It is important to note that the IR here isn't exactly generic: it embeds the assumption of an LZ77 compressor. Now LZ77 compressors have the advantage of bein

It is important to note that the IR here isn't exactly generic: it embeds the assumption of an LZ77 compressor. Now LZ77 compressors have the advantage of being really popular, and many of the leading compressors at various points on the space/time frontier use it, but there a large number of other interesting compression algorithms as well, which this IR cuts out. A big downside of LZ77 is that it is simple enough that we had optimal parsing for it a long time ago, and it isn't likely to be the subject of any big breakthough in the future: much of the interesting work will probably happen in non-LZ77 style compressors. One of the interesting areas for LZ77, however, is coding-sensitive-parsing: i.e., basing your match-finding decisions on the actual code lengths to represent specific matches, literals runs, etc. Since these code lengths are usually dynamic, set by the entropy coding backend, you often want a back-and-forth type of approach between the two components: either multiple passes or two-way feedback. The approach of generating IR once and then feeding it into the DivANS entropy coding backend seems to preclude this.