It's been a while since I've made any major changes to LZHAM (except for minor cmake related stuff). This was a codec I wrote over a few nights and weekends while I was also working my day job. I eventually had to let active dev on LZHAM go to sleep because I got "sidetracked" shipping Portal 2. The codec's alpha has already been successfully deployed in several products, such as Planetside 2 and Titanfall, which isn't bad for a few nights of R&D and implementation work.
I covered what I was thinking of doing with LZHAM in this blog post. I have more interest in improving it again. For the types of products I'm now working on, what matters a lot is the title's retention rate, from first starting the product to the customer actually getting into real gameplay. Slow downloads or updates, loading screens, etc. equals lost users. Lost users=lower monetization. We actually measure the retention rate of every aspect of this in the field. So things like background downloading, streaming, proper organization of asset data into Unity asset bundles, and of course good data compression matter massively to us.
Anyhow, some ideas for LZHAM decompression startup and throughput improvements which I can do pretty quickly:
- After much testing on our game data, I now realize I underestimated how useful the various LZMA settings are. Right now LZHAM always uses the upper 3 MSB's of the prev. two literals for literal/delta literal contexts. Allow the user to control all of this: which prev. literal(s) (if any), say up to 8 bytes back, and which bits from those literals, separately for each type of prediction (literals/delta literals).
- In my quest to get LZHAM's ratio up to be similar to LZMA I made several tradeoffs which can greatly impact decompression perf, especially on uncompressible data. Right now the codec must always init and manage 64*2 Huffman tables. Allow the user to reduce or even increase the # of tables.
- LZHAM was designed for "solid" compression, where you give the codec dozens to hundreds of MB's containing many assets, and you don't restart/reinit the codec in between assets. It's like a slow to start drag racer. So it can suck on small files.
I'm not honestly 100% sure what to do about this yet that won't kill decompression perf. The way LZHAM updates Huffman tables seems like an albatross here. Amortized over many MB's it typically works fine, but on small files they can't be updated (adapted) quickly enough. Less tables are probably good here.
I could just integrate something like miniz into the codec, and try using it on each internal compressor block and using whatever is better. But that seems horrible.
- The Huffman table update frequency needs to be better tuned. If I can't think of anything smarter, allow the user to control the update schedule.
Note if you are very serious about fast, high ratio compression and decompression, Rad's Oodle product is very good. Given what I know about it, it's the best (fastest, highest compression, and most scalable/portable) production class lossless codec I know of.