Objects with duplicate diffIdentifiers but different values break updates

[instagram]

Objects with duplicate diffIdentifiers but different values break updates

[rnystrom]

@jessesquires theoretically yes, but the implementation doesn't enforce it. I think it probably should.

Note though that I think the diffing algorithm should still support duped identifiers.

[rnystrom]

I think we need a system in place to pluck objects with duped diffIdentifiers. There should be asserting when this happens locally.

Recovering from this state is a little trickier because we want to dedupe our objects quickly before entering the diff (I think?). We only have to do this to the toObjects before diffing. Something like:

NSMutableSet *identifiers = [NSMutableSet new];
NSMutableArray *deduped = [NSMutableArray new];
for (id<IGListDiffable> obj in toObjects) {
  if (![identifiers containsObject:[obj diffIdentifier]]) {
    [deduped addObject: obj];
    [identifiers addObject:[obj diffIdentifier]];
  }
}
// then diff w/ array deduped

This adds another O(n) step before diffing, but should be pretty cheap.

[jessesquires]

But isn't diffIdentifier required to be unique across objects?

Seems like this scenario has invalid pre-conditions...