aoc2022/20-a.hs

import Common
import Data.List (elemIndex, intercalate)
import Data.Maybe (fromJust)

wrap :: Int -> Int -> Int
wrap length index
  | index == 0 = length - 1
  | index >= length = wrap length $ index - length + 1
  | index < 0 = wrap length $ index + length - 1
  | otherwise = index

coordinates = [1000, 2000, 3000]

decrypt :: [Int] -> Int
decrypt a = sum $ map test coordinates
  where
    test n = mixed !! (zeroIndex + n)
    zeroIndex = fromJust $ elemIndex 0 mixed
    mixed = cycle $ fst $ foldl step (a, indices) indices

    indices = [0 .. len - 1]
    len = length a

    step :: ([Int], [Int]) -> Int -> ([Int], [Int])
    step (values, indices) i = (mix values index value, mix indices index value)
      where
        value = values !! index
        index = fromJust $ elemIndex i indices

    mix :: [Int] -> Int -> Int -> [Int]
    mix list i x = newList
      where
        newList = listStart ++ value : listEnd
        (listStart, listEnd) = splitAt newIndex list'
        newIndex = wrap len $ i + x
        list' = (\(l, r) -> l ++ tail r) $ splitAt i list
        value = list !! i

main = interact $
  show
  . decrypt
  . map read . lines