Undoing patches about binarydist HOT 2 CLOSED

According to Colin Percival (designer of bsdiff), it is not possible:

In case anyone was wondering: bspatch doesn't have a -R option because making bsdiff patches reversible would make them considerably larger.

But if you wanted, instead of producing one patch (from V0.1 to V0.2), you could produce two patches (going in both directions), and then use whichever one you need in a given situation.

I hope this helps!

from binarydist.

P.S. Here's why.

There are two important operations a binary patch does: insert some bytes and delete some bytes. (There can be other operations, but at least we need these two.)

When inserting bytes, the patch needs to tell us the bytes to be inserted, as well as where to put them. For example, if v1 is go west, the patch "insert south at position 3" would produce go southwest as v2.

When deleting bytes, it can save space by saying how many bytes to be deleted, as well as where to delete them, without repeating the deleted bytes themselves. For example, with v2 go southwest, the patch "delete 4 bytes at position 8" would produce go south as v3.

To reverse a patch, we swap "insert" and "delete". For the first patch above, that's easy. We change it to "delete 5 bytes at position 3". Applying this to v2 gives us v1 again. But for the other patch, what do we do? We know it should look like "insert XXXX at position 8" but what are those 4 bytes? If all we have is v3 and the un-reversed patch, we don't know what to put in the XXXX part.

from binarydist.