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 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: 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-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 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".

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-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.
• Model:
  • Authoritative simulation on the server with a fixed tick rate (default 10 TPS). Tick rate is reconfigurable at runtime by the server operator and changes are broadcast to clients.
• 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.
  • 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.
  • Initial length policy: try to allocate a 3-cell straight strip (head + two body cells) in a legal direction; if not possible, spawn with length 1.
  • If the field appears full, temporarily ignore apples as obstacles for the purpose of finding space. If no free single cell exists, deny the join and inform the client to wait.
• On apple eat: grow by 1; spawn a replacement apple at a free cell.
• 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, 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 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; prefer a safety budget of ~1200 bytes post-compression when partitioning.
• 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, join_deny, input, input_broadcast, state_delta, state_full, part, config_update, 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 | join_deny):

• Client sends: desired name (<=16), optional preferred color id.
• Server replies (join_ack): assigned player id (u8), color id (u8), field size (width u8, height u8), tick rate (u8), wrap_edges (bool), apples_per_snake (u8), apples_cap (u8 up to 255), compression_mode (enum: none|deflate), random seed (u32), palette, and initial full snapshot.
• Server can reply (join_deny) with a reason (e.g., no free cell; please wait).

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.

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)
  • base_tick (u16): server tick for alignment (typically current_tick)
  • events: one or more direction changes, each with rel_tick_offset (u8/varint) from base_tick
  • apply_at_tick (u16): optional absolute apply tick for convenience (both provided; clients may use either)
• 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.
• Per-snake encoding (compressed):
  • Header fields: id (u8), len (u16), head (x,y) (u8,u8).
  • Body TLV framing:
    • Type (T): QUIC varint. Values: 0=body_2bit, 1=body_rle, 0x10=body_2bit_chunk, 0x11=body_rle_chunk.
    • Length (L): QUIC varint; byte size of the Value.
    • Value (V): payload depends on Type (see below). TLV enables easy skipping and robust validation.
  • 2-bit body (T=0): direction stream from head toward tail using 2 bits per step: up=00, right=01, down=10, left=11. Total bits = (len-1)*2. Pack LSB-first within bytes; pad the last byte with zeros. Expected body bytes = ceil(((len-1)*2)/8).
    • Decoder: read exactly len-1 directions; verify body size matches expectation and padding bits are zero.
  • RLE body (T=1): sequence of runs describing straight segments from head toward tail.
    • Each run: dir (u8: 0=up,1=right,2=down,3=left), count (QUIC varint, number of steps, >=1).
    • Decoder: accumulate counts until total equals len-1; reject if over/under.
  • Chunked variants (T=0x10, 0x11): used only when a single snake must be split.
    • Prefix V with start_index (u16, first direction offset from head) and dirs_in_chunk (u16); then the encoding for that range (2-bit or RLE respectively).
    • Client buffers chunks by (update_id, snake_id) and assembles when the full [0..len-2] range is present; then applies.
  • Selection: server chooses per snake whichever TLV (2-bit or RLE) yields fewer bytes.

Compression:

• Primary: domain-specific packing (bits + RLE for segments and apples).
• Optional global DEFLATE mode (server-configured): when enabled, server may set flags.compressed=1 and apply DEFLATE to payloads; this mode is negotiated in join_ack and is not changed on the fly (server restart required; clients must reconnect).
• Client decompresses if set; otherwise reads packed structs directly.

Packet partitioning (if >~1200–1280 bytes after compression):

• Apply to state_full (join and periodic recovery) and large state_delta updates.
• Goal: avoid IP fragmentation. Ensure each compressed datagram payload is <1280 bytes (use ~1200 target for headroom).
• Strategy (whole-snake first):
  • Sort snakes by estimated compressed size (head + body TLV) descending.
  • Greedily pack one or more complete snakes per packet while keeping the compressed payload <1280 bytes.
  • If a snake does not fit with others, send it alone if it fits <1280 bytes.
  • If a single snake still exceeds 1280 bytes by itself, split that snake into multiple similar-sized chunks using the chunked TLV types.
• Framing:
  • Each PART carries update_id (u16), part_index (u8), parts_total (u8), and inner_type (u8: STATE_FULL or STATE_DELTA) followed by the chunk payload.
  • For STATE_FULL: the chunk payload is snakes_count + [per-snake records] + apples. Include apples only in the first part; clients merge across parts using update_id.
  • For STATE_DELTA: the chunk payload is a subset of per-snake changes; include apples_added/removed only in the first part; clients merge across parts using update_id.
  • Clients apply complete per-snake TLVs immediately. Chunk TLVs are buffered and assembled by (update_id, snake_id) using start_index and dirs_in_chunk before applying.

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.
• 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.

Live config updates (server -> clients):

• Packet type: config_update, sent on change and periodically (low frequency).
• Fields: tick_rate (u8), wrap_edges (bool), apples_per_snake (u8), apples_cap (u8 up to 255).
• Clients apply changes at the next tick boundary. Compression mode is not changed via config_update (handshake-only).

Simulation Details

• Tick order per tick:
  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). Wrap-around rules are applied only to the head; if wrap_edges is turned off while a body segment is mid-edge traversal, allow the body to continue, but prevent the head from crossing walls thereafter.
• 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.
• 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 60x40; min 3x3; max 255x255; provided by server in join_ack.
• Tick rate: default 10 TPS; server-controlled, reconfigurable on the fly via config_update (reasonable range 5-30 TPS).
• Apples per snake: default 1; server-controlled, reconfigurable on the fly; min 1/snake, max 12/snake; total apples capped at 255.
• Wrap-around edges: default off (walls are blocking); server-controlled, reconfigurable on the fly; head obeys current rule; legacy body traversal allowed during transitions.
• Compression (DEFLATE): global mode negotiated at join; requires server restart to change; clients must reconnect; when enabled server may set flags.compressed=1.
• Browser targets: must work on latest Firefox; latest Chrome is optional but desirable; others optional.

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-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 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. 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

• Interpolation/smoothing specifics on client for variable tick rates.
• Any additional anti-cheat measures for input_broadcast misuse.
• Exact UI/UX for join denial when field is full (messaging/timing).

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.