module.exports = {
	compareTwoStrings,
	findBestMatch
};

function compareTwoStrings(first, second) {
	first = first.replace(/\s+/g, '')
	second = second.replace(/\s+/g, '')

	if (!first.length && !second.length) return 1;                   // if both are empty strings
	if (!first.length || !second.length) return 0;                   // if only one is empty string
	if (first === second) return 1;       							 // identical
	if (first.length === 1 && second.length === 1) return 0;         // both are 1-letter strings
	if (first.length < 2 || second.length < 2) return 0;			 // if either is a 1-letter string

	let firstBigrams = new Map();
	for (let i = 0; i < first.length - 1; i++) {
		const bigram = first.substr(i, 2);
		const count = firstBigrams.has(bigram)
			? firstBigrams.get(bigram) + 1
			: 1;

		firstBigrams.set(bigram, count);
	};

	let intersectionSize = 0;
	for (let i = 0; i < second.length - 1; i++) {
		const bigram = second.substr(i, 2);
		const count = firstBigrams.has(bigram)
			? firstBigrams.get(bigram)
			: 0;

		if (count > 0) {
			firstBigrams.set(bigram, count - 1);
			intersectionSize++;
		}
	}

	return (2.0 * intersectionSize) / (first.length + second.length - 2);
}

function findBestMatch(mainString, targetStrings) {
	if (!areArgsValid(mainString, targetStrings)) throw new Error('Bad arguments: First argument should be a string, second should be an array of strings');
	
	const ratings = [];
	let bestMatchIndex = 0;

	for (let i = 0; i < targetStrings.length; i++) {
		const currentTargetString = targetStrings[i];
		const currentRating = compareTwoStrings(mainString, currentTargetString)
		ratings.push({target: currentTargetString, rating: currentRating})
		if (currentRating > ratings[bestMatchIndex].rating) {
			bestMatchIndex = i
		}
	}
	
	
	const bestMatch = ratings[bestMatchIndex]
	
	return { ratings, bestMatch, bestMatchIndex };
}

function flattenDeep(arr) {
	return Array.isArray(arr) ? arr.reduce((a, b) => a.concat(flattenDeep(b)), []) : [arr];
}

function areArgsValid(mainString, targetStrings) {
	if (typeof mainString !== 'string') return false;
	if (!Array.isArray(targetStrings)) return false;
	if (!targetStrings.length) return false;
	if (targetStrings.find(s => typeof s !== 'string')) return false;
	return true;
}

function letterPairs(str) {
	const pairs = [];
	for (let i = 0, max = str.length - 1; i < max; i++) pairs[i] = str.substring(i, i + 2);
	return pairs;
}

function wordLetterPairs(str) {
	const pairs = str.toUpperCase().split(' ').map(letterPairs);
	return flattenDeep(pairs);
}