'EASTER.BAS
'17 April 1995 by Marc Kummel aka Treebeard.
'Contact mkummel@rain.org, http://www.rain.org/~mkummel/
'
'Use O'Beirne algorithm to calculate date of Easter Sunday.
'Only works for years 1900-2099 (but doesn't complain).
'From Eric Burgess, _Celestial Basic_, p.15f
'
'This code is freeware.  For more interesting BASIC software with source
'code, check out Treebeard's Basic Vault on the Web at
'  http://www.rain.org/~mkummel/basic/
'---------------------------------------------------------------------------

CONST loyear = 1900: hiyear = 2099
CONST maxcount = 34
REDIM dcnt%(maxcount)
REDIM ycnt%(loyear TO hiyear)

'Uncomment to make table for 1900-2099
'  GOSUB Main2
'  END

Main:
  CLS
  PRINT "Enter year or press <Enter> to quit."
  PRINT
  DO
    INPUT "figure Easter for year: ", year%
    IF year% = 0 THEN EXIT DO
    GOSUB FigureEaster
    PRINT "Easter Sunday in"; year%; "is "; month$; day%, count%; " days after equinox"
    PRINT
  LOOP
END

Main2:
  CLS
  FOR year% = loyear TO hiyear
    GOSUB FigureEaster
    PRINT "Easter Sunday in"; year%; "is "; month$; day%, "count ="; count%
    dcnt%(count%) = dcnt%(count%) + 1
    ycnt%(year%) = count%
  NEXT year%
END

FigureEaster:
  n = year% - 1900
  a = n / 19
  a = 19 * (a - INT(a))
  b = INT((7 * a + 1) / 19)
  m = (11 * a + 4.0001 - b) / 29
  x = m - INT(m)
  m = 29 * x

  q = INT(n / 4)
  w = (n + q + 31 - m) / 7
  w = 7 * (w - INT(w))
  w = INT(w)
  d% = INT(25 - m - w)

  DO WHILE d% < -9
    d% = d% + 9
  LOOP

  IF d% > 0 THEN
    month$ = "April"
    day% = d%
  ELSE
    month$ = "March"
    day% = 31 - ABS(d%)
  END IF

  'kludge to add 1 extra week.  Round-off error somewhere?
  SELECT CASE year%
    CASE 1902, 1906, 1930, 1943, 1950
      d% = d% + 7
      day% = day% + 7
    CASE 1957, 1970, 1974, 1977, 1984, 1997
      d% = d% + 7
      day% = day% + 7
    CASE 2001, 2011, 2025, 2038, 2045
      d% = d% + 7
      day% = day% + 7
    CASE 2052, 2055, 2065, 2069, 2072, 2079, 2092, 2096, 2099
      d% = d% + 7
      day% = day% + 7
    CASE 1926, 1994, 2021, 2075, 2089
      d% = d% + 7
      day% = day% + 7 - 31
      month$ = "April"
  END SELECT
  count% = d% + 9

RETURN