[pcwalton]

Can a XOR-only decompressor do the standard LZ77 trick of encoding many repeated bytes with a copy operation that refers to not-yet-decompressed data? It seems to me that you'd have to have a lot of 0 patch bytes for that. Huffman coding could encode all the 0 bytes in 1 bit each, but that still loses over regular LZ77. It seems to me that you still might want to keep "COPY" around for RLE-style data.

[DannyBee]

yes, since that is about how you handle the offsets most of the time. At most you just have an instruction for source side, and one for target side. I will say however, this is useful for files, but not for deltas, as it makes delta composability much more complicated, and also makes streaming harder.

(IE with only original-source copies, and a->delta1->delta2->delta3->b, composing delta1/2/3 prior to applying them is easy and simple to reason about. If you allow target side copies, it is a lot messier)

Way back in the day, when i upgraded subversion's delta format, i did a lot of work on testing out various mechanisms - in practice, target side copies were much worse, and much more expensive to process, than doing source-only copies and then zlib'ing the delta instructions + new data ;)