[Haskell-beginners] MD2 Implementation

[Sean Bartell]

I've just recently started learning Haskell, and I wrote an MD2
implementation. This version is correct, but crashes on large files
because of the weak evaluation. What I'm wondering is, what's the sort
of thought-process I would go through to find the problem areas and
fix it?

Also, please post any suggestions you have for this code, for style or
whatever :).

{-# LANGUAGE BangPatterns #-}

module MD2 (md2) where

import qualified Data.ByteString as B
import Data.Bits (xor)
import Data.List (foldl', mapAccumL)
import Data.Word (Word8)

table = B.pack [0x29, 0x2E, 0x43, 0xC9, 0xA2, 0xD8, 0x7C, 0x01,
                0x3D, 0x36, 0x54, 0xA1, 0xEC, 0xF0, 0x06, 0x13,
                0x62, 0xA7, 0x05, 0xF3, 0xC0, 0xC7, 0x73, 0x8C,
                0x98, 0x93, 0x2B, 0xD9, 0xBC, 0x4C, 0x82, 0xCA,
                0x1E, 0x9B, 0x57, 0x3C, 0xFD, 0xD4, 0xE0, 0x16,
                0x67, 0x42, 0x6F, 0x18, 0x8A, 0x17, 0xE5, 0x12,
                0xBE, 0x4E, 0xC4, 0xD6, 0xDA, 0x9E, 0xDE, 0x49,
                0xA0, 0xFB, 0xF5, 0x8E, 0xBB, 0x2F, 0xEE, 0x7A,
                0xA9, 0x68, 0x79, 0x91, 0x15, 0xB2, 0x07, 0x3F,
                0x94, 0xC2, 0x10, 0x89, 0x0B, 0x22, 0x5F, 0x21,
                0x80, 0x7F, 0x5D, 0x9A, 0x5A, 0x90, 0x32, 0x27,
                0x35, 0x3E, 0xCC, 0xE7, 0xBF, 0xF7, 0x97, 0x03,
                0xFF, 0x19, 0x30, 0xB3, 0x48, 0xA5, 0xB5, 0xD1,
                0xD7, 0x5E, 0x92, 0x2A, 0xAC, 0x56, 0xAA, 0xC6,
                0x4F, 0xB8, 0x38, 0xD2, 0x96, 0xA4, 0x7D, 0xB6,
                0x76, 0xFC, 0x6B, 0xE2, 0x9C, 0x74, 0x04, 0xF1,
                0x45, 0x9D, 0x70, 0x59, 0x64, 0x71, 0x87, 0x20,
                0x86, 0x5B, 0xCF, 0x65, 0xE6, 0x2D, 0xA8, 0x02,
                0x1B, 0x60, 0x25, 0xAD, 0xAE, 0xB0, 0xB9, 0xF6,
                0x1C, 0x46, 0x61, 0x69, 0x34, 0x40, 0x7E, 0x0F,
                0x55, 0x47, 0xA3, 0x23, 0xDD, 0x51, 0xAF, 0x3A,
                0xC3, 0x5C, 0xF9, 0xCE, 0xBA, 0xC5, 0xEA, 0x26,
                0x2C, 0x53, 0x0D, 0x6E, 0x85, 0x28, 0x84, 0x09,
                0xD3, 0xDF, 0xCD, 0xF4, 0x41, 0x81, 0x4D, 0x52,
                0x6A, 0xDC, 0x37, 0xC8, 0x6C, 0xC1, 0xAB, 0xFA,
                0x24, 0xE1, 0x7B, 0x08, 0x0C, 0xBD, 0xB1, 0x4A,
                0x78, 0x88, 0x95, 0x8B, 0xE3, 0x63, 0xE8, 0x6D,
                0xE9, 0xCB, 0xD5, 0xFE, 0x3B, 0x00, 0x1D, 0x39,
                0xF2, 0xEF, 0xB7, 0x0E, 0x66, 0x58, 0xD0, 0xE4,
                0xA6, 0x77, 0x72, 0xF8, 0xEB, 0x75, 0x4B, 0x0A,
                0x31, 0x44, 0x50, 0xB4, 0x8F, 0xED, 0x1F, 0x1A,
                0xDB, 0x99, 0x8D, 0x33, 0x9F, 0x11, 0x83, 0x14]

-- NOTE: There's a problematic typo in RFC 1319 in section 3.2. It says
-- Set C[j] to S[c xor L], but it should be Set C[j] to C[j] xor S[c xor L].

type Block = B.ByteString

two x = (x, x)
applySnd f (x, y) = (x, f y)

zeroBlock = B.replicate 16 0
checksumInitial = (0, zeroBlock)

foldlBlocks :: (a -> Block -> a) -> a -> B.ByteString -> a
foldlBlocks f v x | B.null x = v
                  | otherwise = foldlBlocks f v' rest
                        where (block, rest) = B.splitAt 16 x
                              !v' = f v block

checksumBlock :: (Word8, Block) -> Block -> (Word8, Block)
checksumBlock (l, cs) bs = applySnd B.pack $ mapAccumL updateL l $ B.zip cs bs
    where updateL l (c, b) = two $ c `xor` table `B.index`
fromIntegral (l `xor` b)

doBlock :: Block -> Block -> Block
doBlock ds bs = B.take 16 xs'
    where xs = ds `B.append` bs `B.append` B.pack (B.zipWith xor ds bs)
          (_, xs') = foldl' doRound (0, xs) [0..17]
          doRound (t, xs) n = let (t', xs') = B.mapAccumL updateT t xs
                              in (t' + n, xs')
          updateT t b = two $ b `xor` table `B.index` fromIntegral t

doBlockAndCS (csState, blockState) block = (checksumBlock csState
block, doBlock blockState block)

padding n = B.replicate n (fromIntegral n)

md2 :: B.ByteString -> Block
md2 bs = let padded = bs `B.append` padding (16 - B.length bs `mod` 16)
             ((_, cksum), state) = foldlBlocks doBlockAndCS
(checksumInitial, zeroBlock) padded
         in doBlock state cksum