From: Paul Duncan <pabs@...>
Date: 2003-04-15T02:28:27+09:00
Subject: Re: Code for random access to compressed IO?


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* coma_killen@fastmail.fm (coma_killen@fastmail.fm) wrote:
> Hi,
>=20
> Anyone seen some code for compressed IO streams that allow=20
> random access (arbitrary seek)? Along the lines of=20
> razip.sf.net?

Zlib itself doesn't allow for arbitrary seek.  SEEK_END (aka seeking
relative to the end of the stream) isn't supported, and seeking forward
is basically implemented by reading forward in a tight for() loop.
Here's what zlib.h has to say about gzseek():

ZEXTERN z_off_t ZEXPORT    gzseek OF((gzFile file,
              z_off_t offset, int whence));
/*=20
      Sets the starting position for the next gzread or gzwrite on the
   given compressed file. The offset represents a number of bytes in the
   uncompressed data stream. The whence parameter is defined as in
   lseek(2); the value SEEK_END is not supported.
     If the file is opened for reading, this function is emulated but
   can be extremely slow. If the file is opened for writing, only
   forward seeks are supported; gzseek then compresses a sequence of
   zeroes up to the new starting position.

      gzseek returns the resulting offset location as measured in bytes
   from the beginning of the uncompressed stream, or -1 in case of
   error, in particular if the file is opened for writing and the new
   starting position would be before the current position.
*/

In order to implement truly random access, you'd need to read till the
end of the file, get the length, then rewind the file.  Then you could
emulate SEEK_END by rewinding the file, and gzseek()ing forward
file_len - seek_amt.  This works, but it's slooooooow. =20

> It seems to me that ruby-zlib does not provide this. It
> has GzipReader and Writer but they don't give random access?
>=20
> Any info appreciated,
>=20
> Karsten

--=20
Paul Duncan <pabs@pablotron.org>        pabs in #gah (OPN IRC)
http://www.pablotron.org/               OpenPGP Key ID: 0x82C29562

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