Haskell

I have no Idea how to optimize this and am looking forward to the other solutions that probably run in sub-single-second times. I like my solution because it was simple to write which I hadn’t managed in the previous days, runs in 17 seconds with no less than 100MB of RAM.

import Control.Arrow
import Data.Bits (xor)
import Data.Ord (comparing)

import qualified Data.List as List
import qualified Data.Map as Map

parse :: String -> [Int]
parse = map read . filter (/= "") . lines

mix = xor 
prune = flip mod 16777216
priceof = flip mod 10

nextSecret step0 = do
        let step1 = prune . mix step0 $ step0 * 64
        let step2 = prune . mix step1 $ step1 `div` 32
        let step3 = prune . mix step2 $ step2 * 2048
        step3

part1 = sum . map (head . drop 2000 . iterate nextSecret)
part2 = map (iterate nextSecret
                >>> take 2001
                >>> map priceof
                >>> (id &&& tail)
                >>> uncurry (zipWith (curry (uncurry (flip (-)) &&& snd)))
                >>> map (take 4) . List.tails
                >>> filter ((==4) . length)
                >>> map (List.map fst &&& snd . List.last)
                >>> List.foldl (\ m (s, p) -> Map.insertWith (flip const) s p m) Map.empty
                )
        >>> Map.unionsWith (+)
        >>> Map.assocs
        >>> List.maximumBy (comparing snd)

main = getContents
        >>= print
        . (part1 &&& part2)
        . parse