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