codexPySnake/PROJECT_PLAN.md

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).

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.
• 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.
• Apples and scoring:
  • Replace score display with current snake length.
  • Eating 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”.

Input Buffer Rules

• Maintain a small buffer of up to 3 upcoming direction changes.
• If the new input is directly opposite to the last buffered direction, replace the last buffered entry instead of appending.
• Drop repeats: Do not add consecutive duplicates to the buffer.
• 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.

Server Architecture (Python 3)

• Runtime: Python 3 with asyncio.
• 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.
• 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.
  • 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.
• 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.
• 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.
• 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).

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.

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).

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.

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.
• Per-snake encoding (compressed):
  • 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.
• 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.

Sequence wrap & ordering:

• Define is_newer(a, b) using signed 16-bit difference: ((a - b) & 0xFFFF) < 0x8000.
• Clients ignore updates older-than last applied per-sender.

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.
  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.

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.
• 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.

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.

Error Handling & Resilience

• 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.

Testing Strategy

• Unit tests: input buffer rules, 180° 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.

Milestones

1. Protocol draft + 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.

Open Questions

• Exact tick rate target and interpolation strategy on client.
• Initial snake length on spawn (3 is suggested) and spawn retry rules in crowded fields.
• Apple count baseline when players are present (fixed count vs proportional to players vs area).
• 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).

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).
• Begin Milestone 2: implement server skeleton and simulation core.