chromium / external / code.launchpad.net / dateutil / 40996e01996096f252ef9d7754edf054e691900d / . / dateutil / easter.py

""" | |

Copyright (c) 2003-2007 Gustavo Niemeyer <gustavo@niemeyer.net> | |

This module offers extensions to the standard Python | |

datetime module. | |

""" | |

__license__ = "Simplified BSD" | |

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: | |

http://users.chariot.net.au/~gmarts/eastalg.htm | |

and | |

http://www.tondering.dk/claus/calendar.html | |

""" | |

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)) | |