Cross-repo recipe package system.

third_party/recipe_engine/third_party/dateutil/easter.py

+"""
+Copyright (c) 2003-2007  Gustavo Niemeyer <gustavo@niemeyer.net>
+
+This module offers extensions to the standard python 2.3+
+datetime module.
+"""
+__author__ = "Gustavo Niemeyer <gustavo@niemeyer.net>"
+__license__ = "PSF License"
+
+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))
+