class_name LocalPredictionHistory
extends RefCounted

const NetBodyState = preload("res://scripts/net_body_state.gd")

# Client-owned local-ship prediction history (multiplayer-todo.md §4.3).
# This is deliberately independent of NetworkedMatch and the scene tree so
# sequence/ring behaviour can be tested from scripted traces. Each entry is
# tagged with its full sequence number: an old value in a wrapped slot is
# never accepted as a prediction for a newer sequence.
#
# Acknowledge and record are separate producer/consumer clocks. The input
# sender can continue producing while snapshots stop arriving, so record()
# explicitly marks overflow once more than RING_SIZE unacknowledged sequence
# positions exist. It still retains the newest representable window, but
# callers can see that an authoritative resync is required instead of
# mistaking a wrapped overwrite for a valid comparison.
#
# resync_required is a live condition, NOT a latch: compare_authoritative()
# clears it again once acknowledgements have genuinely caught back up (see
# that method). This mirrors input_jitter_buffer.gd's `stalled`, which
# likewise drops back to false the moment a normal tick is consumed again.
# A latched flag would mean one transient ~2s stall anywhere in a match
# permanently pinned every later tick into "needs a hard resync", which is
# exactly the behaviour soft correction exists to avoid — and it would also
# cap overflow_count at 1 forever, since a second episode could never
# observe the flag going false again.
#
# Two things a "matched" result does NOT guarantee, flagged for whoever
# builds task 4.3's actual correction logic on top of this:
#
# 1. A "matched" result can still be reporting stale data. The slot-tag
#    equality check in get_prediction() guarantees a match's payload
#    genuinely belongs to the queried seq (never wrong-seq data mislabeled
#    as right), but nothing in the "matched" status itself says HOW OLD
#    that entry is. Under sparse recording (record() is not called with
#    strictly consecutive seqs — see the record() comment below), an entry
#    from well over RING_SIZE ticks ago can still report "matched" for a
#    query landing on its untouched residue. resync_required correctly
#    stays true in that case (the span guard below is exact), but the
#    comparison payload itself carries no matched_stale/age distinction. A
#    caller wanting to reject "matched but ancient" needs to separately
#    check newest_recorded_seq - seq itself.
#
# 2. record() can be called twice for the same seq with a DIFFERENT action,
#    when input_lead_controller's release path resends a duplicated seq
#    (delta == 0) — the later call silently overwrites the ring slot, so
#    the stored action becomes whichever of the two calls happened last.
#    This matches what the WIRE ends up sending for that seq (the resend
#    replaces the redundancy history's front entry — see
#    networked_match.gd's _send_local_input), but if the SERVER had
#    already consumed the seq from the first packet before the resend
#    arrived, the server's applied action and this ring's stored action for
#    that same seq can disagree. Narrow (release only fires after 120 ticks
#    of sustained surplus depth, when the server is least likely to be
#    right on the edge of consuming that exact seq) but real; a future
#    replay-based catch-up (task 4.5) built on this history should not
#    assume the stored action is provably what the server actually applied.

const RING_SIZE := 128

var _ring_seq: PackedInt32Array = PackedInt32Array()
var _ring_entry: Array = []
var _has_recorded := false

var newest_recorded_seq := -1
var last_acknowledged_seq := 0
var overflow_count := 0
var resync_required := false


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


# Stores a private copy of both action and state. Returns true when this
# record crossed the unacknowledged-capacity boundary; the caller does not
# need that return today, but it makes the eviction event observable rather
# than silent when reconciliation starts applying corrections in Phase 4.3.
func record(seq: int, action: ShipAction, state: NetBodyState) -> bool:
	var overflowed_now := false
	if not _has_recorded or seq > newest_recorded_seq:
		if seq - last_acknowledged_seq > RING_SIZE:
			# Only the LEADING edge of an episode counts: resync_required is
			# still true for every subsequent tick of the same stall, and
			# counting those would report one outage as hundreds. Because
			# compare_authoritative() can now clear the flag, a genuinely
			# separate later episode does increment this again.
			overflowed_now = not resync_required
			resync_required = true
			if overflowed_now:
				overflow_count += 1
		newest_recorded_seq = seq
		_has_recorded = true
	var idx := posmod(seq, RING_SIZE)
	_ring_seq[idx] = seq
	_ring_entry[idx] = {
		"action": action.copy(),
		"state": state.copy(),
	}
	return overflowed_now


# Returns independent copies so diagnostic/reconciliation consumers cannot
# mutate a retained prediction by accident.
func get_prediction(seq: int) -> Dictionary:
	var idx := posmod(seq, RING_SIZE)
	if _ring_seq[idx] != seq:
		return {}
	var entry: Dictionary = _ring_entry[idx]
	return {
		"seq": seq,
		"action": (entry["action"] as ShipAction).copy(),
		"state": (entry["state"] as NetBodyState).copy(),
	}


# Produces comparison data only. Applying a snap, teleport, velocity delta,
# or visual offset belongs to later Phase 4 tasks.
func compare_authoritative(seq: int, authoritative: NetBodyState) -> Dictionary:
	if seq > last_acknowledged_seq:
		last_acknowledged_seq = seq
	var prediction := get_prediction(seq)
	if prediction.is_empty():
		return {
			"status": _missing_status(seq),
			"seq": seq,
			"authoritative_state": authoritative.copy(),
		}

	# A successful match is the only evidence that the acknowledgement clock
	# has genuinely caught back up, so it is the only thing allowed to clear
	# resync_required — a "missing_evicted"/"missing_not_recorded" result
	# proves the opposite, and must leave the flag alone.
	#
	# The extra span check is not redundant. record() is not guaranteed to be
	# called with consecutive sequences: input_lead_controller.update() can
	# return 0 or up to 1+3, so the client's seq can skip forward, leaving a
	# ring slot holding a tag OLDER than newest_recorded_seq - RING_SIZE
	# (its residue was simply never rewritten). get_prediction() would still
	# report that as "matched", so matching alone does not imply the
	# outstanding window is back within capacity. Gate on the exact inverse
	# of record()'s own trip inequality instead, which holds regardless of
	# how sparsely sequences were recorded.
	if newest_recorded_seq - last_acknowledged_seq <= RING_SIZE:
		resync_required = false

	var predicted_state: NetBodyState = prediction["state"]
	var position_error := authoritative.position - predicted_state.position
	var rotation_error_radians := predicted_state.rotation.angle_to(authoritative.rotation)
	return {
		"status": "matched",
		"seq": seq,
		"action": prediction["action"],
		"predicted_state": predicted_state,
		"authoritative_state": authoritative.copy(),
		"position_error": position_error,
		"position_error_magnitude": position_error.length(),
		"rotation_error_radians": rotation_error_radians,
		"rotation_error_degrees": rad_to_deg(rotation_error_radians),
		"linear_velocity_error": authoritative.linear_velocity - predicted_state.linear_velocity,
		"angular_velocity_error": authoritative.angular_velocity - predicted_state.angular_velocity,
	}


func _missing_status(seq: int) -> String:
	if _has_recorded and seq <= newest_recorded_seq - RING_SIZE:
		return "missing_evicted"
	return "missing_not_recorded"