import secrets
from datetime import timedelta

from asgiref.sync import async_to_sync
from channels.layers import get_channel_layer
from django.db import transaction
from django.utils import timezone
from rest_framework.exceptions import ValidationError

from .models import BoardSession

BOARD_CODE_ALPHABET = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789"
BOARD_TTL = timedelta(minutes=30)
DRAW_GUESS_TARGETS = (
    "抛物线",
    "三角形",
    "勾股定理",
    "质数",
    "圆",
    "正弦函数",
    "分数",
    "坐标系",
)


def _new_board_code():
    for _ in range(20):
        code = "".join(secrets.choice(BOARD_CODE_ALPHABET) for _ in range(6))
        if not BoardSession.objects.filter(code=code).exists():
            return code
    raise ValidationError("暂时无法生成画板联机码，请稍后重试")


def broadcast_board(session_id, reason):
    channel_layer = get_channel_layer()
    if channel_layer is None:
        return
    async_to_sync(channel_layer.group_send)(
        f"board_{session_id}",
        {
            "type": "board.state",
            "reason": reason,
        },
    )


def board_payload(session, user):
    is_host = session.host_id == user.id
    reveal_target = (
        session.mode == BoardSession.Mode.DRAW_GUESS
        and (is_host or session.status == BoardSession.Status.COMPLETED)
    )
    return {
        "session_id": session.id,
        "code": session.code,
        "mode": session.mode,
        "mode_label": session.get_mode_display(),
        "status": session.status,
        "role": "host" if is_host else "guest",
        "target": session.target if reveal_target else None,
        "host": session.host.nickname,
        "guest": session.guest.nickname if session.guest else None,
        "host_score": session.host_score,
        "guest_score": session.guest_score,
        "expires_at": session.expires_at,
        "websocket_path": f"/ws/v1/toolbox/boards/{session.id}/",
    }


@transaction.atomic
def create_board(user, mode):
    if mode not in BoardSession.Mode.values:
        raise ValidationError({"mode": "不支持的画板联机模式"})
    BoardSession.objects.filter(
        host=user,
        status=BoardSession.Status.WAITING,
    ).update(status=BoardSession.Status.CANCELLED)
    return BoardSession.objects.create(
        code=_new_board_code(),
        mode=mode,
        host=user,
        target=(
            secrets.choice(DRAW_GUESS_TARGETS)
            if mode == BoardSession.Mode.DRAW_GUESS
            else ""
        ),
        expires_at=timezone.now() + BOARD_TTL,
    )


@transaction.atomic
def join_board(user, code):
    normalized = str(code or "").strip().upper()
    if len(normalized) != 6 or any(
        character not in BOARD_CODE_ALPHABET for character in normalized
    ):
        raise ValidationError({"code": "画板联机码应为 6 位大写字母或数字"})
    try:
        session = (
            BoardSession.objects.select_for_update()
            .select_related("host", "guest")
            .get(code=normalized)
        )
    except BoardSession.DoesNotExist as exc:
        raise ValidationError({"code": "画板联机码不存在"}) from exc
    if session.host_id == user.id:
        raise ValidationError({"code": "不能加入自己创建的画板"})
    if session.status != BoardSession.Status.WAITING:
        raise ValidationError({"code": "画板联机码已失效或已被使用"})
    if session.expires_at <= timezone.now():
        session.status = BoardSession.Status.CANCELLED
        session.save(update_fields=["status"])
        raise ValidationError({"code": "画板联机码已经过期"})
    session.guest = user
    session.status = BoardSession.Status.ACTIVE
    session.save(update_fields=["guest", "status"])
    transaction.on_commit(lambda: broadcast_board(session.id, "joined"))
    return session


@transaction.atomic
def submit_guess(user, session_id, raw_guess):
    session = (
        BoardSession.objects.select_for_update()
        .select_related("host", "guest")
        .get(id=session_id)
    )
    if session.guest_id != user.id:
        raise ValidationError("只有猜题方可以提交答案")
    if (
        session.mode != BoardSession.Mode.DRAW_GUESS
        or session.status != BoardSession.Status.ACTIVE
    ):
        raise ValidationError("当前画板不接受猜题")
    guess = str(raw_guess or "").strip()
    if not guess or len(guess) > 40:
        raise ValidationError({"guess": "请输入不超过 40 个字符的数学对象"})
    correct = guess.replace(" ", "").lower() == session.target.replace(" ", "").lower()
    if correct:
        session.guest_score += 1
        session.status = BoardSession.Status.COMPLETED
        session.completed_at = timezone.now()
        session.save(
            update_fields=["guest_score", "status", "completed_at"]
        )
        transaction.on_commit(lambda: broadcast_board(session.id, "completed"))
    return session, correct