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+# -*- 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))