leven/index.js

const array = [];
const characterCodeCache = [];

export default function leven(first, second) {
	if (first === second) {
		return 0;
	}

	const swap = first;

	// Swapping the strings if `a` is longer than `b` so we know which one is the
	// shortest & which one is the longest
	if (first.length > second.length) {
		first = second;
		second = swap;
	}

	let firstLength = first.length;
	let secondLength = second.length;

	// Performing suffix trimming:
	// We can linearly drop suffix common to both strings since they
	// don't increase distance at all
	// Note: `~-` is the bitwise way to perform a `- 1` operation
	while (firstLength > 0 && (first.charCodeAt(~-firstLength) === second.charCodeAt(~-secondLength))) {
		firstLength--;
		secondLength--;
	}

	// Performing prefix trimming
	// We can linearly drop prefix common to both strings since they
	// don't increase distance at all
	let start = 0;

	while (start < firstLength && (first.charCodeAt(start) === second.charCodeAt(start))) {
		start++;
	}

	firstLength -= start;
	secondLength -= start;

	if (firstLength === 0) {
		return secondLength;
	}

	let bCharacterCode;
	let result;
	let temporary;
	let temporary2;
	let index = 0;
	let index2 = 0;

	while (index < firstLength) {
		characterCodeCache[index] = first.charCodeAt(start + index);
		array[index] = ++index;
	}

	while (index2 < secondLength) {
		bCharacterCode = second.charCodeAt(start + index2);
		temporary = index2++;
		result = index2;

		for (index = 0; index < firstLength; index++) {
			temporary2 = bCharacterCode === characterCodeCache[index] ? temporary : temporary + 1;
			temporary = array[index];
			// eslint-disable-next-line no-multi-assign
			result = array[index] = temporary > result ? (temporary2 > result ? result + 1 : temporary2) : (temporary2 > temporary ? temporary + 1 : temporary2);
		}
	}

	return result;
}