1
var SnapshotArray = function (length) {
2
  this.snaps = [];
3
  this.currentIndex = 0;
4
  this.currentSnaps = [];
5
};
6

7
SnapshotArray.prototype.set = function (index, val) {
8
  this.currentSnaps[index] = val;
9
};
10

11
SnapshotArray.prototype.snap = function () {
12
  this.snaps[this.currentIndex] = [...this.currentSnaps];
13

14
  return this.currentIndex++;
15
};
16

17
SnapshotArray.prototype.get = function (index, snap_id) {
18
  const res = this.snaps[snap_id];
19

20
  if (res[index] === undefined) return 0;
21

22
  return res[index];
23
};