{-# OPTIONS_GHC -fglasgow-exts #-}

module Main (main) where

import Prelude hiding (reverse)

import Control.Monad.ST

import GHC.ST
import GHC.Base
import GHC.Prim

import Foreign (sizeOf)

import System.Environment

reverse :: MutableByteArray# s -> Int# -> Int# -> State# s -> State# s
reverse arr i j s
  | i <# j    = case readIntArray#  arr i    s of { (# s, ei #) ->
                case readIntArray#  arr j    s of { (# s, ej #) ->
                case writeIntArray# arr j ei s of { s ->
                case writeIntArray# arr i ej s of { s ->
                reverse arr (i +# 1#) (j -# 1#) s } } } }
  | otherwise = s



fill :: MutableByteArray# s -> Int# -> State# s -> State# s
fill arr n = go 0#
 where
 go i s
   | i <# n    = case writeIntArray# arr i i s of { s ->
                 go (i +# 1#) s }
   | otherwise = s
{-# INLINE fill #-}

bench :: Int -> Int -> ST s ()
bench (I# k) (I# n) = ST go
 where
 go s = case sizeOf (0 :: Int)        of { I# w ->
        case newByteArray# (n *# w) s of { (# s, arr #) ->
        case fill arr n s             of { s ->
        go' arr k s } } }
 go' arr 0# s = (# s, () #)
 go' arr k  s = case reverse arr 0# (n -# 1#) s of { s ->
                go' arr (k -# 1#) s }


main = do (k:n:_) <- map read `fmap` getArgs
          stToIO (bench k n)
          putStrLn "Done."