Zlib support

[pgaskin]

• Figure out issues with headers (see below)
• Test zlib extraction and replacement
• Implement zlib in patchlib
• Decide on zlib replacement instructions
• Test with existing patches
• Figure out cgo cross-compiling (probably something like what I did with seriesmeta), biggest issue will be for macOS (will need to compile osxcross)

[pgaskin]

The hard part about this is figuring out a consistent and concise way to read and patch the zlib strings.

[pgaskin]

So far, I have this: https://gist.github.com/geek1011/55ca3c4e750aa1bcd497cf296ceb8cf0 , which works for extracting the css. The biggest issue is that when recompressing it, the go zlib library uses a different dict (0x8c instead of 0x8d). Once I can get it to use the proper dict, I'll be able to add it to kobopatch and implement find_zlib, replace_zlib and replace_css instructions.

[pgaskin]

Some notes I've made for myself:

kobo      : 1111000 10011100 10001101 10010100 1011111 1101111
            78      9c       8d       94       5f      6f
czlib (go): 1111000 10011100 10001100 10010100 1011111 1101111
            78      9c       8c       94       5f      6f
go stdlib is completely wrong, so I'm using the czlib library instead.
            CMF---- FLG----- DICTID --------------------------

CMF:
 - Deflate compression

FLG:
 - Uses FDICT
 - Default compression (level 6)

DICTID:
 - TODO: figure out what the dictionary is (inconsistent between czlib and kobo).
         if it comes to it, I might just fork out to python, which uses the correct dict

References:
 - https://stackoverflow.com/questions/9050260/what-does-a-zlib-header-look-like
 - https://tools.ietf.org/html/rfc1950

[pgaskin]

I think I might just end up embedding (if I can get it to cross-compile to a static binary or execing python (at least for now). Figuring out the compression issue is getting to be a bit too much work. The other alternative might be to use cgo to run a c program which calls zlib.

[pgaskin]

Nevermind, 😠 I missed something obvious again (just like path chars in kepubify)!

Internally, python uses zlib.compressobj for zlib.compress. This uses deflateInit2. When I used czlib, I used czlib.NewWriterLevel, which internally uses deflateInit, which is different. To fix this, all I had to do was call czlib.Compress instead.

Now, I just need to do some more testing, then this feature should be ready!

[pgaskin]

New gist: https://gist.github.com/geek1011/ac32378471be5c0334cff09567fcd057