1# -*- coding: utf-8 -*- 2""" 3This module offers a generic easter computing method for any given year, using 4Western, Orthodox or Julian algorithms. 5""" 6 7import datetime 8 9__all__ = ["easter", "EASTER_JULIAN", "EASTER_ORTHODOX", "EASTER_WESTERN"] 10 11EASTER_JULIAN = 1 12EASTER_ORTHODOX = 2 13EASTER_WESTERN = 3 14 15 16def easter(year, method=EASTER_WESTERN): 17 """ 18 This method was ported from the work done by GM Arts, 19 on top of the algorithm by Claus Tondering, which was 20 based in part on the algorithm of Ouding (1940), as 21 quoted in "Explanatory Supplement to the Astronomical 22 Almanac", P. Kenneth Seidelmann, editor. 23 24 This algorithm implements three different easter 25 calculation methods: 26 27 1 - Original calculation in Julian calendar, valid in 28 dates after 326 AD 29 2 - Original method, with date converted to Gregorian 30 calendar, valid in years 1583 to 4099 31 3 - Revised method, in Gregorian calendar, valid in 32 years 1583 to 4099 as well 33 34 These methods are represented by the constants: 35 36 * ``EASTER_JULIAN = 1`` 37 * ``EASTER_ORTHODOX = 2`` 38 * ``EASTER_WESTERN = 3`` 39 40 The default method is method 3. 41 42 More about the algorithm may be found at: 43 44 `GM Arts: Easter Algorithms <http://www.gmarts.org/index.php?go=415>`_ 45 46 and 47 48 `The Calendar FAQ: Easter <https://www.tondering.dk/claus/cal/easter.php>`_ 49 50 """ 51 52 if not (1 <= method <= 3): 53 raise ValueError("invalid method") 54 55 # g - Golden year - 1 56 # c - Century 57 # h - (23 - Epact) mod 30 58 # i - Number of days from March 21 to Paschal Full Moon 59 # j - Weekday for PFM (0=Sunday, etc) 60 # p - Number of days from March 21 to Sunday on or before PFM 61 # (-6 to 28 methods 1 & 3, to 56 for method 2) 62 # e - Extra days to add for method 2 (converting Julian 63 # date to Gregorian date) 64 65 y = year 66 g = y % 19 67 e = 0 68 if method < 3: 69 # Old method 70 i = (19*g + 15) % 30 71 j = (y + y//4 + i) % 7 72 if method == 2: 73 # Extra dates to convert Julian to Gregorian date 74 e = 10 75 if y > 1600: 76 e = e + y//100 - 16 - (y//100 - 16)//4 77 else: 78 # New method 79 c = y//100 80 h = (c - c//4 - (8*c + 13)//25 + 19*g + 15) % 30 81 i = h - (h//28)*(1 - (h//28)*(29//(h + 1))*((21 - g)//11)) 82 j = (y + y//4 + i + 2 - c + c//4) % 7 83 84 # p can be from -6 to 56 corresponding to dates 22 March to 23 May 85 # (later dates apply to method 2, although 23 May never actually occurs) 86 p = i - j + e 87 d = 1 + (p + 27 + (p + 6)//40) % 31 88 m = 3 + (p + 26)//30 89 return datetime.date(int(y), int(m), int(d)) 90