java - Find the size of the file inside a GZIP file

Question

Is there a way to find out the size of the original file which is inside a GZIP file in java?

As in, I have a file a.txt of 15 MB which has been GZipped to a.gz of size 3MB. I want to know the size of a.txt present inside a.gz, without unzipping a.gz.

Answer 1

There is no truly reliable way, other than gunzipping the stream. You do not need to save the result of the decompression, so you can determine the size by simply reading and decoding the entire file without taking up space with the decompressed result.

There is an unreliable way to determine the uncompressed size, which is to look at the last four bytes of the gzip file, which is the uncompressed length of that entry modulo 2³² in little endian order.

It is unreliable because a) the uncompressed data may be longer than 2³² bytes, and b) the gzip file may consist of multiple gzip streams, in which case you would find the length of only the last of those streams.

If you are in control of the source of the gzip files, you know that they consist of single gzip streams, and you know that they are less than 2³² bytes uncompressed, then and only then can you use those last four bytes with confidence.

pigz (which can be found at http://zlib.net/pigz/ ) can do it both ways. pigz -l will give you the unreliable length very quickly. pigz -lt will decode the entire input and give you the reliable lengths.