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

14 Apr 2011

      searchForPattern' (and therefore searchForPattern) will read the entire
file, indeed. To check only the first 'n' chunks in a file you'd have to

1) receive a lazy bytestring, check first n*chuckSize bytes.
2) receive the handle, loop over n chunks.

El jue, 14-04-2011 a las 14:50 +0200, Daniel Fischer escribió:
...
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,
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Re: [Haskell-beginners] please critique my first stab at systems programming

MAN