But the preset dic functionality I've seen, in say, LZ4, really is just the saved state of a normal compression run. So once you start compressing, the dictiona

But the preset dic functionality I've seen, in say, LZ4, really is just the saved state of a normal compression run. So once you start compressing, the dictionary eventually evaporates as more data does in and backreferences can no longer point back that far into the dictionary. That's fine if all your content compresses well, but if the dictionary is a far superior state...