Random access gzip stream

FWIW: I've developed a command line tool upon zlib's zran.c source code which can do random access to gzip with the creation of indexes for gzip files: https://github.com/circulosmeos/gztool

It can even create an index for a still-growing gzip file (for example a log created by rsyslog directly in gzip format) thus reducing in the practice to zero the time of index creation. See the -S (Supervise) option.

The BGZF file format, compatible with GZIP was developped by the biologists.

(...) The advantage of BGZF over conventional gzip is that BGZF allows for seeking without having to scan through the entire file up to the position being sought.

In http://picard.svn.sourceforge.net/viewvc/picard/trunk/src/java/net/sf/samtools/util/ , have a look at BlockCompressedOutputStream and BlockCompressedInputStream.java

Have a look at this link (C code example).

/* zran.c -- example of zlib/gzip stream indexing and random access
...

Gzip is just zlib with an envelope.