Re: [Haskell-beginners] please critique my first stab at systems programming

On Thursday 14 April 2011 14:30:59, Felipe Almeida Lessa wrote:

On Thu, Apr 14, 2011 at 8:50 AM, Daniel Fischer

wrote:

...

$ cabal install stringsearch -- faster searches for a pattern than provided by bytestring

import qualified Data.ByteString.Char8 as BC import qualified Data.ByteString.Lazy as L import qualified Data.ByteString.Lazy.Search as L

blockSize = 512

main = do stuff <- L.readFile "blocks" case L.indices (BC.pack "PART") stuff of [] -> putStrLn "Not Found." (i:_) -> putStrLn $ "Found at " ++ show (i `quot` blockSize) ++ "." putStrLn "Done."

This solves a different problem. The original looked for PART only on the beginning of each block,

Well, I would have expected that, but

searchForPattern' index handle pat = do

eof <- hIsEOF handle if eof then return Nothing

else do

bytes <- B.hGet handle chunkSize case BC.breakSubstring pat bytes of

(x, y) | BC.null y -> searchForPattern' (index + 1) handle pat

| otherwise -> return (Just index)

searches the entire block and checks whether the second component is empty to see whether there's any match at all. So I thought that'd be the desired behaviour. If one wants to only look at the start of the blocks, isPrefixOf would be a much more efficient way than breakSubstring. It would probably be still more efficient to read larger chunks than 512 bytes and step through them with 512-byte steps to check for a "PART" there (unless the "PART" appears in the first few blocks).

while your solution will find everything that has PART in it. Because this is I/O bound, probably the real time taken won't be very different, but the program will surely use more CPU time and report more false positives.

Cheers,