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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))
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