Could we have an undoCompose function? For calculating the reverse of a delta when composed with a base delta.

[jaredly]

E.g. this would make undo simpler / faster.

The quilljs history module has a guessUndoDelta, which can only guess the undo of an insert, understandably -- you need the base delta to be able to calculate the undo of a delete, for example.

So I'm envisioning a delta.undoCompose(baseDelta) such that the following holds true for any delta:

const undo = delta.undoCompose(baseDelta);
deepEqual(baseDelta.compose(delta).compose(undo), baseDelta)

[jhchen]

Quill's clipboard module deliberately does not want to store all baseDeltas (which would be the entire document). Also its clipboard deals with insert-only deltas which can use diff() if one is willing to store all baseDeltas. Are there other use cases you have in mind?

[jaredly]

Right, you wouldn't be storing the baseDeltas. I'm envisioning this as a better way to do diff() that is aware of what the delta was that is making the change.
So instead of

const undoDelta = baseDelta.compose(delta).diff(baseDelta);

you can do

const undoDelta = delta.undoCompose(baseDelta)

[jhchen]

I see what you mean now. baseDelta needs to be an insert-only delta I believe but delta can be an arbitrary delta in your examples. I can see its use for undo and could replace guessUndoDelta. Are there other use cases though? If not it should arguably belong in Quill instead of here. Either way is this something you'd be willing to contribute a PR to?

[jaredly]

Yup, I'd definitely be willing to contribute a PR. I'd much rather have it live here, because quill is tied to the dom, and this undoCompose would be data-only

[jhchen]

Great thanks! Let me know when there is something to look at or if you have questions getting started.

[jhchen]

Hey is this something you are still planning on working on? We were planning on adding this soon if not.

[jaredly]

I haven't gotten to this yet, it's a little farther down on my priority list