Items that do not move show as being moved

[Chandlerdea]

I am seeing some weird behavior when diffing arrays which have contents that do not move. Here is the scenario:

I have two arrays, which look like this:

Old array: ([Int]) $R90 = 20 values {
  [0] = 63
  [1] = 5
  [2] = 112
  [3] = 48
  [4] = 100006
  [5] = 121
  [6] = 33
  [7] = 65
  [8] = 100003
  [9] = 57
  [10] = 100007
  [11] = 117
  [12] = 108
  [13] = 1
  [14] = 118
  [15] = 100688
  [16] = 18
  [17] = 54
  [18] = 116
  [19] = 110
}
New array: ([Int]) $R91 = 20 values {
  [0] = 65
  [1] = 118
  [2] = 110
  [3] = 117
  [4] = 100006
  [5] = 112
  [6] = 1
  [7] = 54
  [8] = 116
  [9] = 100688
  [10] = 48
  [11] = 57
  [12] = 108
  [13] = 63
  [14] = 121
  [15] = 33
  [16] = 18
  [17] = 5
  [18] = 100003
  [19] = 100007
}

One of the patches returned the extendedPatch() function looks like this:

 [13] = move {
    move = (from = 20, to = 16)
  }

This is causing a crash in my app, because there is no item at index 20 in the array. You can see that the object at index 16 doesn't move, but for some reason the library is saying that it has moved.

[tonyarnold]

Would you mind showing me the code you're using to diff the arrays? A reproduction in a Playground would be a huge help if you have time.

[georgenavarro]

Hi,

I am seeing the same issue. I can reproduce simply by using this code:

var existingIds = [63, 5,   112, 48,  100006, 121, 33, 65, 100003, 57,     100007, 117, 108, 1,  118, 100688, 18, 54, 116,    110]
let incomingIds = [65, 118, 110, 117, 100006, 112, 1,  54, 116,    100688, 48,     57,  108, 63, 121, 33,     18, 5,  100003, 100007]
let idPatches = extendedPatch(from: existingIds, to: incomingIds)

These are the patches produced:

[M(0,4), M(0,7), M(6,0), M(14,1), M(19,2), M(13,3), M(7,4), M(15,6), M(18,7), M(19,8), M(18,9), M(16,11), M(19,12), M(20,16)]

Like @Chandlerdea says, the last patch would cause a crash due to index out of bounds.

[tonyarnold]

Thanks for the example, @georgenavarro - I appreciate it. I'm pretty busy at the moment, so I'm not sure when I'll have a chance to look at fixing this issue. If either of you gets a chance in the meantime, a PR with tests showing and/or fixing the problem are always welcome.