Zellar

gistfile1.hs

import qualified System.Time as T

type Year   = Int
type Month  = Int
type Day    = Int

-- Ex. $> h 2013 5 21
--     $> Tuesday 
h :: Year -> Month -> Day -> T.Day
h year 1 day        = h year 13 day
h year 2 day        = h year 13 day
h year month day    = f where
    f       = hToDay $ g `mod` 7
    g       = day + i + bigy + j + ganma year
    i       = floor $ realToFrac (26 * (month + 1)) / 10
    j       = floor $ realToFrac bigy / 4
    bigy    = year `mod` 100

ganma :: Year -> Int
ganma = f where
    f year 
        | year >= 1582              = 5 * bigc + g -- Gregorian
        | 4 <= year && year < 1582  = 6 * bigc + 5 -- Julian
        | otherwise             = error "ganma"
        where 
            g       = floor $ realToFrac bigc / 4
            bigc    = floor $ realToFrac year / 100

hToDay :: Int -> T.Day
hToDay  = f where
    f 1 = T.Sunday
    f 2 = T.Monday
    f 3 = T.Tuesday
    f 4 = T.Wednesday
    f 5 = T.Thursday
    f 6 = T.Friday
    f 0 = T.Saturday