28 lines
981 B
Haskell
28 lines
981 B
Haskell
import Data.Monoid
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data Month = Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec deriving (Eq,Ord,Enum,Bounded,Show)
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data Day = Sun | Mon | Tue | Wed | Thu | Fri | Sat deriving (Eq,Ord,Enum,Bounded,Show)
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type Year = Int
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leapYear yr = yr `mod` 400 == 0 || (yr `mod` 4 == 0 && yr `mod` 100 /= 0)
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daysInMonth :: (Month, Year) -> Int
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daysInMonth (mn, yr)
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| mn `elem` [ Sep, Apr, Jun, Nov ] = 30
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| mn == Feb = if leapYear yr then 29 else 28
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| otherwise = 31
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nextMonth (Dec, yr) = (Jan, yr + 1)
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nextMonth (mn, yr) = (succ mn, yr)
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compareMonths (m1,y1) (m2,y2) = compare y1 y2 <> compare m1 m2
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addDays d n = toEnum $ (fromEnum d + n) `mod` 7
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startDay = ((Jan, 1900), Mon) : [ (nextMonth m, addDays d n) | (m, d) <- startDay, let n = daysInMonth m ]
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main = print $ length $ filter ((== Sun) . snd) $
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takeWhile (\(m,_) -> m `compareMonths` (Dec, 2000) /= GT) $
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dropWhile (\(m,_) -> m `compareMonths` (Jan, 1901) == LT) $
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startDay
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