45 lines
1.2 KiB
Haskell
45 lines
1.2 KiB
Haskell
import Text.Printf
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data Date = Date {year :: Int, month :: Int, day :: Int}
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instance Show Date where
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show Date {year = y, month = m, day = d} =
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printf "%4d-%02d-%02d" y m d
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leap :: Int -> Bool
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leap year =
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year `mod` 4 == 0
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&& (year `mod` 100 /= 0 || year `mod` 400 == 0)
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weekday :: Date -> Int
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weekday Date {year = y, month = m, day = d} =
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let doom = (s + t + (t `div` 4) + c_anchor) `mod` 7
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anchor = dooms !! pred m
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c_anchor = (5 * mod c 4 + 2) `mod` 7
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dooms =
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(if leap y then [4, 1] else [3, 7])
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<> [7, 4, 2, 6, 4, 1, 5, 3, 7, 5]
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(c, r) = y `divMod` 100
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(s, t) = r `divMod` 12
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in (doom + d - anchor + 7) `mod` 7
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days :: [String]
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days = words "Sunday Monday Tuesday Wednesday Thursday Friday Saturday"
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dates :: [Date]
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dates =
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[ Date {year = 1800, month = 1, day = 6},
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Date {year = 1875, month = 3, day = 29},
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Date {year = 1915, month = 12, day = 7},
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Date {year = 1970, month = 12, day = 23},
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Date {year = 2043, month = 5, day = 14},
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Date {year = 2077, month = 2, day = 12},
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Date {year = 2101, month = 4, day = 2}
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]
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dateAndDay :: Date -> String
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dateAndDay d = printf "%s: %s" (show d) (days !! weekday d)
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main :: IO ()
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main = putStr $ unlines $ map dateAndDay dates
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