[Haskell-beginners] Calculating SHA1 hashes?

[Stephen Blackheath [to Haskell-Beginners]]

Tom,

The way to lazily produce [Word8] from a file is to unpack a lazy
ByteString, and you're right - it will generally be slow.  I don't know
of any obvious accepted way to do it.  The fast way is to work with
ByteStrings.  The update' function in hopenssl almost does what you
want:  You could unwrap your ByteStrings into Ptr Word8 using code like
this:

import qualified Data.ByteString.Internal as BI
import Foreign.ForeignPtr

...

        let (bsFPtr, bsOffset, bsLength) = BI.toForeignPtr bs
        withForeignPtr bsFPtr $ \bsPtr_ -> do
            let bsPtr = bsPtr_ `plusPtr` bsOffset
            update' digest (bsPtr, fromIntegral bsLength)

I should also mention that lazy I/O has problems - it temporarily leaks
file handles, and doesn't handle errors correctly.  It is generally
better read blocks using hGet from Data.ByteString.


Steve

Tom Tobin wrote:
> I'm trying to figure out the best way to calculate SHA1 hashes for
> files using Haskell.  There are several libraries I tried from
> Hackage, but they all seem to end up computing the hash far too slowly
> vs. OpenSSL.  Some of the libraries seem to be FFI bindings to
> OpenSSL, but I haven't figured out how to efficiently feed these.
> (For instance, hopenssl consumes a [Word8], but I haven't figured out
> how to lazily produce that from a file; using "unpack" on a ByteString
> seems to end up much too slow.)  I also tried mimicking nano-md5 by
> writing a "nano-sha1" that was largely a "replace the name of the
> digest function" hack job, but I couldn't manage to get that working.
> 
> Is there a particular accepted way to generate hashes that I'm missing?
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