transcience/docs/NETCODE.md

Netcode

Model

Authoritative dedicated server, clients send input only, with client-side prediction and reconciliation for the local player.

client                          server (60 Hz)
  |                               |
  |-- InputFrame (60 Hz, x3) ---->|  queue_input(): validate, drop, sort
  |   move, aim, 3 button bits    |  SimWorld.step(): move, AI, emitters,
  |                               |    integrate bullets, resolve hits
  |<-- events (reliable, ch 3) ---|  bullet spawns/despawns, hits, deaths
  |<-- snapshot (unrel., ch 2) ---|  players, enemies, boss @ 20 Hz
  |                               |
  predict locally, reconcile      authoritative for everything

The client has no message that expresses a position, a hit, damage taken, or a completed escape. This is deliberate and stronger than validating such messages after the fact: there is no code path to exploit, only intent to interpret.

Why bullets are not replicated as state

A bullet hell has hundreds of live bullets — this project peaks at ~350 with one boss and four players. Sending a position per bullet per snapshot would be roughly 350 × 8 bytes × 20 Hz ≈ 56 KB/s per client and would grow with the pattern density, which is the one thing you want freedom to increase.

Instead the server sends one spawn event per bullet, once: uid, position, velocity, radius, lifetime, kind, team, acceleration, turn rate (~36 bytes). Both sides then run the identical integration in BulletPool.step(). A bullet that expires or leaves the arena dies on both sides with no traffic at all.

The only bullets that need a despawn message are the ones killed early by a hit, because that is the one outcome a client cannot derive.

Cost at the same peak: a few KB/s, dominated by whatever the patterns are actually firing rather than by how much is in the air.

tests/integration/test_replica_parity.gd is the test that keeps this honest. It runs the server and a replica side by side for 900 ticks of a boss fight — once in-process, once with every event encoded and decoded through the real wire format — and asserts that every live bullet matches within 0.01 px (1 px through float32 on the wire). If that test fails, the bullets on screen are no longer the bullets that can kill you.

Note what this does not require: lockstep determinism. Clients simulate bullets for display only. The server never reads a client's bullet state, so float drift between machines is a cosmetic concern, not a correctness one.

Hit validation

All in SimWorld._resolve_bullet_hits() and _resolve_contact_damage(), server only, circle-vs-circle:

• Enemy bullet → player: skipped entirely while the player has i-frames, so a dense pattern costs one hit, not thirty. The bullet is consumed and a BULLET_DESPAWN goes out.
• Player bullet → boss, then enemies: first overlap consumes the bullet. Boss damage is scaled by the current phase's damage_taken_mult.
• Enemy body → player: contact damage, same i-frame gate.

There is no lag compensation and no rewind. For a bullet hell that is the right call: what the player is dodging is their own position against a bullet field, and rewinding the world to a shooter's view would mean a player who dodged on their screen still gets hit. The cost is that a high-ping player's shots at a moving enemy land where the enemy was on the server. Enemies are large and slow relative to bullet speed, so this reads as latency rather than unfairness. If it becomes a complaint, the fix is to lag-compensate player bullets against enemies only, never enemy bullets against players.

Input timing, and a bug worth remembering

The client numbers its input frames INPUT_TARGET_LEAD ticks ahead of the server's tick; the server accepts anything within INPUT_MAX_LEAD and re-syncs nothing itself. These two numbers must be chosen together.

They originally were not — the server dropped inputs past a lead of 12, while the client only corrected itself past 16. A client whose lead drifted into 13–16 therefore had every input silently rejected, while believing its own timing was fine. The server, receiving nothing, coasted on the last input it had (held_input) and then stopped; the client kept predicting forward. The ship and the authoritative position separated permanently, the reconciler fought it every snapshot, and the player got shoved around. It needed two independent clocks to drift apart, so it appeared only after minutes of play — and never recovered, because nothing in the loop could notice.

Two defences now:

1. INPUT_MAX_LEAD (40) is far wider than the client's correction band (INPUT_LEAD_MIN 3 … INPUT_LEAD_MAX 20), so the client always re-syncs long before the server starts refusing anything. test_input_lead.gd asserts that ordering directly, so narrowing the window fails a test rather than shipping.
2. An ack-stall detector: if last_input_tick does not advance across INPUT_ACK_STALL_LIMIT snapshots, the client concludes its numbering is outside the window and hard re-syncs. Lead alone cannot detect this, because a wrong lead looks perfectly normal from the client's side. This is the backstop that makes the whole class of failure self-healing regardless of cause.

tools/diag_prediction.gd injects a +14 tick drift mid-run and asserts the gap returns to normal; it exits non-zero if it does not.

No invulnerability frames

A hit grants no immunity — every bullet that touches you deals its damage. In a bullet hell the wall is the threat, and i-frames invert that: you are punished for the first bullet of a pattern and gifted the next thirty, which makes dense patterns safer than sparse ones. Measured cost: a stationary player survives ~13.6s of the Warden's opening phase, ~17.5s while drifting aimlessly.

Arrival protection (SPAWN_GRACE_TICKS) is the sole exception, and it is a transition, not a combat mechanic.

Input validation

SimWorld.queue_input() is the single audit point. It drops:

Case	Guard
Replay of a consumed tick	f.tick <= p.last_input_tick
Stale input	f.tick < tick - INPUT_MAX_AGE
Input claiming the future	f.tick > tick + INPUT_MAX_LEAD
Flood	queue capped at INPUT_MAX_AGE, oldest dropped
Leaving the hub early after death	RESPAWN_LOCKOUT_TICKS, server-side

The respawn lockout is worth calling out: the HUD disables its button for the same three seconds, but that is presentation. A client that ignores its own UI and holds the bit down still waits, because the check lives in SimWorld.

Beyond that, the wire format itself constrains the cheat surface: the move vector is two signed bytes at 1/100 precision, and Movement.sanitize_move() clamps to the unit disc, so no packet can express extra speed. Fire rate is a server-side cooldown; holding the button every tick fires at exactly the same rate as tapping it correctly.

A client that stops sending coasts on its last input for INPUT_MAX_AGE ticks and then stops, so a dropped connection does not leave a player sliding.

Leaving a run, and why disconnecting is not an escape

The emergency escape is a one-second server-owned channel that damage does not interrupt. That combination is deliberate and the two halves depend on each other.

An interruptible channel sounds like the right kind of risk until you follow it through: a player about to die under fire can never finish the channel, so killing the game process becomes strictly better than using the button. The escape hatch turns into the exploit.

So a dropped connection runs the same channel. On disconnect the peer is removed from the send list but its player stays in the world (SimPlayer.linkdead), treated as holding the escape button down, still fully killable, for the same second everyone else pays. Only when that resolves is the peer forgotten. Pulling the plug is therefore never cheaper than pressing the key, and there is no timing window where it is.

Two details that are easy to get wrong and are covered by tests:

• An instance must not close while a linkdead body is still resolving (Instance.has_linkdead()), or a solo player's drop would delete their body on the next tick and hand back the exact exploit.
• Dying still ends the channel. That is the one thing the escape cannot beat, and it is what keeps the second a real risk rather than a formality.

tools/smoke.sh SIGKILLs a bot mid-dungeon and asserts the server channels it out rather than dropping it instantly.

Arriving in a run

Entering a dungeon grants SimConfig.SPAWN_GRACE_TICKS (2s) of arrival protection: invulnerable and unable to shoot. Both halves matter — invulnerability alone would make the spawn point a free firing position. It is flagged in the snapshot (Protocol.F_SPAWN_GRACE) so every client can draw it on every protected ship, not just its own.

Prediction and reconciliation

ClientRuntime keeps every unacknowledged InputFrame. Each snapshot echoes last_input_tick per player; on receipt the client discards acknowledged frames, takes the server's authoritative position, and replays the rest through the same Movement.step_player() the server used. Divergence over 24 px snaps; smaller errors lerp at 0.3 so ordinary jitter does not read as rubber-banding.

ClientRuntime sets process_physics_priority = -10 so it samples and sends input before ServerRuntime ticks. This matters only on a listen server, where both live in one process: with the default tree order the server ran first, so input sampled on frame N was not consumed until frame N+1, leaving the drawn ship a permanent one tick (4px at 240 u/s) ahead of the authoritative one — and bullets, which spawn at the authoritative position, visibly trailed behind the ship. tools/diag_prediction.gd measures this gap; it is currently ~0.1px mean, down from a constant 4.00px.

The other half of that fix is SimConfig.PLAYER_MUZZLE_OFFSET, derived from the visual radius rather than the hitbox. A remote client still draws its ship a tick or two ahead of the server, and that margin is what keeps bullets emerging from the nose rather than the middle of the sprite.

Remote players, enemies and the boss are interpolated between the last two snapshots. Bullets are not interpolated — they are simulated, and a spawn event carries the server tick it was generated at so the client fast-forwards the bullet by the packet's flight time (BulletPool.advance_slot()) instead of popping it in at the muzzle a round-trip late.

Channels

ENet channels keep a burst of reliable bullet events from head-of-line blocking the snapshot stream:

Channel	Traffic	Mode
1	handshake, instance transitions	reliable
2	snapshots	unreliable, newest wins
3	bullet spawns/despawns, hits	reliable ordered
4	client input	unreliable ordered

Protocol.VERSION is checked at handshake and mismatched clients are rejected with a reason, rather than desyncing later.

Instances

One server process hosts the lobby hub plus every concurrent dungeon. An Instance is a SimWorld and a peer list; messages are addressed to the peers of one instance, so a player never receives traffic for a world they are not in. At ~0.24 ms/tick per instance, the limit is bandwidth and player count long before it is CPU.

Transfers (portal in, escape out, dungeon cleared) are driven by server-only events that NetCodec refuses to serialise, so the decision cannot leak to a client as something it might try to send back.

The listen server

"Host and play" runs a ServerRuntime and a ClientRuntime in one process. The client is peer 1 and goes through the identical code path — it still sends input and still learns outcomes from snapshots. Only the transport is short-circuited, in Net.send_* / Net.send_input, which check for the local peer and call the handler directly instead of going through ENet. No simulation code knows the difference, so a bug cannot hide in a "host-only" branch.

Known gaps

• No lag compensation (see above — deliberate, with a stated escape hatch).
• No snapshot delta compression. Full state at 20 Hz is fine at this actor count; delta-encoding against the last acknowledged snapshot is the next step if enemy counts grow.
• No encryption or authentication. ENetMultiplayerPeer supports DTLS; wire it up before exposing a server to the internet.
• Bullet spawn events are per-bullet. Sending pattern events instead (emitter id + tick + seed, clients regenerate) would cut bullet traffic by roughly the count of each emitter's shot. Worth doing only if bandwidth becomes the binding constraint — it couples the client to emitter behaviour, which the current design deliberately avoids.