highest value palindrome. https://www.hackerrank.com/challenges/richie-rich/problem

Palindrome.hs

module Lib where

import Data.List (foldr)

-- Complete the highestValuePalindrome function below.

highestValuePalindrome :: String -> Int -> Int -> String
highestValuePalindrome s n changesAllowed =
    if changesToMakePalindrome > changesAllowed
       then "-1"
       else result

    where
        changesToMakePalindrome = sum $ fmap (uncurry palindromeCost) $ zip [0..] $ take half allStats

        -- Cost of transforming a string into a palindrome
        palindromeCost ix (_, isEqual) = if isEqual || isMiddleChar ix then 0 else 1

        -- After making it a palindrome how many more changes can we make?
        extraChanges = changesAllowed - changesToMakePalindrome

        result = take firstHalf withNines ++ reverse (take half withNines)

        half = n `div` 2

        firstHalf = half + n - 2 * half

        -- String with as many characters replaced for '9' as possible
        withNines = reverse $ snd $ foldr withCosts ((1, 0) , []) $ reverse $ take firstHalf allStats

        withCosts stat ((ix, totalCost), list) =
            let
                cost =  nineCost ix stat - palindromeCost ix stat
            in
            if extraChanges >= totalCost + cost
               then ( (ix + 1, totalCost + cost), '9'      : list )
               else ( (ix + 1, totalCost       ), fst stat : list )

        -- pair of target letter and whether it is equal on both sides of the string
        allStats :: [(Char, Bool)]
        allStats = zipWith (\a b -> (max a b, a == b)) s (reverse s)

        isMiddleChar ix = odd n && ix == half + 1

        -- Number of changes required to convert one character into a nine on both sides of the string
        nineCost :: Int -> (Char, Bool) -> Int
        nineCost ix (char, isEqual) =
            let
                cost = if char /= '9'
                       then 2
                       else if isEqual
                            then 0
                            else 1
            in
            if isMiddleChar ix
               then max 0 (cost - 1)
               else cost