vlq/src/index.js

Generate, and decode, base64 VLQ mappings for source maps and other uses

/** @type {Record<string, number>} */
let char_to_integer = {};

/** @type {Record<number, string>} */
let integer_to_char = {};

'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/='
	.split('')
	.forEach(function (char, i) {
		char_to_integer[char] = i;
		integer_to_char[i] = char;
	});

/** @param {string} string */
export function decode(string) {
	/** @type {number[]} */
	let result = [];

	let shift = 0;
	let value = 0;

	for (let i = 0; i < string.length; i += 1) {
		let integer = char_to_integer[string[i]];

		if (integer === undefined) {
			throw new Error('Invalid character (' + string[i] + ')');
		}

		const has_continuation_bit = integer & 32;

		integer &= 31;
		value += integer << shift;

		if (has_continuation_bit) {
			shift += 5;
		} else {
			const should_negate = value & 1;
			value >>>= 1;

			if (should_negate) {
				result.push(value === 0 ? -0x80000000 : -value);
			} else {
				result.push(value);
			}

			// reset
			value = shift = 0;
		}
	}

	return result;
}

/** @param {number | number[]} value */
export function encode(value) {
	if (typeof value === 'number') {
		return encode_integer(value);
	}

	let result = '';
	for (let i = 0; i < value.length; i += 1) {
		result += encode_integer(value[i]);
	}

	return result;
}

/** @param {number} num */
function encode_integer(num) {
	let result = '';

	if (num < 0) {
		num = (-num << 1) | 1;
	} else {
		num <<= 1;
	}

	do {
		let clamped = num & 31;
		num >>>= 5;

		if (num > 0) {
			clamped |= 32;
		}

		result += integer_to_char[clamped];
	} while (num > 0);

	return result;
}