import sys
import os
import struct
from array import array
from collections import namedtuple
from datetime import datetime

ttinfo = namedtuple('ttinfo', ['tt_gmtoff', 'tt_isdst', 'tt_abbrind'])

class TZInfo:
 def __init__(self, transitions, type_indices, ttis, abbrs):
 self.transitions = transitions
 self.type_indices = type_indices
 self.ttis = ttis
 self.abbrs = abbrs

 @classmethod
 def fromfile(cls, fileobj):
 if fileobj.read(4).decode() != "TZif":
 raise ValueError("not a zoneinfo file")
 fileobj.seek(20)
 header = fileobj.read(24)
 tzh = (tzh_ttisgmtcnt, tzh_ttisstdcnt, tzh_leapcnt,
 tzh_timecnt, tzh_typecnt, tzh_charcnt) = struct.unpack(">6l", header)
 transitions = array('i')
 transitions.fromfile(fileobj, tzh_timecnt)
 if sys.byteorder != 'big':
 transitions.byteswap()

 type_indices = array('B')
 type_indices.fromfile(fileobj, tzh_timecnt)

 ttis = []
 for i in range(tzh_typecnt):
 ttis.append(ttinfo._make(struct.unpack(">lbb", fileobj.read(6))))

 abbrs = fileobj.read(tzh_charcnt)

 self = cls(transitions, type_indices, ttis, abbrs)
 self.tzh = tzh

 return self

 def dump(self, stream, start=None, end=None):
 for j, (trans, i) in enumerate(zip(self.transitions, self.type_indices)):
 utc = datetime.utcfromtimestamp(trans)
 tti = self.ttis[i]
 lmt = datetime.utcfromtimestamp(trans + tti.tt_gmtoff)
 abbrind = tti.tt_abbrind
 abbr = self.abbrs[abbrind:self.abbrs.find(0, abbrind)].decode()
 if j > 0:
 prev_tti = self.ttis[self.type_indices[j - 1]]
 shift = " %+g" % ((tti.tt_gmtoff - prev_tti.tt_gmtoff) / 3600)
 else:
 shift = ''
 print("%s UTC = %s %-5s isdst=%d" % (utc, lmt, abbr, tti[1]) + shift, file=stream)

 @classmethod
 def zonelist(cls, zonedir='/usr/share/zoneinfo'):
 zones = []
 for root, _, files in os.walk(zonedir):
 for f in files:
 p = os.path.join(root, f)
 with open(p, 'rb') as o:
 magic = o.read(4)
 if magic == b'TZif':
 zones.append(p[len(zonedir) + 1:])
 return zones

if __name__ == '__main__':
 if len(sys.argv) < 2:
 zones = TZInfo.zonelist()
 for z in zones:
 print(z)
 sys.exit()
 filepath = sys.argv[1]
 if not filepath.startswith('/'):
 filepath = os.path.join('/usr/share/zoneinfo', filepath)
 with open(filepath, 'rb') as fileobj:
 tzi = TZInfo.fromfile(fileobj)
 tzi.dump(sys.stdout)