from __future__ import annotations

import re
from datetime import datetime, timedelta, timezone


TZ_RE = re.compile(r"^(?P<sign>[+-])(?P<h>\d{2})(?::?(?P<m>\d{2}))?$")


def local_timezone() -> timezone:
    offset = datetime.now().astimezone().utcoffset()
    if offset is None:
        return timezone.utc
    return timezone(offset)


def parse_timezone_offset(value: str) -> timezone:
    match = TZ_RE.match(value.strip())
    if not match:
        raise ValueError("timezone must look like +09:00 or -05:30")
    sign = 1 if match.group("sign") == "+" else -1
    hours = int(match.group("h"))
    minutes = int(match.group("m") or "0")
    if minutes not in {0, 15, 30, 45}:
        raise ValueError("timezone minutes must be 00, 15, 30, or 45")
    return timezone(sign * timedelta(hours=hours, minutes=minutes))


def timezone_to_string(tz: timezone) -> str:
    offset = tz.utcoffset(None)
    if offset is None:
        return "+00:00"
    total_minutes = int(offset.total_seconds() // 60)
    sign = "+" if total_minutes >= 0 else "-"
    total_minutes = abs(total_minutes)
    return f"{sign}{total_minutes // 60:02d}:{total_minutes % 60:02d}"