Performance: Convert make intervals to select using array slices

[evelynhathaway]

Instead of pushing pixels repeatedly, I wonder if this would be faster: only track an index for the duration of an interval, and then once it no longer matches, push a slice that represents the interval.

[evelynhathaway]

Surprisingly, the refactor alone isn't consistently that much more performant; however this refactor could allow for a small uplift if the intervals were sorted while making them.

Code for push slices

export const makeIntervals = (pixels: Pixels1D, thresholds: Array<Threshold>): Array<Interval> => {
	const intervals: Array<Interval> = [];
	let startingIndex = 0;
	let didMeetThreshold: boolean | undefined;
	for (let index = 0; index < pixels.length; index++) {
		const pixel = pixels[index];
		const meetsThreshold = checkThresholds(pixel, thresholds);
		const isLastPixel = index === pixels.length - 1;
		if (didMeetThreshold == null) didMeetThreshold = meetsThreshold;
		if (meetsThreshold !== didMeetThreshold || isLastPixel) {
			intervals.push({
				sort: didMeetThreshold,
				pixels: pixels.slice(startingIndex, index),
			});
			startingIndex = index;
			didMeetThreshold = meetsThreshold;
		}
	}
	return intervals;
};

Code for push single unsorted

export const makeIntervals = (pixels: Pixels1D, thresholds: Array<Threshold>): Array<Interval> => {
	const intervals: Array<Interval> = [];
	let didMeetThreshold: boolean | undefined;
	for (const pixel of pixels) {
		const meetsThreshold = checkThresholds(pixel, thresholds);
		if (meetsThreshold === didMeetThreshold) {
			intervals.at(-1)?.pixels.push(pixel);
		} else {
			intervals.push({
				sort: meetsThreshold,
				pixels: [pixel],
			});
			didMeetThreshold = meetsThreshold;
		}
	}
	return intervals;
};