Bitonic Sort in Haskell

比較の仕方がデータによらないソートである Bitonic Sort を Haskell で書いてみた．要素数 2^n 限定．だけ．

import System.Random
randInt :: IO (Int)
randInt = getStdRandom (randomR (0,1000))
randInts n = randInts' n []
 where
 randInts' n xs = 
  if n == 0 then return xs
  else
   randInt >>= (\x -> randInts' (n-1) (x:xs))
  
bimerge xs ys 1 dir = unzip (zipWith (\x y -> if (x < y) == dir then (x,y) else (y,x)) xs ys)
bimerge xs ys n dir = (xxs++xys, yxs++yys)
	where
	(xs', ys') = unzip (zipWith (\x y -> if (x < y) == dir then (x,y) else (y,x)) xs ys)
	n2 = (n `div` 2)
	(xxs, xys) = bimerge (take n2 xs') (drop n2 xs') n2 dir
	(yxs, yys) = bimerge (take n2 ys') (drop n2 ys') n2 dir
  
bisort xs = bisort' xs (length xs) True
	where
	bisort' zs 1 dir = zs
	bisort' zs n dir = xs'' ++ ys''
		where
		n2 = (n `div` 2)
		xs' = bisort' (take n2 zs) n2 dir
		ys' = bisort' (drop n2 zs) n2 (not dir)
		(xs'', ys'') = bimerge xs' ys' n2 dir
  
 -- randInts 1024 >>= (\x -> print (bisort x))
 -- randInts 1024 >>= (\xs -> print (snd (foldl (\(x, f) y -> (y, and [f, (x <= y)])) (-1, True) (bisort xs))))