event-of-code-2018-day5-part1.js

/**
 *
 * part 1
 *
 * @param {string} input
 * @returns {number} length of the polymer after all resulting reactions
 */
function alchemicalReduction(input) {
	let lastAnswer = -1;

	while (true) {
		for (let i = 0; i < input.length - 1; ++i) {
			const current = input[i];
			const next = input[i + 1];

			if (canReact(current, next)) {
				/** react - remove the current & next chars */
				input = input.slice(0, i) + input.slice(i + 1 + 1);

				--i;
			}
		}

		if (input.length === lastAnswer) {
			break; /** done */
		}

		lastAnswer = input.length;
	}

	return input.length - 1;
}

/**
 *
 * why `--i`?
 *
 * decrement the index so that the previous char is also checked:
 *
 *   ||
 * "abBAxxx"
 *
 *   ||
 * "aAxxx"
 *
 * --i
 *
 *  ||
 * "aAxxx"
 *
 *
 */

/**
 *
 * @param {string} current
 * @param {string} next
 *
 * @returns boolean
 *
 * returns true if:
 *
 * "a" "A"
 * "A" "a"
 *
 * where "a" and "A" are the `current` & `next` variables
 */
function canReact(current, next) {
	return (
		((isLower(current) && isUpper(next)) || (isUpper(current) && isLower(next))) &&
		current.toLowerCase() === next.toLowerCase()
	);
}

/**
 *
 * @param {string} s
 */
function isLower(s) {
	return s === s.toLowerCase();
}

/**
 *
 * @param {string} s
 */
function isUpper(s) {
	return s === s.toUpperCase();
}