zipmanip 0.1.0a1

Rewrite zip files, modifying compression type and parameters.

zipmanip

This is a command line utility that rewrites a zip file. It attempts to leave the archive contents metadata and ordering as unchanged as is possible, but recompresses or decompresses the contents.

This was written to use as a git smudge/clean filter to use when using git to keep a history of zip files. (See below for more on that.)

It may be useful in other cases as well.

Usage with Git

Zipmanip can be used as a clean/smudge filter with git so that zip archives are stored uncompressed in the git index.

(The motivation is that if the zip contents are not compressed, git should be able to more efficiently pack the deltas between revisions.)

To set this up:

git config filter.zipmanip.clean "zipmanip --compression-method=store"
git config filter.zipmanip.smudge "zipmanip -9"
# optionally, for diff formatting
git config diff.unzip.textconv "unzip -c -a"

Then, edit .gitattributes to set the filter=zipmanip (and, optionally diff=unzip) on any zip files that you want to store uncompressed. E.g.

*.FCStd binary filter=zipmanip diff=unzip
*.3mf binary filter=zipmanip diff=unzip
*.amf binary filter=zipmanip diff=unzip

Bugs

On Round-trip Idempotentency

Currently if a zip archive is round tripped — converted to uncompressed, then recompressed — the result will not be byte-wise identical to the original. This is due to (at least) a couple of issues.

Differing compression algorithm and parameters

It may be possible to improve this situation, at least partially, by storing information on the original compression type in the uncompressed archives.

Note that data on compression level may be available from bits 1, 2 and possibly 4 of the ZipInfo.flags. (See section 4.4.4 of PKZIP Application Note.)

Differing use of "zip64" extended header

Also the use of "extended local header" is not preserved. (This manifests in .3mf files written by PrusaSlicer. PrusaSlicer always writes extended headers. (This, I think could be fixed with appropriate use of the force_zip64 parameters to ZipFile.open.)

Author

Jeff Dairiki [e-mail]