from datetime import datetime, time, timedelta
from zoneinfo import ZoneInfo

from fastapi import HTTPException, status
from sqlalchemy import select
from sqlalchemy.ext.asyncio import AsyncSession

from app.core.config import settings
from app.models.daily_usage import DailyUsage

SHANGHAI_TZ = ZoneInfo("Asia/Shanghai")


class QuotaService:
    def today(self):
        return datetime.now(SHANGHAI_TZ).date()

    def reset_at(self) -> datetime:
        tomorrow = self.today() + timedelta(days=1)
        return datetime.combine(tomorrow, time.min, tzinfo=SHANGHAI_TZ)

    async def get_quota(self, db: AsyncSession, user_id: str) -> dict:
        usage = await self._get_usage(db, user_id)
        used = usage.count if usage else 0
        return self._quota_payload(used)

    async def consume(self, db: AsyncSession, user_id: str) -> dict:
        usage = await self._get_usage(db, user_id)
        if usage is None:
            usage = DailyUsage(user_id=user_id, usage_date=self.today(), count=0)
            db.add(usage)
            await db.flush()

        if usage.count >= settings.daily_reading_limit:
            quota = self._quota_payload(usage.count)
            raise HTTPException(
                status_code=status.HTTP_429_TOO_MANY_REQUESTS,
                detail=f"今日 {settings.daily_reading_limit} 次解读机会已用完，请明天 0 点后再来。",
                headers={
                    "X-RateLimit-Limit": str(quota["limit"]),
                    "X-RateLimit-Remaining": str(quota["remaining"]),
                    "X-RateLimit-Reset": quota["reset_at"].isoformat(),
                },
            )

        usage.count += 1
        await db.flush()
        return self._quota_payload(usage.count)

    async def _get_usage(self, db: AsyncSession, user_id: str) -> DailyUsage | None:
        result = await db.execute(
            select(DailyUsage).where(DailyUsage.user_id == user_id, DailyUsage.usage_date == self.today())
        )
        return result.scalar_one_or_none()

    def _quota_payload(self, used: int) -> dict:
        limit = settings.daily_reading_limit
        return {
            "limit": limit,
            "used": used,
            "remaining": max(0, limit - used),
            "reset_at": self.reset_at(),
        }