41 lines
1.2 KiB
Haskell
41 lines
1.2 KiB
Haskell
import Data.List
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type YInt = Int
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collatz :: YInt -> YInt
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collatz n
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| n < 0 = 1
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| even n = n `quot` 2
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| otherwise = 3 * n + 1
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-- The l `seq` is necessary to not be lazy (non-strict)
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-- And not to fill the stack
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lencollatz' :: YInt -> YInt -> YInt
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lencollatz' l 1 = l
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lencollatz' l n = l `seq` lencollatz' (l + 1) (collatz n)
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lencollatz = lencollatz' 0
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-- The j `seq` is necessary to not be lazy (non-strict)
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-- And not to fill the stack
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maximalIndex :: (YInt, YInt) -> YInt -> [YInt] -> (YInt, YInt)
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maximalIndex (m, i) j [] = (m, i)
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maximalIndex (m, i) j (x : xs) = j `seq` maximalIndex res (j + 1) xs
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where res = if x > m then (x, j) else (m, i)
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showCollatz :: YInt -> String
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showCollatz 1 = "1"
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showCollatz n = show n ++ " → " ++ showCollatz ( collatz n )
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pure (Just x) = x
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main = do
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putStrLn $ "max len: " ++ show m
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putStrLn $ "max value: " ++ show i
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-- print mrec
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-- print irec
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-- putStrLn $ showCollatz $ toInteger i
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where
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array = map lencollatz [1 .. 1000000]
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m = maximum array
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i = 1 + pure (elemIndex m array)
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(mrec, irec) = maximalIndex (0, 0) 1 array
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