molecules-to-atoms.js

molecules-to-atoms.js

// codewars exercise
const str = 'As2{Be4C5[BCo3(CO2)3]2}4Cu5';

function parseMolecule(formula) {
	const BRACKET_OPENER_REGEXP = /\[|\{|\(/;
	const BRACKET_CLOSER_REGEXP = /\]|\}|\)/;
	const VALID_ATOM_REGEXP = /^[A-Z][a-z]?$/;
	const MULTIPLIER_REGEXP = /^\d+/;

	function createGroup(parent = {}) {
		return {atoms : {}, groups : [], multiplier : 1, parent};
	}

	function extractGroups(str) {
		const collection = createGroup();
		let currentGroup = collection;

		for (let i = 0; i < str.length; i++) {
			const currentChar = str[i];
			let isCloser = false;
			let atom;
			let multiplier = 1;

			if (BRACKET_OPENER_REGEXP.test(currentChar)) {
				// move down one level
				const parentGroup = currentGroup;
				currentGroup = createGroup(parentGroup);
				parentGroup.groups.push(currentGroup);

				continue;
			}
			else if (BRACKET_CLOSER_REGEXP.test(currentChar)) {
				// move up one level
				isCloser = true;
			}
			else if (VALID_ATOM_REGEXP.test(currentChar)) {
				const currentChar = str[i];
				const extendedChars = currentChar + str[i + 1];
				atom = currentChar;

				if (VALID_ATOM_REGEXP.test(extendedChars)) {
					atom = extendedChars;
					i++;
				}
			}

			// look at following chars for multiplier
			const multiplierMatch = str.slice(i + 1).match(MULTIPLIER_REGEXP);

			if (multiplierMatch) {
				const multiplierStr = multiplierMatch[0];
				multiplier = +multiplierStr;
				i += multiplierStr.length;
			}

			if (isCloser) {
				currentGroup.multiplier = multiplier;
				currentGroup = currentGroup.parent;
			}
			else {
				const currentAtomCount = currentGroup.atoms[atom] || 0;
				currentGroup.atoms[atom] = currentAtomCount + multiplier;
			}
		}

		return collection;
	}

	function sumGroup(group, cumulatedMultiplier = 1, acc = {}) {
		const {groups, multiplier, atoms} = group;

		cumulatedMultiplier *= multiplier;

		for (let i = 0; i < groups.length; i++) {
			sumGroup(groups[i], cumulatedMultiplier, acc);
		}

		// Object.entries cannot be used yet on codewars :(
		Object.keys(atoms).forEach(atom => {
			const prevCount = acc[atom] || 0;
			const count = atoms[atom];

			acc[atom] = prevCount + (count * cumulatedMultiplier);
		});

		return acc;
	}

	const collection = extractGroups(formula);
	return sumGroup(collection);
}