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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)) | |