Had a shower-thought this morning: what if we change the result produced by IGListDiff(...) to be a generic result object with an array of operations (delete/insert/reload/move)?

Why?

There are currently too many things doing essentially the same stuff:

• IGListIndexSetResult and IGListIndexPathResult
  • NSIndexSet or NSArray<NSIndexPath *>
  • Have multiple delete, insert, update, and moves
  • Keep a map tracking from/to for identifiers
• IGListMoveIndex and IGListMoveIndexPath
  • NSInteger or NSIndexPath
  • Has a to/from
• IGListReloadIndexPath
  • NSIndexPath
  • Has a to/from

There's a lot of duplication going on here. I think we can clean it up:

Proposal

Create a generic "operation" object:

@interface IGListOperation<ObjectType>: NSObject
@property (nonatomic, strong, readonly) ObjectType from;
@property (nonatomic, strong, readonly) ObjectType to;
@property (nonatomic, assign, readonly) IGListOperationType type; // delete, insert, move, update
@end

Then we create a new result object:

@interface IGListDiffResult<ObjectType>: NSObject
@property (nonatomic, strong, readonly) NSArray<IGListOperation<ObjectType> *> *operations;
@end

I'd also love to restore diff statistics (like clocking the time it takes to diff). We used to collect this data.

We could also add back APIs like -[IGListDiffResult inserts] that just query an internal map of operation types to operations. Might be convenient?

Considerations

• We merge diff results with other coalesced operations. This still has to happen.
• Operations should implement isEqual: and hash so we can add and unique them in NSSets

Questions

• Can we intermix NSIndexPath and NSInteger with generic types?
  • I don't think so, and if even if we could, I don't think we could right a proper hash and isEqual for the operation
• Is unpacking NSNumber with integerValue when enumerating operations a concern at all? The perf overhead should be minuscule, but it does exist