genequivyear.py

#!/usr/bin/env python2
#
#  Generate equivalent year table needed by duk_bi_date.c.  Based on:
#
#    http://code.google.com/p/v8/source/browse/trunk/src/date.h#146
#

import datetime
import pytz

def isleapyear(year):
    if (year % 4) != 0:
        return False
    if (year % 100) != 0:
        return True
    if (year % 400) != 0:
        return False
    return True

def eqyear(weekday, isleap):
    # weekday: 0=Sunday, 1=Monday, ...

    if isleap:
        recent_year = 1956
    else:
        recent_year = 1967
    recent_year += (weekday * 12) % 28
    year = 2008 + (recent_year + 3 * 28 - 2008) % 28

    # some assertions
    #
    # Note that ECMAScript internal weekday (0=Sunday) matches neither
    # Python weekday() (0=Monday) nor isoweekday() (1=Monday, 7=Sunday).
    # Python isoweekday() % 7 matches the ECMAScript weekday.
    # https://docs.python.org/2/library/datetime.html#datetime.date.isoweekday

    dt = datetime.datetime(year, 1, 1, 0, 0, 0, 0, pytz.UTC)  # Jan 1 00:00:00.000 UTC
    #print(weekday, isleap, year, dt.isoweekday(), isleapyear(year))
    #print(repr(dt))
    #print(dt.isoformat())

    if isleap != isleapyear(year):
        raise Exception('internal error: equivalent year does not have same leap-year-ness')
        pass

    if weekday != dt.isoweekday() % 7:
        raise Exception('internal error: equivalent year does not begin with the same weekday')
        pass

    return year

def main():
    for i in xrange(14):
        print(eqyear(i % 7, i >= 7))

if __name__ == '__main__':
    main()