# -*- coding: utf-8 -*-
"""
This module offers a generic Easter computing method
for any given year, using
Western, Orthodox
or Julian algorithms.
"""
import datetime
__all__ = [
"easter",
"EASTER_JULIAN",
"EASTER_ORTHODOX",
"EASTER_WESTERN"]
EASTER_JULIAN = 1
EASTER_ORTHODOX = 2
EASTER_WESTERN = 3
def easter(year, method=EASTER_WESTERN):
"""
This method was ported
from the work done by GM Arts,
on top of the algorithm by Claus Tondering, which was
based
in part on the algorithm of Ouding (1940),
as
quoted
in "Explanatory Supplement to the Astronomical
Almanac
", P. Kenneth Seidelmann, editor.
This algorithm implements three different Easter
calculation methods:
1. Original calculation
in Julian calendar, valid
in
dates after 326 AD
2. Original method,
with date converted to Gregorian
calendar, valid
in years 1583 to 4099
3. Revised method,
in Gregorian calendar, valid
in
years 1583 to 4099
as well
These methods are represented by the constants:
* ``EASTER_JULIAN = 1``
* ``EASTER_ORTHODOX = 2``
* ``EASTER_WESTERN = 3``
The default method
is method 3.
More about the algorithm may be found at:
`GM Arts: Easter Algorithms <
http://www.gmarts.org/index.php?go=415>`_
and
`The Calendar FAQ: Easter <
https://www.tondering.dk/claus/cal/easter.php>`_
"""
if not (1 <= method <= 3):
raise ValueError(
"invalid method")
# g - Golden year - 1
# c - Century
# h - (23 - Epact) mod 30
# i - Number of days from March 21 to Paschal Full Moon
# j - Weekday for PFM (0=Sunday, etc)
# p - Number of days from March 21 to Sunday on or before PFM
# (-6 to 28 methods 1 & 3, to 56 for method 2)
# e - Extra days to add for method 2 (converting Julian
# date to Gregorian date)
y = year
g = y % 19
e = 0
if method < 3:
# Old method
i = (19*g + 15) % 30
j = (y + y//4 + i) % 7
if method == 2:
# Extra dates to convert Julian to Gregorian date
e = 10
if y > 1600:
e = e + y//100 - 16 - (y//100 - 16)//4
else:
# New method
c = y//100
h = (c - c//4 - (8*c + 13)//25 + 19*g + 15) % 30
i = h - (h//28)*(1 - (h//28)*(29//(h + 1))*((21 - g)//11))
j = (y + y//4 + i + 2 - c + c//4) % 7
# p can be from -6 to 56 corresponding to dates 22 March to 23 May
# (later dates apply to method 2, although 23 May never actually occurs)
p = i - j + e
d = 1 + (p + 27 + (p + 6)//40) % 31
m = 3 + (p + 26)//30
return datetime.date(int(y), int(m), int(d))
