'''
This module provides some datetime.tzinfo implementations. 

All those classes are taken from the Python documentation.
'''
from datetime import timedelta, tzinfo
import time

ZERO = timedelta(0)
# constant for zero time offset.    

class Utc(tzinfo):
    '''UTC
    
    Universal time coordinated time zone.
    '''
    
    def utcoffset(self, dt):
        '''
        Return offset from UTC in minutes east of UTC, which is ZERO for UTC.
        '''
        return ZERO

    def tzname(self, dt):
        '''
        Return the time zone name corresponding to the datetime object dt, as a string.
        '''
        return "UTC"

    def dst(self, dt):
        '''
        Return the daylight saving time (DST) adjustment, in minutes east of UTC.
        '''
        return ZERO

UTC = Utc()
# the default instance for UTC.

class FixedOffset(tzinfo):
    '''
    A class building tzinfo objects for fixed-offset time zones.
    
    Note that FixedOffset(0, "UTC") is a different way to build a
    UTC tzinfo object.
    '''
    
    def __init__(self, offset_hours, offset_minutes, name):
        '''
        Initialise an instance with time offset and name.
        The time offset should be positive for time zones east of UTC
        and negate for time zones west of UTC. 
        '''
        self.__offset = timedelta(hours=offset_hours, minutes=offset_minutes)
        self.__name = name

    def utcoffset(self, dt):
        '''
        Return offset from UTC in minutes of UTC.
        '''
        return self.__offset

    def tzname(self, dt):
        '''
        Return the time zone name corresponding to the datetime object dt, as a
        string.
        '''
        return self.__name

    def dst(self, dt):
        '''
        Return the daylight saving time (DST) adjustment, in minutes east of 
        UTC.
        '''
        return ZERO
    
    def __repr__(self):
        '''
        Return nicely formatted repr string.
        '''
        return "<FixedOffset %r>" % self.__name


STDOFFSET = timedelta(seconds = -time.timezone)
# locale time zone offset

# calculate local daylight saving offset if any.
if time.daylight:
    DSTOFFSET = timedelta(seconds = -time.altzone)
else:
    DSTOFFSET = STDOFFSET

DSTDIFF = DSTOFFSET - STDOFFSET
# difference between local time zone and local DST time zone

class LocalTimezone(tzinfo):
    """
    A class capturing the platform's idea of local time.
    """

    def utcoffset(self, dt):
        '''
        Return offset from UTC in minutes of UTC.
        '''
        if self._isdst(dt):
            return DSTOFFSET
        else:
            return STDOFFSET

    def dst(self, dt):
        '''
        Return daylight saving offset.
        '''
        if self._isdst(dt):
            return DSTDIFF
        else:
            return ZERO

    def tzname(self, dt):
        '''
        Return the time zone name corresponding to the datetime object dt, as a
        string.
        '''
        return time.tzname[self._isdst(dt)]

    def _isdst(self, dt):
        '''
        Returns true if DST is active for given datetime object dt.
        '''
        tt = (dt.year, dt.month, dt.day,
              dt.hour, dt.minute, dt.second,
              dt.weekday(), 0, -1)
        stamp = time.mktime(tt)
        tt = time.localtime(stamp)
        return tt.tm_isdst > 0

LOCAL = LocalTimezone()
# the default instance for local time zone.
