Solution to https://open.kattis.com/problems/substitution (fails second secret test case)

Substitution.hs

module Main where

import           Prelude
import           Control.Arrow
import           Data.List.Split                ( chunksOf )
import           Data.Maybe                     ( fromJust )
import qualified Data.ByteString.Lazy.Char8    as C
import qualified Data.Vector.Unboxed           as V

-- Zero index
parse :: [C.ByteString] -> [V.Vector Int]
parse =
  map (V.fromList . map (flip (-) 1 . (fst . fromJust . C.readInt)) . C.words)

-- Vector's unsafeBackpermute does the heavy lifting for us
solve :: [V.Vector Int] -> Int
solve [ms, cs, ps] =
  (length . takeWhile (/= cs) . iterate (V.unsafeBackpermute ps)) ms
solve _ = error "Malformed test case."

main :: IO ()
main =
  C.interact
    $   C.lines        -- Each line is an element of the list
    >>> drop 1       -- We don't need the number of test cases
    >>> chunksOf 4   -- Each test case has four lines
    >>> map
          (   drop 1 -- We don't need the first line of the test case
          >>> parse  -- Transform the input to data
          >>> solve  -- Manipulate the data
          >>> show   -- Transform back to text
          >>> C.pack -- Transform to ByteString
          )
    >>> C.unlines      -- Print each result on a different line