# 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.