Description Emilian Miron 2011-05-19 14:47:48 EDT

When trying to use JGit on Android the platform limits the window size of ZLib deflators to less than the usual 2^15. Therefore, the loose objects written by JGit do not have the signature first byte equal to 0x78 because of the decreased window size.

In class org.eclipse.jgit.storage.file.UnpackedObject the following method checks the signature for loose objects versus packs:

	private static boolean isStandardFormat(final byte[] hdr) {
		// Try to determine if this is a standard format loose object or
		// a pack style loose object. The standard format is completely
		// compressed with zlib so the first byte must be 0x78 (15-bit
		// window size, deflated) and the first 16 bit word must be
		// evenly divisible by 31. Otherwise its a pack style object.
		//
		final int fb = hdr[0] & 0xff;
		return fb == 0x78 && (((fb << 8) | hdr[1] & 0xff) % 31) == 0;
	}


If fb == 0x78 is modified to ((fb & 0x0f) == 0x08), this will only check the CM (compression method) flag in the ZLib header for 0x8 which is DEFLATE and will not restrict the window size which is in the other nibble. 

For reference, this is the specification for Zlib:
http://www.gzip.org/zlib/rfc-zlib.html

Git's format is described here:
http://book.git-scm.com/7_how_git_stores_objects.html
http://book.git-scm.com/7_the_packfile.html

Since pack files start with "PACK", the check above does not yield positive for packs.

Also note that this conforms to Git's storage format guidelines, since there is no mention of an exact window size anywhere.