The biggest issue with this approach is that it's bit-aligned, and the alignment is determined by fields within the data. It's hard to make this run fast on mos

The biggest issue with this approach is that it's bit-aligned, and the alignment is determined by fields within the data. It's hard to make this run fast on most CPUs; a typical implementation ends up having to maintain a bit buffer, and shifts bits in/out of it a couple times for every codeword. I bet you could improve the performance of this algorithm significantly, without harming its compression too badly, by one or both of: A) Adjusting the sizes of some bit fields to make them evenly divide the system word size, e.g. requiring the run length of the 11-prefix to be a multiple of 4, so that it can be encoded in 4 bits instead of 6. (This is a net space savings of half a bit; you have to include an average of 1.5 more bits of nonzero data, but the length is two bits shorter.) B) Writing the streams of bits for e.g. "mode", "number of leading zeroes", and "bit subsequences" separately for each block of compressed data, rather than combining them into a single stream. As a bonus, each of those streams will now be more amenable to compression by generic compressors.