colorjs.io/src/angles.js

Let’s get serious about color

export function constrain (angle) {
	return ((angle % 360) + 360) % 360;
}

export function adjust (arc, angles) {
	if (arc === "raw") {
		return angles;
	}

	let [a1, a2] = angles.map(constrain);

	let angleDiff = a2 - a1;

	if (arc === "increasing") {
		if (angleDiff < 0) {
			a2 += 360;
		}
	}
	else if (arc === "decreasing") {
		if (angleDiff > 0) {
			a1 += 360;
		}
	}
	else if (arc === "longer") {
		if (-180 < angleDiff && angleDiff < 180) {
			if (angleDiff > 0) {
				a2 += 360;
			}
			else {
				a1 += 360;
			}
		}
	}
	else if (arc === "shorter") {
		if (angleDiff > 180) {
			a1 += 360;
		}
		else if (angleDiff < -180) {
			a2 += 360;
		}
	}

	return [a1, a2];
}