"""AI 功能用量与配额（批次 B2，为批次 C billing 预留接口）。

设计：每次 AI 调用记一条 `AiUsage` 流水；配额从 `quota_for()` 取——
批次 C 的 billing 落地后，把 `apps/billing/quota.py` 放进来即自动接管，
本模块无需改动（导入失败则回落到 settings.AI_FREE_MONTHLY_QUOTA）。
"""

from __future__ import annotations

from datetime import date

from django.conf import settings
from django.db import models
from django.db.models import Count

from apps.core.base_models import TenantScopedModel

# 用量类型
KIND_PARSE_ORDER = "parse_order"
KIND_ASK = "ask"
KIND_ADVICE = "advice"


try:  # billing（批次 C1）已安装时复用其异常类，视图层只需 catch 一处
    from apps.billing.quota import QuotaExceeded as _QuotaBase
except Exception:  # pragma: no cover - billing 未安装时的兜底基类
    _QuotaBase = Exception


class QuotaExceeded(_QuotaBase):
    """AI 用量超出套餐配额（调用方翻译为 403 + 升级引导）。"""

    def __init__(self, kind: str, used: int, quota: int, **extra):
        self.kind = kind
        self.used = used
        self.quota = quota
        self.extra = extra
        # 直接调 Exception.__init__：父类（billing）的签名不同，不适合链式调用
        Exception.__init__(self, f"ai quota exceeded for {kind}: {used}/{quota}")

    def as_dict(self) -> dict:
        return {
            "code": "quota_exceeded",
            "kind": self.kind,
            "detail": f"AI 功能本月已用 {self.used}/{self.quota} 次，升级套餐可解除限制",
            "used": self.used,
            "limit": self.quota,
            "upgrade_url": "/#/pricing",
        }


class AiUsage(TenantScopedModel):
    """AI 调用流水（一次调用一条）。"""

    kind = models.CharField(max_length=32, help_text="parse_order/ask/advice")
    period = models.CharField(max_length=7, help_text="账期 YYYY-MM（便于按月聚合）")
    detail = models.JSONField(default=dict, blank=True, help_text="调用元数据（不含敏感原文）")

    class Meta:
        db_table = "ai_usage"
        verbose_name_plural = "AI 调用流水"
        indexes = [
            models.Index(fields=["tenant", "kind", "period"]),
        ]
        ordering = ["-created_at"]

    def __str__(self):
        return f"{self.kind}@{self.period}"


def current_period(today: date | None = None) -> str:
    return (today or date.today()).strftime("%Y-%m")


def quota_for(tenant, kind: str) -> int:
    """该租户该 AI 功能的月度配额（0 = 不限）。

    优先走批次 C 的 billing（若已安装），否则用全局免费额度。
    """
    try:
        from apps.billing.quota import quota_for as billing_quota  # type: ignore

        return int(billing_quota(tenant, f"ai_{kind}"))
    except Exception:
        return int(getattr(settings, "AI_FREE_MONTHLY_QUOTA", 10))


def monthly_count(tenant, kind: str, *, period: str | None = None) -> int:
    """本月该功能的已用次数。"""
    return AiUsage.objects.filter(
        tenant=tenant, kind=kind, period=period or current_period()
    ).count()


def check_quota(tenant, kind: str, *, period: str | None = None) -> dict:
    """检查并返回配额状态 {used, quota, remaining, allowed}。

    超限抛 QuotaExceeded（不在此处吞掉，让视图层翻译 HTTP 语义）。
    """
    period = period or current_period()
    quota = quota_for(tenant, kind)
    used = monthly_count(tenant, kind, period=period)
    if quota > 0 and used >= quota:
        raise QuotaExceeded(kind, used, quota)
    return {
        "used": used,
        "quota": quota,
        "remaining": (quota - used) if quota > 0 else None,
        "allowed": True,
    }


def record_usage(tenant, kind: str, *, detail: dict | None = None) -> AiUsage:
    """登记一次调用。"""
    return AiUsage.objects.create(
        tenant=tenant,
        kind=kind,
        period=current_period(),
        detail=detail or {},
    )


def usage_summary(tenant) -> dict:
    """该租户各 AI 功能本月用量总览（前端展示"免费版 10 次/月"进度）。"""
    period = current_period()
    rows = (
        AiUsage.objects.filter(tenant=tenant, period=period)
        .values("kind")
        .annotate(n=Count("id"))
    )
    used = {r["kind"]: r["n"] for r in rows}
    kinds = [KIND_PARSE_ORDER, KIND_ASK, KIND_ADVICE]
    return {
        "period": period,
        "items": [
            {
                "kind": k,
                "used": used.get(k, 0),
                "quota": quota_for(tenant, k),
            }
            for k in kinds
        ],
    }