from __future__ import annotations

from dataclasses import dataclass
from enum import IntEnum
from typing import Iterable, List, Sequence, Tuple


class PacketType(IntEnum):
    JOIN = 0
    JOIN_ACK = 1
    JOIN_DENY = 2
    INPUT = 3
    INPUT_BROADCAST = 4
    STATE_DELTA = 5
    STATE_FULL = 6
    PART = 7
    CONFIG_UPDATE = 8
    PING = 9
    PONG = 10
    ERROR = 11


class BodyTLV(IntEnum):
    BODY_2BIT = 0x00
    BODY_RLE = 0x01
    BODY_2BIT_CHUNK = 0x10
    BODY_RLE_CHUNK = 0x11


class Direction(IntEnum):
    UP = 0
    RIGHT = 1
    DOWN = 2
    LEFT = 3


def quic_varint_encode(value: int) -> bytes:
    """Encode QUIC varint (RFC 9000)."""
    if value < 0:
        raise ValueError("varint must be non-negative")
    if value <= 0x3F:  # 6 bits, 1 byte, 00xx xxxx
        return bytes([value & 0x3F])
    if value <= 0x3FFF:  # 14 bits, 2 bytes, 01xx xxxx
        v = 0x4000 | value
        return v.to_bytes(2, "big")
    if value <= 0x3FFFFFFF:  # 30 bits, 4 bytes, 10xx xxxx
        v = 0x80000000 | value
        return v.to_bytes(4, "big")
    if value <= 0x3FFFFFFFFFFFFFFF:  # 62 bits, 8 bytes, 11xx xxxx
        v = 0xC000000000000000 | value
        return v.to_bytes(8, "big")
    raise ValueError("varint too large")


def quic_varint_decode(buf: bytes, offset: int = 0) -> Tuple[int, int]:
    """Decode QUIC varint starting at offset. Returns (value, next_offset)."""
    first = buf[offset]
    prefix = first >> 6
    if prefix == 0:
        return (first & 0x3F, offset + 1)
    if prefix == 1:
        v = int.from_bytes(buf[offset : offset + 2], "big") & 0x3FFF
        return (v, offset + 2)
    if prefix == 2:
        v = int.from_bytes(buf[offset : offset + 4], "big") & 0x3FFFFFFF
        return (v, offset + 4)
    v = int.from_bytes(buf[offset : offset + 8], "big") & 0x3FFFFFFFFFFFFFFF
    return (v, offset + 8)


def pack_header(version: int, ptype: PacketType, flags: int, seq: int, tick: int | None) -> bytes:
    """Pack common header fields.

    Layout:
    - ver: u8
    - type: u8
    - flags: u8
    - seq: u16 (network order)
    - tick: optional u16
    """
    if not (0 <= version <= 255):
        raise ValueError("version out of range")
    if not (0 <= flags <= 255):
        raise ValueError("flags out of range")
    if not (0 <= seq <= 0xFFFF):
        raise ValueError("seq out of range")
    parts = bytearray()
    parts.append(version & 0xFF)
    parts.append(int(ptype) & 0xFF)
    parts.append(flags & 0xFF)
    parts.extend(seq.to_bytes(2, "big"))
    if tick is not None:
        if not (0 <= tick <= 0xFFFF):
            raise ValueError("tick out of range")
        parts.extend(tick.to_bytes(2, "big"))
    return bytes(parts)


def unpack_header(buf: bytes, expect_tick: bool) -> Tuple[int, PacketType, int, int, int | None, int]:
    """Unpack header; returns (ver, type, flags, seq, tick, next_offset)."""
    if len(buf) < 5:
        raise ValueError("buffer too small for header")
    ver = buf[0]
    ptype = PacketType(buf[1])
    flags = buf[2]
    seq = int.from_bytes(buf[3:5], "big")
    off = 5
    tick = None
    if expect_tick:
        if len(buf) < 7:
            raise ValueError("buffer too small for tick")
        tick = int.from_bytes(buf[5:7], "big")
        off = 7
    return ver, ptype, flags, seq, tick, off


# Message builders/parsers (subset)

def build_config_update(
    *,
    version: int,
    seq: int,
    tick: int,
    tick_rate: int,
    wrap_edges: bool,
    apples_per_snake: int,
    apples_cap: int,
) -> bytes:
    header = pack_header(version, PacketType.CONFIG_UPDATE, 0, seq, tick)
    body = bytearray()
    body.append(tick_rate & 0xFF)  # u8
    body.append(1 if wrap_edges else 0)  # bool u8
    body.append(apples_per_snake & 0xFF)  # u8
    body.append(apples_cap & 0xFF)  # u8
    return header + bytes(body)


@dataclass
class InputEvent:
    rel_tick_offset: int  # relative to base_tick
    direction: Direction


def build_input_broadcast(
    *,
    version: int,
    seq: int,
    tick: int,
    player_id: int,
    input_seq: int,
    base_tick: int,
    events: Sequence[InputEvent],
    apply_at_tick: int | None = None,
) -> bytes:
    header = pack_header(version, PacketType.INPUT_BROADCAST, 0, seq, tick)
    body = bytearray()
    body.append(player_id & 0xFF)
    body.extend(int(input_seq & 0xFFFF).to_bytes(2, "big"))
    body.extend(int(base_tick & 0xFFFF).to_bytes(2, "big"))
    # number of events as QUIC varint
    body.extend(quic_varint_encode(len(events)))
    for ev in events:
        # rel offset as QUIC varint, direction as u8 (low 2 bits)
        body.extend(quic_varint_encode(int(ev.rel_tick_offset)))
        body.append(int(ev.direction) & 0x03)
    # Optional absolute apply_at_tick presence flag + value
    if apply_at_tick is None:
        body.append(0)
    else:
        body.append(1)
        body.extend(int(apply_at_tick & 0xFFFF).to_bytes(2, "big"))
    return header + bytes(body)


def pack_body_tlv(t: BodyTLV, payload: bytes) -> bytes:
    return quic_varint_encode(int(t)) + quic_varint_encode(len(payload)) + payload


def bitpack_2bit_directions(directions: Iterable[Direction]) -> bytes:
    out = bytearray()
    acc = 0
    bits = 0
    for d in directions:
        acc |= (int(d) & 0x03) << bits
        bits += 2
        if bits >= 8:
            out.append(acc & 0xFF)
            acc = acc >> 8
            bits -= 8
    if bits:
        out.append(acc & 0xFF)  # zero-padded high bits
    return bytes(out)


# Join / Ack / Deny

def build_join(name_utf8: bytes, preferred_color_id: int | None = None) -> bytes:
    # Client-side helper (not used in server)
    raise NotImplementedError


def parse_join(buf: bytes, offset: int) -> Tuple[str, int | None, int]:
    name_len, off = quic_varint_decode(buf, offset)
    name_b = buf[off : off + name_len]
    off += name_len
    preferred = None
    if off < len(buf):
        preferred = buf[off]
        off += 1
    name = name_b.decode("utf-8", errors="ignore")
    return name, preferred, off


def build_join_ack(
    *,
    version: int,
    seq: int,
    player_id: int,
    color_id: int,
    width: int,
    height: int,
    tick_rate: int,
    wrap_edges: bool,
    apples_per_snake: int,
    apples_cap: int,
    compression_mode: int,
) -> bytes:
    header = pack_header(version, PacketType.JOIN_ACK, 0, seq, None)
    body = bytearray()
    body.append(player_id & 0xFF)
    body.append(color_id & 0xFF)
    body.append(width & 0xFF)
    body.append(height & 0xFF)
    body.append(tick_rate & 0xFF)
    body.append(1 if wrap_edges else 0)
    body.append(apples_per_snake & 0xFF)
    body.append(apples_cap & 0xFF)
    body.append(compression_mode & 0xFF)
    return header + bytes(body)


def build_join_deny(*, version: int, seq: int, reason: str) -> bytes:
    header = pack_header(version, PacketType.JOIN_DENY, 0, seq, None)
    rb = reason.encode("utf-8")[:64]
    return header + quic_varint_encode(len(rb)) + rb


# Input (client -> server)

def parse_input(buf: bytes, offset: int) -> Tuple[int, int, int, List[InputEvent], int]:
    ack_seq = int.from_bytes(buf[offset : offset + 2], "big")
    offset += 2
    input_seq = int.from_bytes(buf[offset : offset + 2], "big")
    offset += 2
    base_tick = int.from_bytes(buf[offset : offset + 2], "big")
    offset += 2
    n_ev, offset = quic_varint_decode(buf, offset)
    events: List[InputEvent] = []
    for _ in range(n_ev):
        rel, offset = quic_varint_decode(buf, offset)
        d = Direction(buf[offset] & 0x03)
        offset += 1
        events.append(InputEvent(rel_tick_offset=int(rel), direction=d))
    return ack_seq, input_seq, base_tick, events, offset


# State snapshot (server -> client)

def build_state_full(
    *,
    version: int,
    seq: int,
    tick: int,
    snakes: Sequence[Tuple[int, int, int, int, Sequence[Direction]]],
    apples: Sequence[Tuple[int, int]],
) -> bytes:
    """Build a minimal state_full: per-snake header + BODY_2BIT TLV; apples list.

    snakes: sequence of (snake_id, len, head_x, head_y, body_dirs_from_head)
    apples: sequence of (x, y)
    """
    header = pack_header(version, PacketType.STATE_FULL, 0, seq, tick & 0xFFFF)
    body = bytearray()
    # snakes count
    body.extend(quic_varint_encode(len(snakes)))
    for sid, slen, hx, hy, dirs in snakes:
        body.append(sid & 0xFF)
        body.extend(int(slen & 0xFFFF).to_bytes(2, "big"))
        body.append(hx & 0xFF)
        body.append(hy & 0xFF)
        payload = bitpack_2bit_directions(dirs)
        tlv = pack_body_tlv(BodyTLV.BODY_2BIT, payload)
        body.extend(tlv)
    # apples
    body.extend(quic_varint_encode(len(apples)))
    for ax, ay in apples:
        body.append(ax & 0xFF)
        body.append(ay & 0xFF)
    return header + bytes(body)