Function interpolate

packages/svelte/tests/animation-helpers.js:170–197

* @param {string} a * @param {string} b * @param {number} p

(a, b, p)

 * @param {number} p
 */
function interpolate(a, b, p) {
	if (a === b) return a;

	const fallback = p < 0.5 ? a : b;

	const a_match = a.match(/[\d.]+|[^\d.]+/g);
	const b_match = b.match(/[\d.]+|[^\d.]+/g);

	if (!a_match || !b_match) return fallback;
	if (a_match.length !== b_match.length) return fallback;

	let result = '';

	for (let i = 0; i < a_match.length; i += 2) {
		const a_num = parseFloat(a_match[i]);
		const b_num = parseFloat(b_match[i]);
		result += a_num + (b_num - a_num) * p;

		if (a_match[i + 1] !== b_match[i + 1]) {
			// bail
			return fallback;
		}

		result += a_match[i + 1] ?? '';
	}

	return result;
}

/**
 * @param {Keyframe[]} keyframes