tactical-shooter/server/scripts/combat/lag_compensation.gd

## LagCompensation — Server-side lag compensation for hit-scan weapons.
##
## Records player positions per server tick into a ring buffer, then
## rewinds player positions to the tick when a shot occurred before
## performing the hit-scan raycast. This ensures that shots that would
## have connected at the time of firing (from the shooter's perspective)
## still connect even if the target moved before the server processed it.
##
## Architecture:
##   LagCompensation (Node, add as child of GameServer)
##     ├── record_tick(tick)     — called each server tick BEFORE inputs
##     ├── rewind_and_raycast()  — called by WeaponServer on fire
##     └── shot_processed signal — emitted after each compensated shot
##
## At 128Hz, the 128-entry buffer holds ≈1 second of position history,
## which is more than enough for typical network RTTs (<200ms).
##
class_name LagCompensation
extends Node

# ---------------------------------------------------------------------------
# Constants
# ---------------------------------------------------------------------------
## Number of ticks to keep in the position history ring buffer.
## At 128Hz: 128 entries ≈ 1 second of history.
const HISTORY_SIZE: int = 128

# ---------------------------------------------------------------------------
# Signals
# ---------------------------------------------------------------------------
## Emitted after a shot has been processed through lag compensation.
## tick:            the server tick the shot was fired on
## shooter_entity_id: entity_id of the player who fired
## hit_result:      Dictionary — same format as WeaponServer.fire() hit results
##                  {hit: bool, position: Vector3, target_id: int, damage: float, weapon_id: String}
signal shot_processed(tick: int, shooter_entity_id: int, hit_result: Dictionary)

# ---------------------------------------------------------------------------
# State
# ---------------------------------------------------------------------------
## Ring buffer of position snapshots per tick.
## Indexed by tick % HISTORY_SIZE.
## Each entry is a Dictionary {entity_id (int): position (Vector3)}.
var _position_history: Array = []

## Maps entity_id → Node3D for all tracked player physics bodies.
var _player_nodes: Dictionary = {}  # entity_id (int) → Node3D

# ---------------------------------------------------------------------------
# Lifecycle
# ---------------------------------------------------------------------------
func _init() -> void:
    _position_history.resize(HISTORY_SIZE)

# ---------------------------------------------------------------------------
# Public API — Player node registration
# ---------------------------------------------------------------------------

## Register a player node for position tracking.
## entity_id: the simulation entity ID assigned by GameServer.
## node: the Node3D (CharacterBody3D) whose position to track.
func register_player_node(entity_id: int, node: Node3D) -> void:
    _player_nodes[entity_id] = node

## Remove a player node from position tracking (on disconnect / respawn).
func unregister_player_node(entity_id: int) -> void:
    _player_nodes.erase(entity_id)

# ---------------------------------------------------------------------------
# Public API — Tick recording
# ---------------------------------------------------------------------------

## Record the current positions of all tracked player nodes for [tick].
## Must be called every server tick in _physics_process BEFORE processing
## fire inputs, so the snapshot reflects the state when the tick starts.
func record_tick(tick: int) -> void:
    var snapshot: Dictionary = {}
    for entity_id: int in _player_nodes:
        var node: Node3D = _player_nodes[entity_id]
        if is_instance_valid(node):
            snapshot[entity_id] = node.global_position
    _position_history[tick % HISTORY_SIZE] = snapshot

# ---------------------------------------------------------------------------
# Public API — Rewind & raycast
# ---------------------------------------------------------------------------

## Rewind player positions to the state at [tick], perform a single
## raycast, restore original positions, and return the hit result.
##
## Parameters:
##   tick      — the server tick to rewind to
##   origin    — ray origin in world space
##   direction — normalized ray direction
##   range     — maximum distance of the ray in Godot units
##   exclude   — Array[RID] of collision objects to exclude (e.g. shooter)
##
## Returns:
##   Dictionary from PhysicsDirectSpaceState3D.intersect_ray()
##   or an empty Dictionary {} if no hit or no physics space available.
##
## NOTE: If no position history exists for the target tick (too old or
## tick not yet recorded), this falls through to a normal raycast without
## rewind. This keeps weapons functional even during brief history gaps.
func rewind_and_raycast(
    tick: int,
    origin: Vector3,
    direction: Vector3,
    range: float,
    exclude: Array[RID]
) -> Dictionary:
    var space_state: PhysicsDirectSpaceState3D = _get_space_state()
    if space_state == null:
        return {}

    # Look up positions for the target tick
    var rewound_positions: Dictionary = _position_history[tick % HISTORY_SIZE]

    # No history for this tick — fall through to normal raycast
    if rewound_positions == null or rewound_positions.is_empty():
        return _normal_raycast(space_state, origin, direction, range, exclude)

    # --- Rewind phase ---
    # Save current positions and move tracked nodes to their tick-time positions.
    # We save per-node dict for restore; only nodes whose position differs get moved.
    var saved_positions: Dictionary = {}  # entity_id → Vector3 (original)

    for entity_id: int in rewound_positions:
        if not _player_nodes.has(entity_id):
            continue
        var node: Node3D = _player_nodes[entity_id]
        if not is_instance_valid(node):
            continue

        var original_pos: Vector3 = node.global_position
        var rewound_pos: Vector3 = rewound_positions[entity_id]

        # Skip if the node is already at the rewound position
        if original_pos.is_equal_approx(rewound_pos):
            continue

        saved_positions[entity_id] = original_pos
        node.global_position = rewound_pos

    # --- Raycast phase ---
    # Perform the raycast with targets at their rewound positions.
    var result: Dictionary = _normal_raycast(
        space_state, origin, direction, range, exclude
    )

    # --- Restore phase ---
    # Move all saved nodes back to their original positions.
    for entity_id: int in saved_positions:
        if not _player_nodes.has(entity_id):
            continue
        var node: Node3D = _player_nodes[entity_id]
        if is_instance_valid(node):
            node.global_position = saved_positions[entity_id]

    return result

# ---------------------------------------------------------------------------
# Internal — helpers
# ---------------------------------------------------------------------------

## Perform a single raycast without any rewind.
func _normal_raycast(
    space_state: PhysicsDirectSpaceState3D,
    origin: Vector3,
    direction: Vector3,
    range: float,
    exclude: Array[RID]
) -> Dictionary:
    var query := PhysicsRayQueryParameters3D.create(
        origin, origin + direction * range
    )
    query.exclude = exclude
    query.collide_with_bodies = true
    query.collide_with_areas = false
    return space_state.intersect_ray(query)

## Get the PhysicsDirectSpaceState3D from the current world.
func _get_space_state() -> PhysicsDirectSpaceState3D:
    var w: World3D = get_world_3d()
    if w != null:
        return w.direct_space_state
    return null