edit: I can't tell if you edited this since I read it, or I just skipped the last few paragraphs outright, my apologies. I'll leave this here for posterity anyway.

To add something amusing but constructive - it turns out that one pass of Brotli is even better than using LZ4+zstd-1 for predicting this, but integrating a compression algorithm _just_ as an early pass filter seemed like overkill. Heh.

It's actually even funnier than that.

It tries LZ4, and if that fails, then it tries zstd-1, and if that fails, then it doesn't try your higher level. If either of the first two succeed, it just jumps to the thing you actually requested.

Because it turns out, just using LZ4 is insanely faster than using zstd-1 is insanely faster than any of the higher levels, and you burn a _lot_ of CPU time on incompressible data if you're wrong; using zstd-1 as a second pass helps you avoid the false positive rate from LZ4 having different compression characteristics sometimes.

Source: I wrote the feature.