Can't memoize segments for sorting

[adi518]

I have to memoize a sorted segments array. To do that efficiently, I need a clear indicator for when the array has updated, which means I need a timestamp field that gets updated each time a segment is added or removed to/from the array. I'd like to hear other ideas as well.

Meanwhile, I hacked it like so:

const originalAdd = peaksInstance.segments.add;
peaksInstance.segments.add = function (segments: SegmentAddOptions | SegmentAddOptions[]) {
  this.timestamp = Date.now();
  originalAdd.call(this, segments);
};

const originalRemove = peaksInstance.segments.remove;
peaksInstance.segments.remove = function (segment: Segment) {
  this.timestamp = Date.now();
  return originalRemove.call(this, segment);
};

And now I can do this:

const getPeaksSortedSegmentsMemo = memoizee((_timestamp: number, segments: Segments) =>
  sortBy((segment) => segment.startTime, segments)
);

function getPeaksSortedSegments() {
  const segments = getPeaks('main')?.segments.getSegments();
  const timestamp = getPeaks('main')?.segments.timestamp!;
  if (segments?.length) {
    return getPeaksSortedSegmentsMemo(timestamp, segments);
  } else {
    return segments;
  }
}

[chrisn]

Would you also want to update the timestamp if any of the segments properties are changed, e.g., startTime and endTime. Another possibility could be to use the segments.add, segments.remove, segments.update, segments.dragged events.

[adi518]

Supposedly, you're right and it'd be prone to bugs otherwise. However, I just tested it with dragging some segments without adding or removing any and it still works as expected, so I guess the segments array still gets called with add/remove. Actually, how does Peaks maintain its array of segments? does it do it linearly?

[chrisn]

They're just stored internally in an array in the order that you add them. Dragging a segment doesn't change it's position in the array.

[adi518]

Closing since it didn't really matter at the end of it. We manage a memoized and sorted source of truth which peaks reads from, so it's all good.