import Data.Array ( listArray, (!) )
import Data.List ( elemIndex )

collatzLengthList m = collatzLengthArray
  where
    collatzLengthArray
      = listArray (1,m) $ 1:map collatzLength [2..m]
      where
        collatzLength n
          | b<=m = 1 + collatzLengthArray ! b
          | otherwise = 1 + collatzLength b
          where
            b
              | even n = n `div` 2 
              | otherwise = 3*n+1

e14 :: (Maybe Int, Integer)
e14 = (n, m)
  where
    m = maximum cArray
    cArray = collatzLengthList p
    p = 1000000
    n = elemIndex m cList
      where
        cList = 0:[cArray ! x | x <- [1..p]]

λ> :set +s
λ> e14
(Just 837799,525)
(1.74 secs, 2,193,219,920 bytes)

— Me@2023-07-25 11:55:41 PM

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2023.07.26 Wednesday (c) All rights reserved by ACHK