The indexed_gzip project is a Python extension which aims to provide a drop-in
replacement for the built-in Python gzip.GzipFile class, the IndexedGzipFile.
indexed_gzip was written to allow fast random access of compressed NIFTI image
files (for which GZIP is the de-facto compression standard), but will work with
any GZIP file. indexed_gzip is easy to use with nibabel.
The standard gzip.GzipFile class exposes a random access-like interface (via
its seek and read methods), but every time you seek to a new point in the
uncompressed data stream, the GzipFile instance has to start decompressing from
the beginning of the file, until it reaches the requested location.
An IndexedGzipFile instance gets around this performance limitation by building
an index, which contains *seek points*, mappings between corresponding
locations in the compressed and uncompressed data streams. Each seek point is
accompanied by a chunk (32KB) of uncompressed data which is used to initialize
the decompression algorithm, allowing us to start reading from any seek point.
If the index is built with a seek point spacing of 1MB, we only have to
decompress (on average) 512KB of data to read from any location in the file.
