CosmicClash/Game/scripts/input_jitter_buffer.gd

class_name InputJitterBuffer
extends RefCounted

# Per-player server-side input state (multiplayer-todo.md §3, task 3.2).
# Deliberately a standalone RefCounted with no scene/RPC dependency — same
# reason net_codec.gd and net_interpolator.gd are pure classes — so task
# 3.5's unit tests can drive it with scripted arrival traces with no live
# match. NetworkedMatch owns one instance per connected slot and is the only
# thing that talks to the network layer; this class only knows about
# sequence numbers and ShipActions.
#
# Ring is fixed-size and slot-tagged (§3.1 step 5's "a client can never make
# the server allocate"): ingest() writes seq % RING_SIZE regardless of how
# large or malicious seq is, and consume() only ever trusts a slot whose
# stored seq exactly matches the one it expects — a stale or wrapped-around
# entry is indistinguishable from an empty one. Range/rate validation of seq
# against the current server tick is the CALLER's job (task 3.4), not this
# class's, since only the caller knows the current server tick.

const RING_SIZE := 32
# 500ms at 60Hz (multiplayer-todo.md §3.2's own numbers) — a duration, not a
# tick-rate-derived constant, so left as a literal rather than pulling in
# SimConstants for one number.
const STARVE_ZERO_TICKS := 30

var last_applied_seq := -1   # -1: consume() has never been called yet
var last_action := ShipAction.new()
var starved_ticks := 0
var stalled := false

var _ring_action: Array = []
var _ring_seq: PackedInt32Array = PackedInt32Array()
# True once ingest() has ever been called for real. Consumption is a no-op
# (no starvation counted, no advancement) until then — see ingest()'s own
# comment for why an un-seeded buffer would otherwise never converge with
# what the client is actually sending.
var _seeded := false
# Highest seq ever seen by ingest(), regardless of whether it's still in the
# ring — consume()'s only way to tell "the data is gone because the ring
# overflowed" apart from "the data just hasn't arrived yet". See consume()'s
# own comment for why this exists: an adversarial review found that without
# it, a backlog bigger than RING_SIZE (a host stall, or persistent client/
# server clock drift) permanently zeroed a connected player's input for the
# rest of the match.
#
# Deliberately public (no underscore), same as last_applied_seq: the
# networked_match.gd caller's seq-range guard (§3.1 step 4) must bound
# against THIS, not against last_applied_seq. A second adversarial review
# found that bounding against last_applied_seq caps every accepted seq at
# last_applied_seq + RING_SIZE, which in turn caps this field at the same
# ceiling — making the resync condition below (which needs this field to
# reach expected + RING_SIZE) arithmetically unreachable on the only call
# path that exists in production. The two fixes looked independent but
# shared a variable and silently cancelled each other out. highest_ingested
# tracks the client's own send epoch instead, which the guard can safely
# let run ahead of a lagging consumer.
var highest_ingested_seq := -1


func _init() -> void:
	_ring_action.resize(RING_SIZE)
	_ring_seq.resize(RING_SIZE)
	for i in RING_SIZE:
		_ring_seq[i] = -1


# newest_seq/actions match NetCodec.unpack_input's own "seq"/"actions"
# fields directly: actions[i] is the action for sequence (newest_seq - i),
# newest-first. Already-consumed or stale entries are silently discarded
# (§3.1 step 5) — this is what makes redundant re-delivery of an already-
# applied tick harmless.
func ingest(newest_seq: int, actions: Array) -> void:
	if not _seeded:
		# The server starts calling consume() every tick the instant this
		# slot exists — well before this player's first packet has had time
		# to arrive (connection handshake, arena/ship spawn, first
		# _physics_process tick on the client all take real time first). An
		# un-seeded last_applied_seq of -1 would have consume() "expecting"
		# sequence 0, 1, 2, ... via pure starvation the whole time, racing
		# arbitrarily far ahead of whatever the client's own from-1
		# numbering has actually reached by the time real packets show up —
		# and since both sides only ever advance monotonically with no
		# resync mechanism, that gap would never close. Seed to align
		# "expected" with reality the moment real data first exists.
		last_applied_seq = newest_seq - actions.size()
		_seeded = true
	if newest_seq > highest_ingested_seq:
		highest_ingested_seq = newest_seq
	for i in actions.size():
		var seq: int = newest_seq - i
		if seq <= last_applied_seq:
			continue
		var idx := seq % RING_SIZE
		_ring_seq[idx] = seq
		_ring_action[idx] = actions[i]


# Contiguous run of not-yet-applied entries starting right after
# last_applied_seq — reported as input_buffer_depth in every snapshot
# (§3.3) and consumed client-side by the input_lead control loop (task 3.3).
func depth() -> int:
	if not _seeded or last_applied_seq < 0:
		return 0
	var d := 0
	var seq := last_applied_seq + 1
	while d < RING_SIZE and _ring_seq[seq % RING_SIZE] == seq:
		d += 1
		seq += 1
	return d


# Called once per server physics tick, before the step (§3.2). A no-op
# (returns the zero-initialized last_action, no starvation counted) until
# this player's first real packet has ever arrived — see ingest()'s comment.
func consume() -> ShipAction:
	if not _seeded:
		return last_action
	var expected := last_applied_seq + 1
	var idx := expected % RING_SIZE

	# Ring-overflow resync. A fixed-size ring can only ever hold RING_SIZE
	# ticks of not-yet-consumed data at once — if the caller has fallen
	# further behind the newest data actually arriving than that (a host
	# stall, or persistent client/server clock drift), every tick between
	# "expected" and "highest_ingested_seq - RING_SIZE" has already been
	# irrecoverably overwritten by more recent arrivals landing on the same
	# ring slots. Waiting for it tick-by-tick would starve — and, past
	# STARVE_ZERO_TICKS, zero this player's ship — for the ENTIRE gap even
	# though fresh, real input already exists in the ring right now. An
	# adversarial review found and reproduced this exact failure (a ~0.7s
	# host freeze permanently zeroed a connected player's input for the
	# rest of the match, with no self-recovery). Skip the unrecoverable
	# span and resync directly to what the ring can still actually provide.
	if highest_ingested_seq - expected >= RING_SIZE:
		last_applied_seq = highest_ingested_seq - RING_SIZE
		expected = last_applied_seq + 1
		idx = expected % RING_SIZE

	if _ring_seq[idx] == expected:
		last_action = _ring_action[idx]
		starved_ticks = 0
		stalled = false
	else:
		# Repeat-last, not zero: inputs are heavily autocorrelated at 60Hz,
		# and the client already predicted with the real input either way,
		# so repeating minimises expected divergence (§3.2). Only zero after
		# a sustained stall, so a disconnecting player's ship doesn't fly
		# into a wall at full throttle forever.
		starved_ticks += 1
		if starved_ticks > STARVE_ZERO_TICKS:
			last_action = ShipAction.new()
			stalled = true
	last_applied_seq = expected
	return last_action