Array diffs entirely on order and not contents
antgonzales opened this issue · comments
When I diff ['a', 'b', 'c']
and ['b', 'c']
, the result is:
{
added: {},
deleted: { '2': undefined },
updated: { '0': 'b', '1': 'c' }
}
I would expect the result to be:
{
added: {},
deleted: { '0': undefined },
updated: { '0': 'b', '1': 'c' }
}
I think this is by design.
How should your expected diff be applied to ['a', 'b', 'c']
to get ['b', 'c']
?
If the deleted
property corresponds to the delete
operator, the result is incorrect:
x = ['a', 'b', 'c']
delete x[0] // -> x = [undefined, 'b', 'c']
x[0] = 'b' // -> x = ['b', 'b', 'c']
x[1] = 'c' // -> x = ['b', 'c', 'c']
If the deleted
property corresponds to Array.splice(n, 1)
, the result would depend on the order in which the operations are applied:
-
If
deleted
is applied first, the result is correct but theupdated
part is completely redundant:x = ['a', 'b', 'c'] x.splice(0,1) // -> x = ['b', 'c'] x[0] = 'b' // -> x = ['b', 'c'] x[1] = 'c' // -> x = ['c', 'c']
-
If
updated
is applied first, the result is incorrect:x = ['a', 'b', 'c'] x[0] = 'b' // -> x = ['b', 'b', 'c'] x[1] = 'c' // -> x = ['b', 'c', 'c'] x.splice(0,1) // -> x = ['c', 'c']
The operations specified by the diff currently output by this library would be applied with these operations (which work in any order):
x = ['a', 'b', 'c']
delete x[2] // -> x = ['a', 'b']
x[0] = 'b' // -> x = ['b', 'b']
x[1] = 'c' // -> x = ['b', 'c']
This applies correctly, but it is indeed a lot of work for what could be 1 Array operation. I imagine this is also why diffing large Arrays takes so long (#56).
Making an Array.splice
-based diff for arrays (without the redundant updated
property) would often be shorter than a per-key diff, especially when large arrays are pushed or popped and many keys shift (it is effectively a run length encoding), but it would require special cases both when calculating the diff and when applying it. It would require care with non-index numeric keys like { '0': 'xyz' }
.
Certainly possible to make. Would have to consider if it's a good fit for this library. If my idea for custom comparisons is implemented in the future, plugging in a separate library to do more compact Array diffs when necessary would be easy.