Parse tzfile for timezone information

parse-tzfile.py

import sys
import struct
import pathlib


def read_till_null(b, start):
    end = start
    while b[end] != 0:
        end += 1
    return b[start:end].decode("utf-8")


def main(tzfile_path: str):
    """python parse-tzfile.py /usr/share/zoneinfo/Asia/Taipei"""

    tzfile = pathlib.Path(tzfile_path)
    if not tzfile.exists():
        print(f"File {tzfile_path} does not exist.")
        sys.exit(1)

    with tzfile.open("rb") as f:
        # Headers
        magic = f.read(4)
        if magic != b"TZif":
            print(f"File {tzfile_path} is not a valid tzfile.")
            sys.exit(1)

        version = f.read(1)
        _ = f.read(15)  # reserved

        tzh_ttisutcnt = struct.unpack(">l", f.read(4))[0]
        tzh_ttisstdcnt = struct.unpack(">l", f.read(4))[0]
        tzh_leapcnt = struct.unpack(">l", f.read(4))[0]
        tzh_timecnt = struct.unpack(">l", f.read(4))[0]
        tzh_typecnt = struct.unpack(">l", f.read(4))[0]
        tzh_charcnt = struct.unpack(">l", f.read(4))[0]

        print(
            tzh_ttisutcnt,
            tzh_ttisstdcnt,
            tzh_leapcnt,
            tzh_timecnt,
            tzh_typecnt,
            tzh_charcnt,
        )

        # Timezone Data
        tzh_transition_times = struct.unpack(
            f"!{tzh_timecnt}l", f.read(4 * tzh_timecnt)
        )
        tzh_time_local_types = struct.unpack(
            f"!{tzh_timecnt}B", f.read(1 * tzh_timecnt)
        )

        for tt in tzh_transition_times:
            print(tt)

        # Timezone Types
        for i in range(tzh_typecnt):
            tt_utoff = struct.unpack("!l", f.read(4))[0]
            tt_isdst = struct.unpack(">B", f.read(1))[0]
            tt_desigidx = struct.unpack(">B", f.read(1))[0]
            print(tt_utoff, tt_isdst, tt_desigidx)

        tzh_typechars = f.read(tzh_charcnt)


if __name__ == "__main__":
    main(sys.argv[1])