class_name NetCodec

# Wire-format constants, quantisers, and pack/unpack for the two hot-path
# packets (§2 of MULTIPLAYER_SPEC.md). Pure functions only — no networking,
# no autoload state — so they're testable head-on by tests/test_runner.tscn
# without a live connection.
#
# Referenced from elsewhere via preload(), not the bare class_name, per the
# same global-script-class-cache caveat documented in tests/test_case.gd and
# sim_constants.gd.

const SimConstants = preload("res://scripts/sim_constants.gd")
const ShipAction = preload("res://scripts/ship_action.gd")
const NetBodyState = preload("res://scripts/net_body_state.gd")

# --- Protocol ---
const PROTOCOL_VERSION := 1
const TICK_HZ: int = SimConstants.TICK_HZ

# --- Channels (logical intent; NetworkManager may need to offset these on
# top of ENet's own reserved channels — verify empirically, see §2.1) ---
const CHANNEL_CONTROL := 0
const CHANNEL_INPUT := 1
const CHANNEL_SNAPSHOT := 2

# --- Packet type/version byte: high nibble = type, low nibble = protocol version ---
enum PacketType { INPUT = 0, SNAPSHOT = 1 }

# --- Input packet (§2.3) ---
const MAX_REDUNDANCY := 4
# type_version u8 + seq u32 + count u8 + ack_snapshot_tick u32 + client_send_ms u16
const INPUT_HEADER_SIZE := 12
const INPUT_ENTRY_SIZE := 7    # thrust i8x3 + rotation i8x3 + flags u8
const INPUT_FLAG_TURBO := 1 << 0

# --- Snapshot packet (§2.4) ---
# last_input_seq u32 + input_buffer_depth i8 + echo_client_send_ms u16
const SNAPSHOT_CLIENT_HEADER_SIZE := 7
# type_version u8 + server_tick u32 + match_state u8 + reset_gen u8 + body_count u8
const SNAPSHOT_BODY_HEADER_SIZE := 8
const SNAPSHOT_BODY_SIZE := 22

const BODY_FLAG_FROZEN := 1 << 0
const BODY_FLAG_TURBO := 1 << 1
const BODY_FLAG_THRUST_Z_SHIFT := 2
const BODY_FLAG_THRUST_Z_MASK := 0x1C   # bits 2-4
const BODY_FLAG_STALLED := 1 << 5
const BODY_FLAG_QUAT_W_SIGN := 1 << 6

# --- Quantisation ranges (§2.4 — derived from arena/gameplay constants, not
# restated prose; see MULTIPLAYER_SPEC.md for the ArenaBoundary/Ship/Ball
# constants these are sized against) ---
const POS_RANGE := 64.0        # metres, ±
const VEL_RANGE := 64.0        # m/s, ±
const QUAT_COMPONENT_RANGE := 1.0
const SHIP_AVEL_RANGE := 4.0   # rad/s, ±
const BALL_AVEL_RANGE := 32.0  # rad/s, ±

const I16_MAX := 32767
const I8_MAX := 127
const THRUST_Z_BIN_MAX := 7    # 3 bits


# ============================================================
# Quantisers — pure, reusable, independently testable.
# ============================================================

static func quantize_i16(value: float, range_max: float) -> int:
	var scaled := clampf(value / range_max, -1.0, 1.0) * I16_MAX
	return clampi(roundi(scaled), -I16_MAX, I16_MAX)

static func dequantize_i16(raw: int, range_max: float) -> float:
	return (float(raw) / I16_MAX) * range_max

static func quantize_i8(value: float, range_max: float) -> int:
	var scaled := clampf(value / range_max, -1.0, 1.0) * I8_MAX
	return clampi(roundi(scaled), -I8_MAX, I8_MAX)

static func dequantize_i8(raw: int, range_max: float) -> float:
	return (float(raw) / I8_MAX) * range_max

static func quantize_thrust_z_bin(thrust_z: float) -> int:
	var t := clampf((thrust_z + 1.0) * 0.5, 0.0, 1.0)
	return clampi(roundi(t * THRUST_Z_BIN_MAX), 0, THRUST_Z_BIN_MAX)

static func dequantize_thrust_z_bin(bin_value: int) -> float:
	return (float(bin_value) / THRUST_Z_BIN_MAX) * 2.0 - 1.0


static func type_version_byte(type: PacketType) -> int:
	return ((int(type) & 0x0F) << 4) | (PROTOCOL_VERSION & 0x0F)

static func packet_type_of(type_version: int) -> int:
	return (type_version >> 4) & 0x0F

static func protocol_version_of(type_version: int) -> int:
	return type_version & 0x0F


# ============================================================
# Input packet — client -> server, channel 1 (§2.3)
# ============================================================

# actions: newest-first, 1..MAX_REDUNDANCY ShipAction instances.
static func pack_input(seq: int, ack_snapshot_tick: int, client_send_ms: int, actions: Array) -> PackedByteArray:
	var count: int = clampi(actions.size(), 1, MAX_REDUNDANCY)
	var buf := StreamPeerBuffer.new()
	buf.put_u8(type_version_byte(PacketType.INPUT))
	buf.put_u32(seq)
	buf.put_u8(count)
	buf.put_u32(ack_snapshot_tick)
	buf.put_u16(client_send_ms & 0xFFFF)
	for i in count:
		var action: ShipAction = actions[i]
		buf.put_8(quantize_i8(action.thrust.x, 1.0))
		buf.put_8(quantize_i8(action.thrust.y, 1.0))
		buf.put_8(quantize_i8(action.thrust.z, 1.0))
		buf.put_8(quantize_i8(action.rotation.x, 1.0))
		buf.put_8(quantize_i8(action.rotation.y, 1.0))
		buf.put_8(quantize_i8(action.rotation.z, 1.0))
		var flags := 0
		if action.turbo:
			flags |= INPUT_FLAG_TURBO
		buf.put_u8(flags)
	return buf.data_array


# Returns a Dictionary: type_version, seq, count, ack_snapshot_tick,
# client_send_ms, actions (Array[ShipAction], newest first).
static func unpack_input(bytes: PackedByteArray) -> Dictionary:
	var buf := StreamPeerBuffer.new()
	buf.data_array = bytes
	var type_version := buf.get_u8()
	var seq := buf.get_u32()
	var count := buf.get_u8()
	var ack_snapshot_tick := buf.get_u32()
	var client_send_ms := buf.get_u16()
	var actions: Array[ShipAction] = []
	for i in count:
		var a := ShipAction.new()
		a.thrust = Vector3(
			dequantize_i8(buf.get_8(), 1.0),
			dequantize_i8(buf.get_8(), 1.0),
			dequantize_i8(buf.get_8(), 1.0)
		)
		a.rotation = Vector3(
			dequantize_i8(buf.get_8(), 1.0),
			dequantize_i8(buf.get_8(), 1.0),
			dequantize_i8(buf.get_8(), 1.0)
		)
		var flags := buf.get_u8()
		a.turbo = (flags & INPUT_FLAG_TURBO) != 0
		actions.append(a)
	return {
		"type_version": type_version,
		"seq": seq,
		"count": count,
		"ack_snapshot_tick": ack_snapshot_tick,
		"client_send_ms": client_send_ms,
		"actions": actions,
	}


# ============================================================
# Snapshot packet — server -> client, channel 2 (§2.4)
# ============================================================

# Shared across every peer this tick — build once, reuse (§2.4's stated
# intent). Returns type_version + server_tick + match_state + reset_gen +
# body_count + body_count * SNAPSHOT_BODY_SIZE bytes.
static func pack_snapshot_body_segment(server_tick: int, match_state: int, reset_gen: int, bodies: Array) -> PackedByteArray:
	var buf := StreamPeerBuffer.new()
	buf.put_u8(type_version_byte(PacketType.SNAPSHOT))
	buf.put_u32(server_tick)
	buf.put_u8(match_state & 0xFF)
	buf.put_u8(reset_gen & 0xFF)
	buf.put_u8(bodies.size())
	for body in bodies:
		var b: NetBodyState = body
		buf.put_16(quantize_i16(b.position.x, POS_RANGE))
		buf.put_16(quantize_i16(b.position.y, POS_RANGE))
		buf.put_16(quantize_i16(b.position.z, POS_RANGE))
		buf.put_16(quantize_i16(b.rotation.x, QUAT_COMPONENT_RANGE))
		buf.put_16(quantize_i16(b.rotation.y, QUAT_COMPONENT_RANGE))
		buf.put_16(quantize_i16(b.rotation.z, QUAT_COMPONENT_RANGE))
		buf.put_16(quantize_i16(b.linear_velocity.x, VEL_RANGE))
		buf.put_16(quantize_i16(b.linear_velocity.y, VEL_RANGE))
		buf.put_16(quantize_i16(b.linear_velocity.z, VEL_RANGE))
		buf.put_8(quantize_i8(b.angular_velocity.x, b.avel_range))
		buf.put_8(quantize_i8(b.angular_velocity.y, b.avel_range))
		buf.put_8(quantize_i8(b.angular_velocity.z, b.avel_range))
		var flags := 0
		if b.frozen:
			flags |= BODY_FLAG_FROZEN
		if b.turbo:
			flags |= BODY_FLAG_TURBO
		flags |= (quantize_thrust_z_bin(b.thrust_z) << BODY_FLAG_THRUST_Z_SHIFT) & BODY_FLAG_THRUST_Z_MASK
		if b.stalled:
			flags |= BODY_FLAG_STALLED
		if b.rotation.w < 0.0:
			flags |= BODY_FLAG_QUAT_W_SIGN
		buf.put_u8(flags)
	return buf.data_array


static func pack_snapshot_client_header(last_input_seq: int, input_buffer_depth: int, echo_client_send_ms: int) -> PackedByteArray:
	var buf := StreamPeerBuffer.new()
	buf.put_u32(last_input_seq)
	buf.put_8(clampi(input_buffer_depth, -128, 127))
	buf.put_u16(echo_client_send_ms & 0xFFFF)
	return buf.data_array


# Convenience: one full per-client packet = per-client header + shared body segment.
static func pack_snapshot(last_input_seq: int, input_buffer_depth: int, echo_client_send_ms: int, body_segment: PackedByteArray) -> PackedByteArray:
	var header := pack_snapshot_client_header(last_input_seq, input_buffer_depth, echo_client_send_ms)
	var out := PackedByteArray()
	out.append_array(header)
	out.append_array(body_segment)
	return out


# Returns a Dictionary: last_input_seq, input_buffer_depth, echo_client_send_ms,
# type_version, server_tick, match_state, reset_gen, bodies (Array[NetBodyState]).
static func unpack_snapshot(bytes: PackedByteArray) -> Dictionary:
	var buf := StreamPeerBuffer.new()
	buf.data_array = bytes
	var last_input_seq := buf.get_u32()
	var input_buffer_depth := buf.get_8()
	var echo_client_send_ms := buf.get_u16()
	var type_version := buf.get_u8()
	var server_tick := buf.get_u32()
	var match_state := buf.get_u8()
	var reset_gen := buf.get_u8()
	var body_count := buf.get_u8()
	var bodies: Array[NetBodyState] = []
	for i in body_count:
		var b := NetBodyState.new()
		b.position = Vector3(
			dequantize_i16(buf.get_16(), POS_RANGE),
			dequantize_i16(buf.get_16(), POS_RANGE),
			dequantize_i16(buf.get_16(), POS_RANGE)
		)
		var qx := dequantize_i16(buf.get_16(), QUAT_COMPONENT_RANGE)
		var qy := dequantize_i16(buf.get_16(), QUAT_COMPONENT_RANGE)
		var qz := dequantize_i16(buf.get_16(), QUAT_COMPONENT_RANGE)
		b.linear_velocity = Vector3(
			dequantize_i16(buf.get_16(), VEL_RANGE),
			dequantize_i16(buf.get_16(), VEL_RANGE),
			dequantize_i16(buf.get_16(), VEL_RANGE)
		)
		# avel_range is unknown to the codec at this point (it isn't on the
		# wire — see net_body_state.gd) — decode at SHIP_AVEL_RANGE and let
		# the caller, which knows this slot's body kind, rescale if it's the
		# ball's slot. Storing the raw i8 would avoid this, but every other
		# field in this struct is already physical units; consistency wins.
		b.angular_velocity = Vector3(
			dequantize_i8(buf.get_8(), SHIP_AVEL_RANGE),
			dequantize_i8(buf.get_8(), SHIP_AVEL_RANGE),
			dequantize_i8(buf.get_8(), SHIP_AVEL_RANGE)
		)
		var flags := buf.get_u8()
		b.frozen = (flags & BODY_FLAG_FROZEN) != 0
		b.turbo = (flags & BODY_FLAG_TURBO) != 0
		var bin_value := (flags & BODY_FLAG_THRUST_Z_MASK) >> BODY_FLAG_THRUST_Z_SHIFT
		b.thrust_z = dequantize_thrust_z_bin(bin_value)
		b.stalled = (flags & BODY_FLAG_STALLED) != 0
		var w_sq := 1.0 - qx * qx - qy * qy - qz * qz
		var w := sqrt(maxf(w_sq, 0.0))
		if (flags & BODY_FLAG_QUAT_W_SIGN) != 0:
			w = -w
		b.rotation = Quaternion(qx, qy, qz, w)
		bodies.append(b)
	return {
		"last_input_seq": last_input_seq,
		"input_buffer_depth": input_buffer_depth,
		"echo_client_send_ms": echo_client_send_ms,
		"type_version": type_version,
		"server_tick": server_tick,
		"match_state": match_state,
		"reset_gen": reset_gen,
		"bodies": bodies,
	}


# Rescales an already-decoded body's angular_velocity from the SHIP_AVEL_RANGE
# assumption unpack_snapshot() decoded it with to the range it was actually
# quantised at (BALL_AVEL_RANGE for the ball). Call once per non-ship body
# immediately after unpack_snapshot(), using slot order to know which.
static func rescale_avel(body: NetBodyState, actual_range: float) -> void:
	if is_equal_approx(actual_range, SHIP_AVEL_RANGE):
		body.avel_range = actual_range
		return
	body.angular_velocity = (body.angular_velocity / SHIP_AVEL_RANGE) * actual_range
	body.avel_range = actual_range