textutils.Change.end is confusing

[Akuli]

Bugs with handling change events are difficult to reproduce, so it's important that using them correctly is easy. Porcupine has a dataclass textutils.Change that contains:

    # These should be Tuple[int, int], but they can't be because converting to
    # json and back turns tuples to lists
    start: List[int]
    end: List[int]
    old_text_len: int
    new_text: str

Let's say you replace hello with hi on line 7 with a 4-space indentation before it:

• old_text_len is 5
• new_text is "hi"
• start is [7, 4]
• What is end? Is it the "old end" [7, 9] (after spaces and hello) or the "new end" [7, 6] (after spaces and hi)?

Turns out that end is the "old end". It kinda makes sense: the "new end" can be computed from start and new_text, but the "old end" can't when this edit erases text from multiple lines.

Instead of just renaming the field to old_end, I think I should have two end fields, old_end and new_end. I usually dislike redundant fields because it's easy to update one and forget to update the other, but that doesn't really apply here because Changes are not meant to be mutated.