import _winreg
import struct
import datetime

handle = _winreg.ConnectRegistry(None, _winreg.HKEY_LOCAL_MACHINE)
tzparent = _winreg.OpenKey(handle,
                           "SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion\\Time Zones")
parentsize = _winreg.QueryInfoKey(tzparent)[0]

localkey = _winreg.OpenKey(handle,
                           "SYSTEM\\CurrentControlSet\\Control\\TimeZoneInformation")
WEEKS = datetime.timedelta(7)


def list_timezones():
    """Return a list of all time zones known to the system."""
    l = []
    for i in xrange(parentsize):
        l.append(_winreg.EnumKey(tzparent, i))
    return l


class win32tz(datetime.tzinfo):
    """tzinfo class based on win32's timezones available in the registry.

    >>> local = win32tz('Central Standard Time')
    >>> oct1 = datetime.datetime(month=10, year=2004, day=1, tzinfo=local)
    >>> dec1 = datetime.datetime(month=12, year=2004, day=1, tzinfo=local)
    >>> oct1.dst()
    datetime.timedelta(0, 3600)
    >>> dec1.dst()
    datetime.timedelta(0)
    >>> braz = win32tz('E. South America Standard Time')
    >>> braz.dst(oct1)
    datetime.timedelta(0)
    >>> braz.dst(dec1)
    datetime.timedelta(0, 3600)
    """

    def __init__(self, name):
        self.data = win32tz_data(name)

    def utcoffset(self, dt):
        if self._isdst(dt):
            return datetime.timedelta(minutes=self.data.dstoffset)
        else:
            return datetime.timedelta(minutes=self.data.stdoffset)

    def dst(self, dt):
        if self._isdst(dt):
            minutes = self.data.dstoffset - self.data.stdoffset
            return datetime.timedelta(minutes=minutes)
        else:
            return datetime.timedelta(0)

    def tzname(self, dt):
        if self._isdst(dt):
            return self.data.dstname
        else:
            return self.data.stdname

    def _isdst(self, dt):
        dat = self.data
        dston = pickNthWeekday(dt.year, dat.dstmonth, dat.dstdayofweek,
                               dat.dsthour, dat.dstminute, dat.dstweeknumber)
        dstoff = pickNthWeekday(dt.year, dat.stdmonth, dat.stddayofweek,
                                dat.stdhour, dat.stdminute, dat.stdweeknumber)
        if dston < dstoff:
            return (dston <= dt.replace(tzinfo=None) < dstoff)
        else:
            return not (dstoff <= dt.replace(tzinfo=None) < dston)

    def __repr__(self):
        return "<win32tz - {0!s}>".format(self.data.display)


def pickNthWeekday(year, month, dayofweek, hour, minute, whichweek):
    """dayofweek == 0 means Sunday, whichweek > 4 means last instance"""
    first = datetime.datetime(year=year, month=month, hour=hour, minute=minute,
                              day=1)
    weekdayone = first.replace(day=((dayofweek - first.isoweekday()) % 7 + 1))
    for n in xrange(whichweek - 1, -1, -1):
        dt = weekdayone + n * WEEKS
        if dt.month == month:
            return dt


class win32tz_data(object):
    """Read a registry key for a timezone, expose its contents."""

    def __init__(self, path):
        """Load path, or if path is empty, load local time."""
        if path:
            keydict = valuesToDict(_winreg.OpenKey(tzparent, path))
            self.display = keydict['Display']
            self.dstname = keydict['Dlt']
            self.stdname = keydict['Std']

            #see http://ww_winreg.jsiinc.com/SUBA/tip0300/rh0398.htm
            tup = struct.unpack('=3l16h', keydict['TZI'])
            self.stdoffset = -tup[0] - tup[1]  # Bias + StandardBias * -1
            self.dstoffset = self.stdoffset - tup[2]  # + DaylightBias * -1

            offset = 3
            self.stdmonth = tup[1 + offset]
            self.stddayofweek = tup[2 + offset]  # Sunday=0
            self.stdweeknumber = tup[3 + offset]  # Last = 5
            self.stdhour = tup[4 + offset]
            self.stdminute = tup[5 + offset]

            offset = 11
            self.dstmonth = tup[1 + offset]
            self.dstdayofweek = tup[2 + offset]  # Sunday=0
            self.dstweeknumber = tup[3 + offset]  # Last = 5
            self.dsthour = tup[4 + offset]
            self.dstminute = tup[5 + offset]

        else:
            keydict = valuesToDict(localkey)

            self.stdname = keydict['StandardName']
            self.dstname = keydict['DaylightName']

            sourcekey = _winreg.OpenKey(tzparent, self.stdname)
            self.display = valuesToDict(sourcekey)['Display']

            self.stdoffset = -keydict['Bias'] - keydict['StandardBias']
            self.dstoffset = self.stdoffset - keydict['DaylightBias']

            #see http://ww_winreg.jsiinc.com/SUBA/tip0300/rh0398.htm
            tup = struct.unpack('=8h', keydict['StandardStart'])

            offset = 0
            self.stdmonth = tup[1 + offset]
            self.stddayofweek = tup[2 + offset]  # Sunday=0
            self.stdweeknumber = tup[3 + offset]  # Last = 5
            self.stdhour = tup[4 + offset]
            self.stdminute = tup[5 + offset]

            tup = struct.unpack('=8h', keydict['DaylightStart'])
            self.dstmonth = tup[1 + offset]
            self.dstdayofweek = tup[2 + offset]  # Sunday=0
            self.dstweeknumber = tup[3 + offset]  # Last = 5
            self.dsthour = tup[4 + offset]
            self.dstminute = tup[5 + offset]


def valuesToDict(key):
    """Convert a registry key's values to a dictionary."""
    d = {}
    size = _winreg.QueryInfoKey(key)[1]
    for i in xrange(size):
        d[_winreg.EnumValue(key, i)[0]] = _winreg.EnumValue(key, i)[1]
    return d


def _test():
    import win32tz, doctest
    doctest.testmod(win32tz, verbose=0)

if __name__ == '__main__':
    _test()