Personal Haskell library for Lexographic Permutations

Lexos.hs

module Data.List.Lexos (lexoPermutation, lexoList) where

getKHelper :: Ord a => [a] -> Int -> Maybe Int -> Maybe Int
getKHelper (x:y:ys) currIndex maybeIndex = if (x < y) then getKHelper (y:ys) (currIndex + 1) (Just currIndex) else getKHelper (y:ys) (currIndex + 1) maybeIndex
getKHelper _ _ maybeIndex = maybeIndex

getK :: Ord a => [a] -> Maybe Int
getK xs = getKHelper xs 0 Nothing

getLHelper :: Ord a => [a] -> a -> Int -> Int -> Int
getLHelper (x:xs) workingValue index resIndex = if (x > workingValue) then (getLHelper xs workingValue (index + 1) index) else (getLHelper xs workingValue (index + 1) resIndex)
getLHelper _ _ _ resIndex = resIndex

getL k xs = getLHelper xs (xs !! k) 0 0

swap :: Int -> Int -> [a] -> [a]
swap x y xs = let (l1, e1:ys) = splitAt x xs; (l2, e2:l3) = splitAt (y - x - 1) ys in (l1 ++ [e2] ++ l2 ++ [e1] ++ l3)

reverseSeqToEnd i xs = let (front, back) = splitAt i xs in front ++ reverse back

lexoPermutation :: Ord a => [a] -> Maybe [a]
lexoPermutation xs = do 
    k <- getK xs  
    let l = getL k xs
    let swapped = swap k l xs
    let reversed = reverseSeqToEnd (k+1) swapped
    return reversed

lexoList :: Ord a => [a] -> [[a]]
lexoList xs = maybe [xs] ((xs :) . lexoList) (lexoPermutation xs) -- lexos array = array ++ ifthen lexographicPermutation xs