from __future__ import annotations

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

from app.services.credit.utils import ensure_aware

BUSINESS_TZ = ZoneInfo("Asia/Shanghai")


def to_business_time(value: datetime) -> datetime:
    return ensure_aware(value).astimezone(BUSINESS_TZ)


def add_natural_months(anchor_at: datetime, months: int) -> datetime:
    """始终基于传入锚点计算自然月，月底压缩后下月恢复原锚点日。"""
    anchor = to_business_time(anchor_at)
    month_index = anchor.year * 12 + anchor.month - 1 + int(months)
    year, month_zero = divmod(month_index, 12)
    month = month_zero + 1
    target_day = min(anchor.day, calendar.monthrange(year, month)[1])
    return anchor.replace(year=year, month=month, day=target_day)


def natural_month_period(anchor_at: datetime, sequence: int) -> tuple[datetime, datetime]:
    start = add_natural_months(anchor_at, sequence)
    end = add_natural_months(anchor_at, sequence + 1)
    return start, end


def next_local_midnight(value: datetime) -> datetime:
    local = to_business_time(value)
    next_day = local.date() + timedelta(days=1)
    return datetime.combine(next_day, time.min, tzinfo=BUSINESS_TZ)


def last_usable_at(expires_at: datetime) -> datetime:
    return ensure_aware(expires_at) - timedelta(microseconds=1)