"""
core/tz.py - Timezone-aware date/time helpers

The frontend sends X-Timezone-Offset (minutes from UTC, same sign as
JavaScript's getTimezoneOffset — positive means behind UTC).
These helpers convert server UTC to the user's local date/time.
"""

from datetime import datetime, date, timezone, timedelta


def _get_offset_minutes():
    """Read the timezone offset header from the current Flask request.
    Returns 0 if outside a request context or header is absent."""
    try:
        import flask
        return int(flask.request.headers.get("X-Timezone-Offset", 0))
    except (ValueError, TypeError, RuntimeError):
        # RuntimeError: outside of request context
        return 0


def _offset_to_tz(offset_minutes):
    """Convert JS-style offset (positive = behind UTC) to a timezone object."""
    return timezone(timedelta(minutes=-offset_minutes))


def user_now(offset_minutes=None):
    """Current datetime in the user's timezone.
    If offset_minutes is provided, uses that instead of the request header."""
    if offset_minutes is None:
        offset_minutes = _get_offset_minutes()
    tz = _offset_to_tz(offset_minutes)
    return datetime.now(tz)


def user_today(offset_minutes=None):
    """Current date in the user's timezone."""
    return user_now(offset_minutes).date()