EPUB files are completely uncompressed

[bmaupin]

Environment

• Operating System: Ubuntu 22.04
• node --version: v20.11.0
• npm --version: 10.2.4
• yarn --version, if using Yarn: N/A
• percollate --version: 4.1.1

Description

I just generated an EPUB file and I was surprised how big it is. When I examined it, it appeared that all of the files in the EBUB were uncompressed.

It's been a while since I've dug into the EPUB specification, but from what I remember the only restriction is that the mimetype file must come first. There are no restrictions regarding compression, so the EPUB files generated by this tool are unnecessarily large.

For now as a workaround I'm repackaging the EPUB files myself, e.g.

unzip before.epub -d tmp-epub
cd tmp-epub
zip -X ../after.epub mimetype
zip -rX ../after.epub * -x mimetype
cd ..
rm -rf tmp-epub

Here's a quick idea of the size difference for just one example file after compressing it with standard zip compression:

-rw-rw-r-- 1 user user 559K May  6 13:38 after.epub
-rw-rw-r-- 1 user user 2.4M May  6 10:15 before.epub

And lest I forget to mention it, thank you for this tool! This is exactly what I was looking for.

[danburzo]

Thanks for opening this issue. According to the EPUB 3.3 spec, the entries can be either stored (uncompressed) or Deflate-compressed. In addition, the mimetype file must not be compressed.

I don’t remember the circumstances exactly, but maybe I couldn’t figure out at the time how to achieve this variable compression with archiver.js? In any case, it’s worth revisiting.

[danburzo]

I don’t think there’s anything else to it than that, made tentative release percollate@v4.2.0. I’ll investigate further if these settings indeed offer the best compression, or if there’s something more that can be done.