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docs/ai/cooperate_network.md

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+# 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.