Changes in project plans: added client-side opponen movement prediction.

PROJECT_PLAN.md

@@ -1,25 +1,25 @@
-# Multiplayer Snake (WebTransport) — Project Plan
+# Multiplayer Snake (WebTransport) - Project Plan
 
 ## Overview
 - Goal: Build a real-time, networked multiplayer Snake game with a Python 3 authoritative server and a web client using WebTransport datagrams for low-latency updates.
-- Gameplay: Continuous, persistent world (no rounds). Players can join/leave anytime. Display each snake’s current length; the longest snake is the momentary “leader”.
-- Field: Default 60×40 grid; protocol supports 3×3 up to 255×255.
-- Networking: Unreliable, unordered datagrams with sequence numbers and wraparound handling. Compression and packet partitioning to fit MTU (~1280–1500 bytes).
+- Gameplay: Continuous, persistent world (no rounds). Players can join/leave anytime. Display each snake's current length; the longest snake is the momentary leader.
+- Field: Default 60x40 grid; protocol supports 3x3 up to 255x255.
+- Networking: Unreliable, unordered datagrams with sequence numbers and wraparound handling. Compression and packet partitioning to fit MTU (~1280-1500 bytes).
 
 ## Core Mechanics
-- Collision handling: When the snake head would hit an obstacle (wall, self, other snake), the head stays in place (blocked) and the tail shrinks by 1 per tick until the player turns to a free direction.
+- Collision handling: If the head would hit an obstacle (wall, self, other snake), the head stays in place (blocked) and the tail shrinks by 1 per tick until the player turns to a free direction.
 - Self-collision: Temporary; tail shrink can clear the blocking segment.
 - Other-snake collision: Clears when the other snake moves or shrinks away.
 - Minimum length: A snake cannot be shorter than its head; minimum length is 1.
 - Turning rules:
-  - Length > 1: 180° turns are invalid and ignored at consumption time.
-  - Length = 1 (only head left): 180° turns are valid.
+  - Length > 1: 180-degree turns are invalid and ignored at consumption time.
+  - Length = 1 (only head left): 180-degree turns are valid.
 - Apples and scoring:
   - Replace score display with current snake length.
-  - Eating apple grows snake by 1 immediately.
+  - Eating an apple grows snake by 1 immediately.
   - When 0 players remain connected, pre-populate field with exactly 3 apples.
 - Colors: Assign a color when a client connects and keep it stable for that session.
-- Spectator join: On initial connection, show current gameplay and overlay “press space to join”.
+- Spectator join: On initial connection, show current gameplay and overlay "press space to join".
 
 ## Input Buffer Rules
 - Maintain a small buffer of up to 3 upcoming direction changes.
@@ -28,117 +28,131 @@
 - Overflow policy: If buffer is full, replace the last element with the new input.
 - At each tick, before moving:
   - Consume at most one input from the buffer.
-  - If length > 1 and it is a 180° turn, ignore it and immediately try the next buffered input; if none valid, keep current direction.
+  - If length > 1 and it is a 180-degree turn, ignore it and immediately try the next buffered input; if none valid, keep current direction.
 
 ## Server Architecture (Python 3)
 - Runtime: Python 3 with asyncio.
-- Transport: WebTransport (HTTP/3 datagrams). Candidate library: `aioquic` for server-side H3 and datagrams.
+- Transport: WebTransport (HTTP/3 datagrams). Candidate library: aioquic for server-side H3 and datagrams.
 - Model:
-  - Authoritative simulation on the server with a fixed tick rate (target 15–20 TPS; start with 15 for network headroom).
-  - Each tick: process inputs, update snakes, resolve collisions (blocked/shrink), move apples, generate state deltas.
+  - Authoritative simulation on the server with a fixed tick rate (target 15-20 TPS; start with 15 for network headroom).
+  - Each tick: process inputs, update snakes, resolve collisions (blocked/shrink), manage apples, generate state deltas.
 - Entities:
   - Field: width, height, random seed, apple set.
-  - Snake: id, name (≤16 chars), color id (0–31), deque of cells (head-first), current direction, input buffer, blocked flag, last-move tick, last-known client seq.
+  - Snake: id, name (<=16 chars), color id (0-31), deque of cells (head-first), current direction, input buffer, blocked flag, last-move tick, last-known client seq.
   - Player session: transport bindings, last-received input seq per player, anti-spam counters.
 - Spawn:
   - New snake spawns at a random free cell with a random legal starting direction, length 3 by default (configurable).
   - On apple eat: grow by 1; spawn a replacement apple at a free cell.
-- Disconnect: Immediately remove snake and its score/length; apples persist. If all disconnect, ensure field contains 3 apples.
+- Disconnect: Immediately remove snake and its length; apples persist. If all disconnect, ensure field contains 3 apples.
 - Rate limiting: Cap input datagrams per second and buffer growth per client.
 
 ## Client Architecture (Web)
-- Tech: Browser WebTransport (H3 datagrams), Canvas/WebGL rendering, vanilla or minimal framework for UI overlay.
+- Tech: Browser WebTransport (H3 datagrams), Canvas/WebGL rendering, minimal UI framework.
 - Modes:
-  - Spectator: Render world, show overlay “press space to join”.
-  - Player: Capture keyboard, build input buffer client-side, send inputs with sequence numbers; predict local movement for smoothness but reconcile to server.
+  - Spectator: Render world, show overlay "press space to join".
+  - Player: Capture keyboard, build input buffer client-side, send inputs with sequence numbers; predict local movement for smoothness and reconcile to server.
 - Rendering:
   - Gridless pixel or cell-based canvas.
   - Colors: 32-color predefined palette; deterministic mapping from player id to color id.
   - HUD: Current length; leaderboard (top N by length).
+- Opponent prediction: When server state updates are late, step other snakes using server input broadcasts from their last known state; maintain mirrored per-opponent buffers and reconcile on the next authoritative update.
 
 ## Networking & Protocol
 Constraints and goals:
 - Use datagrams (unreliable, unordered). Include a packet sequence number for late-packet discard with wraparound handling.
-- Keep payloads ≤1280 bytes to avoid fragmentation; if larger, partition logically.
-- Support up to 32 concurrent players; names limited to 16 bytes UTF‑8.
+- Keep payloads <=1280 bytes to avoid fragmentation; if larger, partition logically.
+- Support up to 32 concurrent players; names limited to 16 bytes UTF-8.
 
 Header (all packets):
-- `ver` (u8): protocol version.
-- `type` (u8): packet type (join, join_ack, input, state_delta, state_full, part, ping, pong, error).
-- `flags` (u8): bit flags (compression, is_last_part, etc.).
-- `seq` (u16): sender’s monotonically incrementing sequence number (wraps at 65535). Newer-than logic uses half-range window.
-- Optional `tick` (u16): simulation tick the packet refers to (on server updates).
+- ver (u8): protocol version.
+- type (u8): packet type (join, join_ack, input, input_broadcast, state_delta, state_full, part, ping, pong, error).
+- flags (u8): bit flags (compression, is_last_part, etc.).
+- seq (u16): sender's monotonically incrementing sequence number (wraps at 65535). Newer-than logic uses half-range window.
+- Optional tick (u16): simulation tick the packet refers to (on server updates).
 
-Handshake (join → join_ack):
-- Client sends: desired name (≤16), optional preferred color id.
+Handshake (join -> join_ack):
+- Client sends: desired name (<=16), optional preferred color id.
 - Server replies: assigned player id (u8), color id (u8), field size (width u8, height u8), tick rate (u8), random seed (u32), palette, and initial full snapshot.
 
-Inputs (client → server):
-- Packet `input` includes: last-acknowledged server seq (u16), local `input_seq` (u16), one or more direction events with timestamps or relative tick offsets. Client pre-filters per buffer rules.
+Inputs (client -> server):
+- Packet input includes: last-acknowledged server seq (u16), local input_seq (u16), one or more direction events with timestamps or relative tick offsets. Client pre-filters per buffer rules.
 
-State updates (server → client):
+Input broadcast (server -> clients):
+- Upon receiving a player's input, the server immediately relays those input events to all other clients as input_broadcast packets.
+- Contents: player_id (u8), the player's input_seq (u16), direction events, and apply_at_tick (u16) assigned by the server (typically current_tick+1) so clients can align predictions.
+- Purpose: Enable client-side opponent prediction during late or lost state updates. Broadcasts are small; apply rate limits if needed.
+
+State updates (server -> client):
 - Types:
-  - `state_full`: rare (on join or periodic recovery); includes all snakes and apples.
-  - `state_delta`: frequent; contains only changed snakes (head move, tail shrink/grow) and apple changes since last acked tick.
+  - state_full: rare (on join or periodic recovery); includes all snakes and apples.
+  - state_delta: frequent; contains only changed snakes (head move, tail shrink/grow) and apple changes since last acked tick.
 - Per-snake encoding (compressed):
-  - `id` (u8), `len` (u16), head `(x,y)` (u8,u8), direction (2 bits), and a compact body representation:
+  - id (u8), len (u16), head (x,y) (u8,u8), direction (2 bits), and a compact body representation:
     - Option A: Run-length of axis-aligned segments (dir, run u16) starting at head.
     - Option B: Delta-coded cells with RLE for straight runs.
 
 Compression:
-- Primary: domain-specific packing (bits + RLE for segments and apples), then optional DEFLATE flag (`flags.compressed=1`) if still large.
+- Primary: domain-specific packing (bits + RLE for segments and apples), then optional DEFLATE flag (flags.compressed=1) if still large.
 - Client decompresses if set; otherwise reads packed structs directly.
 
 Packet partitioning (if >1280 bytes):
-- Split `state_full`/`state_delta` by snakes into multiple independent sub-updates.
-- Each part has: `update_id` (u16), `part_index` (u8), `parts_total` (u8). Clients apply parts as they arrive; missing parts simply leave some snakes stale until the next update.
-- Extremely long single-snake updates: split that snake’s body into balanced segments by RLE chunks.
+- Split state_full/state_delta by snakes into multiple independent sub-updates.
+- Each part has: update_id (u16), part_index (u8), parts_total (u8). Clients apply parts as they arrive; missing parts simply leave some snakes stale until the next update.
+- Extremely long single-snake updates: split that snake's body into balanced segments by RLE chunks.
 
 Sequence wrap & ordering:
-- Define `is_newer(a, b)` using signed 16-bit difference: `((a - b) & 0xFFFF) < 0x8000`.
+- Define is_newer(a, b) using signed 16-bit difference: ((a - b) & 0xFFFF) < 0x8000.
 - Clients ignore updates older-than last applied per-sender.
+- For input_broadcast, deduplicate per (player_id, input_seq) and apply in order per player; if apply_at_tick is in the past, apply immediately up to current tick.
 
 ## Simulation Details
 - Tick order per tick:
-  1) Incorporate at most one valid buffered input per snake (with 180° rule).
-  2) Compute intended head step; if blocked, set `blocked=true` and keep head stationary.
-  3) If `blocked=true`: shrink tail by 1 (to min length 1). If the blocking cell becomes free (due to own shrink or others moving), the next tick’s step in current direction proceeds.
+  1) Incorporate at most one valid buffered input per snake (with 180-degree rule).
+  2) Compute intended head step; if blocked, set blocked=true and keep head stationary.
+  3) If blocked=true: shrink tail by 1 (to min length 1). If the blocking cell becomes free (due to own shrink or others moving), the next tick's step in current direction proceeds.
   4) If moving into an apple: grow by 1 (do not shrink tail that tick) and respawn an apple.
   5) Emit per-snake delta (move, grow, or shrink) and global apple changes.
 - Blocking detection: walls (0..width-1, 0..height-1), any occupied snake cell (including heads and tails that are not about to vacate this tick).
-- 180° turn allowance when length==1 only.
+- 180-degree turn allowance when length == 1 only.
+
+### Client-side Opponent Prediction Heuristics
+- Use last authoritative state per opponent as baseline; apply input_broadcast events at apply_at_tick boundaries.
+- Apply the same input buffer rules and 180-degree constraints as the server.
+- Approximate blocking: if predicted head step hits an occupied cell in the last known occupancy, mark blocked and simulate tail shrink until an alternate direction arrives; reconciliation will correct drift.
 
 ## Data Model & Structures
-- Field cells: Represent coordinates as `(x:u8, y:u8)`.
-- Snakes: Deque of cells or segment list; store length as `deque.size()` and maintain head index.
+- Field cells: Represent coordinates as (x:u8, y:u8).
+- Snakes: Deque of cells or segment list; store length as deque.size() and maintain head index.
 - Apples: Hash set of cells for O(1) lookup; spawn on empty cells only.
-- Occupancy: Sparse set/map from cell→(snake_id, index) for O(1) collision checks.
+- Occupancy: Sparse set/map from cell -> (snake_id, index) for O(1) collision checks.
 
 ## Limits & Config
 - Players: max 32 (ids 0..31); deny joins beyond capacity.
-- Names: UTF‑8, truncate to 16 bytes; filter control chars.
-- Field: default 60×40; min 3×3; max 255×255; provided by server in join_ack.
-- Tick rate: default 15 TPS; configurable 10–30 TPS.
+- Names: UTF-8, truncate to 16 bytes; filter control chars.
+- Field: default 60x40; min 3x3; max 255x255; provided by server in join_ack.
+- Tick rate: default 15 TPS; configurable 10-30 TPS.
 
 ## Error Handling & Resilience
-- Invalid packets: drop and optionally send `error` with code.
+- Invalid packets: drop and optionally send error with code.
 - Flooding: server-side rate limits and strike-based disconnects.
-- Out-of-order: discard using `is_newer` check.
-- Desync recovery: server sends `state_full` periodically or when client reports large gaps.
+- Out-of-order: discard using is_newer check.
+- Desync recovery: server sends state_full periodically or when client reports large gaps.
 
 ## Testing Strategy
-- Unit tests: input buffer rules, 180° logic, blocking/shrink, collision resolution, sequence wrap comparison.
+- Unit tests: input buffer rules, 180-degree logic, blocking/shrink, collision resolution, sequence wrap comparison.
 - Property tests: snake movement invariants (no duplicates in body except expected at head on block), apple placement safety.
 - Soak tests: bot clients sending random but rate-limited inputs; measure packet size and latency.
+- Prediction tests: delayed/missing state updates with input_broadcast streams; assert bounded drift and successful reconciliation.
 
 ## Milestones
-1) Protocol draft + wire structs (this plan).
+1) Protocol draft and wire structs (this plan).
 2) Server skeleton: tick loop, entities, occupancy, apples, basic WebTransport datagrams.
 3) Input handling: buffer logic and per-tick consumption; collision/blocking/shrink rules.
-4) State encoding: deltas, compression, and partitioning; client renderer (spectator).
-5) Player flow: join overlay, name/color, spawn, HUD and leaderboard.
-6) Performance pass: packet size tuning, RLE, optional DEFLATE, rate limits.
-7) Polishing: error messages, reconnect, telemetry/logging, Docker/dev scripts.
+4) Input relay and opponent prediction: server input_broadcast and client prediction + reconciliation.
+5) State encoding: deltas, compression, and partitioning; client renderer (spectator).
+6) Player flow: join overlay, name/color, spawn, HUD and leaderboard.
+7) Performance pass: packet size tuning, RLE, optional DEFLATE, rate limits.
+8) Polishing: error messages, reconnect, telemetry/logging, Docker/dev scripts.
 
 ## Open Questions
 - Exact tick rate target and interpolation strategy on client.
@@ -147,10 +161,11 @@ Sequence wrap & ordering:
 - Whether to allow wrap-around at field edges (currently: walls are blocking).
 - Compression choice tradeoffs: custom bitpacking only vs optional DEFLATE.
 - Minimum viable browser targets (WebTransport support varies).
+- Should input_broadcast include server-assigned apply_at_tick or relative tick offsets only?
 
 ## Next Steps
 - Validate the protocol choices and mechanics with stakeholders.
 - Decide on tick rate and initial lengths.
-- Confirm library choices (`aioquic` server; client-side `WebTransport` API usage).
+- Confirm library choices (aioquic server; client-side WebTransport API usage).
 - Begin Milestone 2: implement server skeleton and simulation core.