    // Flush decoder
+    avcodec_send_packet(dec, nullptr);
+    while (avcodec_receive_frame(dec, frame) >= 0) {
+        const int64_t delay = swr_get_delay(swr, dec->sample_rate);
+        const int dstNbSamples = (int)av_rescale_rnd(delay + frame->nb_samples, outRateConst, dec->sample_rate, AV_ROUND_UP);
+        std::vector<uint8_t> outBytes;
+        outBytes.resize((size_t)dstNbSamples * (size_t)outCh * sizeof(int16_t));
+        uint8_t* outData[1] = { outBytes.data() };
+        const uint8_t** inData = (const uint8_t**)frame->data;
+        const int converted = swr_convert(swr, outData, dstNbSamples, inData, frame->nb_samples);
+        if (converted > 0) {
+            const size_t samplesOut = (size_t)converted * (size_t)outCh;
+            const int16_t* asS16 = (const int16_t*)outBytes.data();
+            const size_t oldSize = outPcm.size();
+            outPcm.resize(oldSize + samplesOut);
+            std::memcpy(outPcm.data() + oldSize, asS16, samplesOut * sizeof(int16_t));
+        }
+        av_frame_unref(frame);
+    }
+
+    av_frame_free(&frame);
+    av_packet_free(&pkt);
+    swr_free(&swr);
+    av_channel_layout_uninit(&inLayout);
+    av_channel_layout_uninit(&outLayout);
+    avcodec_free_context(&dec);
+    avformat_close_input(&fmt);
+
+    outRate = outRateConst;
+    outChannels = outCh;
+
+    return !outPcm.empty();
+}

src/states/VideoState.h

@@ -0,0 +1,67 @@
+// VideoState.h
+#pragma once
+
+#include "State.h"
+
+#include <atomic>
+#include <memory>
+#include <string>
+#include <thread>
+#include <vector>
+
+class VideoPlayer;
+
+class VideoState : public State {
+public:
+    explicit VideoState(StateContext& ctx);
+    ~VideoState() override;
+
+    void onEnter() override;
+    void onExit() override;
+    void handleEvent(const SDL_Event& e) override;
+    void update(double frameMs) override;
+    void render(SDL_Renderer* renderer, float logicalScale, SDL_Rect logicalVP) override;
+
+    // Called from the App's on-enter hook so we can create textures.
+    bool begin(SDL_Renderer* renderer, const std::string& path);
+
+private:
+    enum class Phase {
+        FadeInFirstFrame,
+        Playing,
+        FadeOutToBlack,
+        Done
+    };
+
+    void startAudioIfReady();
+    void stopAudio();
+
+    static bool decodeAudioPcm16Stereo44100(
+        const std::string& path,
+        std::vector<int16_t>& outPcm,
+        int& outRate,
+        int& outChannels
+    );
+
+    std::unique_ptr<VideoPlayer> m_player;
+    std::string m_path;
+
+    Phase m_phase = Phase::FadeInFirstFrame;
+    double m_phaseClockMs = 0.0;
+
+    static constexpr double FADE_IN_MS = 900.0;
+    static constexpr double FADE_OUT_MS = 450.0;
+
+    // Audio decoding runs in the background while we fade in.
+    std::atomic<bool> m_audioDecoded{false};
+    std::atomic<bool> m_audioDecodeFailed{false};
+    std::vector<int16_t> m_audioPcm;
+    int m_audioRate = 44100;
+    int m_audioChannels = 2;
+    bool m_audioStarted = false;
+
+    std::unique_ptr<std::jthread> m_audioThread;
+
+    // Render-time overlay alpha (0..1) for fade stages.
+    float m_blackOverlayAlpha = 1.0f;
+};

src/video/VideoPlayer.cpp

@@ -0,0 +1,172 @@
+#include "VideoPlayer.h"
+
+#include <iostream>
+#include <chrono>
+
+extern "C" {
+#include <libavformat/avformat.h>
+#include <libavcodec/avcodec.h>
+#include <libswscale/swscale.h>
+#include <libavutil/imgutils.h>
+}
+
+VideoPlayer::VideoPlayer() {}
+
+VideoPlayer::~VideoPlayer() {
+    if (m_texture) SDL_DestroyTexture(m_texture);
+    if (m_rgbBuffer) av_free(m_rgbBuffer);
+    if (m_frame) av_frame_free(&m_frame);
+    if (m_sws) sws_freeContext(m_sws);
+    if (m_dec) avcodec_free_context(&m_dec);
+    if (m_fmt) avformat_close_input(&m_fmt);
+}
+
+bool VideoPlayer::open(const std::string& path, SDL_Renderer* renderer) {
+    m_path = path;
+    avformat_network_init();
+    if (avformat_open_input(&m_fmt, path.c_str(), nullptr, nullptr) != 0) {
+        std::cerr << "VideoPlayer: failed to open " << path << "\n";
+        return false;
+    }
+    if (avformat_find_stream_info(m_fmt, nullptr) < 0) {
+        std::cerr << "VideoPlayer: failed to find stream info\n";
+        return false;
+    }
+    // Find video stream
+    m_videoStream = -1;
+    for (unsigned i = 0; i < m_fmt->nb_streams; ++i) {
+        if (m_fmt->streams[i]->codecpar->codec_type == AVMEDIA_TYPE_VIDEO) { m_videoStream = (int)i; break; }
+    }
+    if (m_videoStream < 0) { std::cerr << "VideoPlayer: no video stream\n"; return false; }
+
+    AVCodecParameters* codecpar = m_fmt->streams[m_videoStream]->codecpar;
+    const AVCodec* codec = avcodec_find_decoder(codecpar->codec_id);
+    if (!codec) { std::cerr << "VideoPlayer: decoder not found\n"; return false; }
+    m_dec = avcodec_alloc_context3(codec);
+    if (!m_dec) { std::cerr << "VideoPlayer: failed to alloc codec ctx\n"; return false; }
+    if (avcodec_parameters_to_context(m_dec, codecpar) < 0) { std::cerr << "VideoPlayer: param to ctx failed\n"; return false; }
+    if (avcodec_open2(m_dec, codec, nullptr) < 0) { std::cerr << "VideoPlayer: open codec failed\n"; return false; }
+
+    m_width = m_dec->width;
+    m_height = m_dec->height;
+    m_frame = av_frame_alloc();
+    m_sws = sws_getContext(m_width, m_height, m_dec->pix_fmt, m_width, m_height, AV_PIX_FMT_RGBA, SWS_BILINEAR, nullptr, nullptr, nullptr);
+    m_rgbBufferSize = av_image_get_buffer_size(AV_PIX_FMT_RGBA, m_width, m_height, 1);
+    m_rgbBuffer = (uint8_t*)av_malloc(m_rgbBufferSize);
+
+    if (renderer) {
+        m_texture = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA32, SDL_TEXTUREACCESS_STREAMING, m_width, m_height);
+        if (!m_texture) { std::cerr << "VideoPlayer: failed create texture\n"; }
+    }
+
+    m_finished = false;
+    m_textureReady = false;
+    m_started = false;
+    m_frameAccumulatorMs = 0.0;
+
+    // Estimate frame interval.
+    m_frameIntervalMs = 33.333;
+    if (m_fmt && m_videoStream >= 0) {
+        AVRational fr = m_fmt->streams[m_videoStream]->avg_frame_rate;
+        if (fr.num > 0 && fr.den > 0) {
+            const double fps = av_q2d(fr);
+            if (fps > 1.0) {
+                m_frameIntervalMs = 1000.0 / fps;
+            }
+        }
+    }
+
+    // Seek to start
+    av_seek_frame(m_fmt, m_videoStream, 0, AVSEEK_FLAG_BACKWARD);
+    if (m_dec) avcodec_flush_buffers(m_dec);
+    return true;
+}
+
+bool VideoPlayer::decodeOneFrame() {
+    if (m_finished || !m_fmt) return false;
+
+    AVPacket* pkt = av_packet_alloc();
+    if (!pkt) {
+        m_finished = true;
+        return false;
+    }
+
+    int ret = 0;
+    while (av_read_frame(m_fmt, pkt) >= 0) {
+        if (pkt->stream_index == m_videoStream) {
+            ret = avcodec_send_packet(m_dec, pkt);
+            if (ret < 0) {
+                av_packet_unref(pkt);
+                continue;
+            }
+
+            while (ret >= 0) {
+                ret = avcodec_receive_frame(m_dec, m_frame);
+                if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF) break;
+                if (ret < 0) break;
+
+                uint8_t* dstData[4] = { m_rgbBuffer, nullptr, nullptr, nullptr };
+                int dstLinesize[4] = { m_width * 4, 0, 0, 0 };
+                sws_scale(m_sws, m_frame->data, m_frame->linesize, 0, m_height, dstData, dstLinesize);
+                m_textureReady = true;
+                if (m_texture) {
+                    SDL_UpdateTexture(m_texture, nullptr, m_rgbBuffer, dstLinesize[0]);
+                }
+                av_frame_unref(m_frame);
+
+                av_packet_unref(pkt);
+                av_packet_free(&pkt);
+                return true;
+            }
+        }
+        av_packet_unref(pkt);
+    }
+
+    av_packet_free(&pkt);
+    m_finished = true;
+    return false;
+}
+
+bool VideoPlayer::decodeFirstFrame() {
+    if (!m_fmt || m_finished) return false;
+    if (m_textureReady) return true;
+    // Ensure we are at the beginning.
+    av_seek_frame(m_fmt, m_videoStream, 0, AVSEEK_FLAG_BACKWARD);
+    if (m_dec) avcodec_flush_buffers(m_dec);
+    return decodeOneFrame();
+}
+
+void VideoPlayer::start() {
+    m_started = true;
+}
+
+bool VideoPlayer::update(double deltaMs) {
+    if (m_finished || !m_fmt) return false;
+    if (!m_started) return true;
+
+    m_frameAccumulatorMs += deltaMs;
+
+    // Decode at most a small burst per frame to avoid spiral-of-death.
+    int framesDecoded = 0;
+    const int maxFramesPerTick = 4;
+    while (m_frameAccumulatorMs >= m_frameIntervalMs && framesDecoded < maxFramesPerTick) {
+        m_frameAccumulatorMs -= m_frameIntervalMs;
+        if (!decodeOneFrame()) {
+            return false;
+        }
+        ++framesDecoded;
+    }
+    return !m_finished;
+}
+
+bool VideoPlayer::update() {
+    // Legacy behavior: decode exactly one frame.
+    return decodeOneFrame();
+}
+
+void VideoPlayer::render(SDL_Renderer* renderer, int winW, int winH) {
+    if (!m_textureReady || !m_texture || !renderer) return;
+    if (winW <= 0 || winH <= 0) return;
+    SDL_FRect dst = { 0.0f, 0.0f, (float)winW, (float)winH };
+    SDL_RenderTexture(renderer, m_texture, nullptr, &dst);
+}

src/video/VideoPlayer.h

@@ -0,0 +1,59 @@
+// Minimal FFmpeg-based video player (video) that decodes into an SDL texture.
+// Audio for the intro is currently handled outside this class.
+#pragma once
+
+#include <string>
+#include <SDL3/SDL.h>
+
+struct AVFormatContext;
+struct AVCodecContext;
+struct SwsContext;
+struct AVFrame;
+
+class VideoPlayer {
+public:
+    VideoPlayer();
+    ~VideoPlayer();
+
+    // Open video file and attach to SDL_Renderer for texture creation
+    bool open(const std::string& path, SDL_Renderer* renderer);
+    // Decode the first frame immediately so it can be used for fade-in.
+    bool decodeFirstFrame();
+
+    // Start time-based playback.
+    void start();
+
+    // Update playback using elapsed time in milliseconds.
+    // Returns false if finished or error.
+    bool update(double deltaMs);
+
+    // Compatibility: advance by one decoded frame.
+    bool update();
+
+    // Render video frame fullscreen to the given renderer using provided output size.
+    void render(SDL_Renderer* renderer, int winW, int winH);
+    bool isFinished() const { return m_finished; }
+    bool isTextureReady() const { return m_textureReady; }
+
+    double getFrameIntervalMs() const { return m_frameIntervalMs; }
+    bool isStarted() const { return m_started; }
+
+private:
+    bool decodeOneFrame();
+
+    AVFormatContext* m_fmt = nullptr;
+    AVCodecContext* m_dec = nullptr;
+    SwsContext* m_sws = nullptr;
+    AVFrame* m_frame = nullptr;
+    int m_videoStream = -1;
+    double m_frameIntervalMs = 33.333;
+    double m_frameAccumulatorMs = 0.0;
+    bool m_started = false;
+    int m_width = 0, m_height = 0;
+    SDL_Texture* m_texture = nullptr;
+    uint8_t* m_rgbBuffer = nullptr;
+    int m_rgbBufferSize = 0;
+    bool m_textureReady = false;
+    bool m_finished = true;
+    std::string m_path;
+};

vcpkg.json

@@ -6,8 +6,10 @@
 			"name": "sdl3-image",
 			"features": ["jpeg", "png", "webp"]
 		},
+		"enet",
 		"catch2",
 		"cpr",
-		"nlohmann-json"
+		"nlohmann-json",
+		"ffmpeg"
 	]
 }