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92 lines
2.5 KiB
Python
92 lines
2.5 KiB
Python
"""
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Copyright (c) 2003-2007 Gustavo Niemeyer <gustavo@niemeyer.net>
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This module offers extensions to the standard Python
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datetime module.
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"""
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__license__ = "Simplified BSD"
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import datetime
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__all__ = ["easter", "EASTER_JULIAN", "EASTER_ORTHODOX", "EASTER_WESTERN"]
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EASTER_JULIAN = 1
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EASTER_ORTHODOX = 2
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EASTER_WESTERN = 3
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def easter(year, method=EASTER_WESTERN):
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"""
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This method was ported from the work done by GM Arts,
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on top of the algorithm by Claus Tondering, which was
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based in part on the algorithm of Ouding (1940), as
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quoted in "Explanatory Supplement to the Astronomical
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Almanac", P. Kenneth Seidelmann, editor.
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This algorithm implements three different easter
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calculation methods:
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1 - Original calculation in Julian calendar, valid in
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dates after 326 AD
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2 - Original method, with date converted to Gregorian
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calendar, valid in years 1583 to 4099
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3 - Revised method, in Gregorian calendar, valid in
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years 1583 to 4099 as well
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These methods are represented by the constants:
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EASTER_JULIAN = 1
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EASTER_ORTHODOX = 2
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EASTER_WESTERN = 3
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The default method is method 3.
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More about the algorithm may be found at:
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http://users.chariot.net.au/~gmarts/eastalg.htm
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and
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http://www.tondering.dk/claus/calendar.html
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"""
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if not (1 <= method <= 3):
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raise ValueError("invalid method")
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# g - Golden year - 1
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# c - Century
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# h - (23 - Epact) mod 30
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# i - Number of days from March 21 to Paschal Full Moon
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# j - Weekday for PFM (0=Sunday, etc)
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# p - Number of days from March 21 to Sunday on or before PFM
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# (-6 to 28 methods 1 & 3, to 56 for method 2)
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# e - Extra days to add for method 2 (converting Julian
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# date to Gregorian date)
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y = year
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g = y % 19
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e = 0
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if method < 3:
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# Old method
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i = (19*g+15)%30
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j = (y+y//4+i)%7
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if method == 2:
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# Extra dates to convert Julian to Gregorian date
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e = 10
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if y > 1600:
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e = e+y//100-16-(y//100-16)//4
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else:
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# New method
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c = y//100
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h = (c-c//4-(8*c+13)//25+19*g+15)%30
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i = h-(h//28)*(1-(h//28)*(29//(h+1))*((21-g)//11))
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j = (y+y//4+i+2-c+c//4)%7
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# p can be from -6 to 56 corresponding to dates 22 March to 23 May
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# (later dates apply to method 2, although 23 May never actually occurs)
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p = i-j+e
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d = 1+(p+27+(p+6)//40)%31
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m = 3+(p+26)//30
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return datetime.date(int(y), int(m), int(d))
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