CodeWars/5kyu/pick_peaks/solution.hs

module PickPeak.JorgeVS.Kata where

data PickedPeaks = PickedPeaks { pos :: [Int], peaks :: [Int] } deriving (Eq, Show)

windowed :: Int -> [a] -> [[a]]
windowed n xs
  | null xs || length window < n = []
  | otherwise = window : (windowed n $ tail xs)
  where window = take n xs

addIfPeak :: PickedPeaks -> [(Int, Int)] -> PickedPeaks
addIfPeak ps [l, mid, r]
  | (snd mid > snd l) && (snd mid > snd r) = PickedPeaks { pos = fst mid : pos ps, peaks = snd mid : peaks ps }
  | otherwise = ps
addIfPeak _ _ = error "Invalid state"

removeConsecutiveDuplicates :: [(Int, Int)] -> [(Int, Int)]
removeConsecutiveDuplicates = foldr addIfNotDuplicate []
  where addIfNotDuplicate :: (Int, Int) -> [(Int, Int)] -> [(Int, Int)]
        addIfNotDuplicate pair [] = [pair]
        addIfNotDuplicate pair@(i, x) pairs
          | x == (snd . head) pairs = pair : tail pairs
          | otherwise = pair : pairs

pickPeaks :: [Int] -> PickedPeaks
pickPeaks = foldl addIfPeak PickedPeaks { pos = [], peaks = [] }
          . windowed 3
          . reverse
          . removeConsecutiveDuplicates
          . zip [0..]