from datetime import date, datetime, timezone, timedelta

CST = timezone(timedelta(hours=8))  # China Standard Time


def today_cst() -> date:
    """Get today's date in Asia/Shanghai timezone."""
    return datetime.now(timezone.utc).astimezone(CST).date()


def parse_date(s: str) -> date:
    """Parse a date string that may be YYYY-MM-DD or a full ISO datetime."""
    return date.fromisoformat(s[:10])


def is_working_day(d: date | None = None, holidays: set[date] | None = None, workdays: set[date] | None = None) -> bool:
    """Check if a date is a Chinese working day.

    Returns False for:
    - Saturdays and Sundays (weekday >= 5), UNLESS in the workdays set (调休)
    - Dates in the holidays set (weekday holidays like 春节/国庆)

    Returns True for:
    - Monday-Friday, unless in holidays set
    - Saturday/Sunday that are in the workdays set (调休 makeup days)
    """
    d = d or today_cst()
    is_weekend = d.weekday() >= 5

    # 调休: weekend that becomes a workday
    if is_weekend and workdays and d in workdays:
        return True

    # Normal weekend
    if is_weekend:
        return False

    # Weekday holiday
    if holidays and d in holidays:
        return False

    return True