QuackSort?

quacksort.hs

-- merge + quick sort
-- quite good for random or semi-sorted lists
-- terrible for reverse sorted lists

import Debug.Trace
import Data.List
import Data.Word

randomList :: Word32 -> Word32 -> [Word32]
randomList seed 0    = []
randomList seed size = seed : randomList (seed * 1664525 + 1013904223) (size - 1)

quacksort :: [Word32] -> [Word32]
quacksort []           = []
quacksort [x]          = [x]
quacksort (p : x : xs) = split p (p : x : xs) [] [] where

  -- Splits the list in two halves of elements smaller/bigger than a pivot
  split p []       as bs = merge (quacksort as) (quacksort bs)
  split p (x : xs) as bs = quack p (p < x) x xs as bs

  -- Helper function for `split`
  quack p False x xs as bs = split p xs (x : as) bs
  quack p True  x xs as bs = split p xs as (x : bs)

  -- Merges two lists as a sorted one
  merge []       ys       = ys
  merge xs       []       = xs
  merge (x : xs) (y : ys) = place (x < y) x xs y ys

  -- Helper function for `merge`
  place False x xs y ys = y : merge (x : xs) ys
  place True  x xs y ys = x : merge xs (y : ys)

main :: IO ()
main = do
  let l = randomList 0 2000000
  let b = quacksort l
  print $ sum b