#!/usr/bin/env python3
"""
Generates configs/eventnotifier_timezones.cfg from the system tzdata.

Each zone lists "<utc timestamp>" "<offset seconds> <abbreviation>" pairs; the
"0" entry is the offset in effect before the first transition in range.
Regenerate before END_YEAR runs out (32-bit timestamps end in 2038 anyway).
"""

import re
import zoneinfo
from datetime import datetime, timedelta, timezone
from pathlib import Path

START_YEAR = 2025
END_YEAR = 2037

OUT = Path(__file__).resolve().parent.parent / "configs" / "eventnotifier_timezones.cfg"


def utc(ts):
    return datetime.fromtimestamp(ts, timezone.utc)


def state(zone, ts):
    local = utc(ts).astimezone(zone)
    return int(local.utcoffset().total_seconds()), local.tzname()


def abbreviation(name, offset):
    # tzdata uses numeric names like "+03" / "-0330" for zones without a real abbreviation.
    if name and re.fullmatch(r"[A-Za-z]+", name):
        return name
    sign = "+" if offset >= 0 else "-"
    hours, rem = divmod(abs(offset), 3600)
    minutes = rem // 60
    if hours == 0 and minutes == 0:
        return "UTC"
    return f"UTC{sign}{hours}:{minutes:02d}" if minutes else f"UTC{sign}{hours}"


def transitions(zone):
    start = int(datetime(START_YEAR, 1, 1, tzinfo=timezone.utc).timestamp())
    end = int(datetime(END_YEAR, 12, 31, tzinfo=timezone.utc).timestamp())
    step = int(timedelta(days=1).total_seconds())

    periods = [(0, *state(zone, start))]
    prev_ts, prev = start, periods[0][1:]
    ts = start + step
    while ts <= end:
        cur = state(zone, ts)
        if cur != prev:
            # Bisect to the first second of the new state.
            lo, hi = prev_ts, ts
            while hi - lo > 1:
                mid = (lo + hi) // 2
                if state(zone, mid) == prev:
                    lo = mid
                else:
                    hi = mid
            periods.append((hi, *state(zone, hi)))
            prev = cur
        prev_ts = ts
        ts += step
    return periods


def main():
    lines = ["// Generated by tools/gen_timezones.py, do not edit by hand.", '"Timezones"', "{"]
    for name in sorted(zoneinfo.available_timezones()):
        if name.startswith(("posix/", "right/")) or name in ("Factory", "localtime"):
            continue
        zone = zoneinfo.ZoneInfo(name)
        lines.append(f'\t"{name}"')
        lines.append("\t{")
        for ts, offset, tzname in transitions(zone):
            lines.append(f'\t\t"{ts}"\t"{offset} {abbreviation(tzname, offset)}"')
        lines.append("\t}")
    lines.append("}")

    OUT.parent.mkdir(parents=True, exist_ok=True)
    OUT.write_text("\n".join(lines) + "\n")
    print(f"Wrote {OUT}")


if __name__ == "__main__":
    main()