merge.hs

merge.hs

merge :: Integer -> Integer -> Integer
merge i1 i2 = merge' i1 i2 (length i1) (length i2) 0
  where
    merge' _ _ 0 0 acc   = acc
    merge' _ i2' 0 l2 acc  = acc * 10 ^ l2 + i2'
    merge' i1' _ l1 0 acc  = acc * 10 ^ l1 + i1'
    merge' i1' i2' l1 l2 acc = merge' rest1 rest2 (l1 - 1) (l2 - 1) acc'
      where
        (first1, rest1) = split i1' l1
        (first2, rest2) = split i2' l2
        acc' = acc * 10 ^ 2 + first1 * 10 + first2

    split :: Integer -> Integer -> (Integer, Integer)
    split i l = (first, rest)
      where
        rest  = (i `mod` 10 ^ (l - 1))
        first = (i - rest) `div` 10 ^ (l - 1)

    length :: Integer -> Integer
    length i  = length' i 1
    length' i acc = case i - 10 ^ acc of
      d | d < 0 -> acc
      _ -> length' i (acc + 1)

main :: IO ()
main = print $ merge 111111111111111111111111111111111111111111111111 222222222222222222222222222222222222222222222222222222222222

-- $ stack ghci
-- Configuring GHCi with the following packages:
-- GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
-- Loaded GHCi configuration from /private/var/folders/v5/16cv223x5lb09z8cw__tk5v80000gn/T/ghci7780/ghci-script
-- Prelude> :l merge.hs
-- [1 of 1] Compiling Main             ( merge.hs, interpreted )
-- Ok, modules loaded: Main.
-- *Main> main
-- 121212121212121212121212121212121212121212121212121212121212121212121212121212121212121212121212222222222222