const re = /^([0-9a-f]{8})-([0-9a-f]{4})-7[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}$/i;

/**
 * Convert a byte array to an hexadecimal string.
 */
function toHexString(a) {
    return Array.from(a, b => b.toString(16).padStart(2, '0')).join('');
}

/**
 * Generate a UUIDv7 (Time-Sortable Identifier).
 * @returns {string}
 * The generated UUIDv7 (`00000000-0000-0000-0000-000000000000`).
 * @example
 * let id; console.log(id = generateId());
 * console.log(new Date(validateId(id)).toLocaleString());
 */
export function generateId() {
    const timestamp = Date.now();

    // Convert the timestamp to a 6-byte hexadecimal string.
    const timestampHex = timestamp.toString(16).padStart(12, '0');

    // Generate 10 random bytes for the remaining parts.
    const randomBytes = new Uint8Array(10);
    crypto.getRandomValues(randomBytes);

    const randomPart1 = randomBytes.subarray(0, 2);
    const randomPart2 = randomBytes.subarray(2, 4);
    const randomPart3 = randomBytes.subarray(4);

    randomPart1[0] = (randomPart1[0] & 0x0F) | 0x70; // version
    randomPart2[0] = (randomPart2[0] & 0x3F) | 0x80; // variant

    return [
        timestampHex.slice(0, 8), // first 4 bytes of timestamp
        timestampHex.slice(8, 12), // last 2 bytes of timestamp
        toHexString(randomPart1), // version 7 (2 bytes)
        toHexString(randomPart2), // variant+random (2 bytes)
        toHexString(randomPart3), // remaining 6 bytes
    ].join('-'); // 00000000-0000-0000-0000-000000000000
}

/**
 * Validate a UUIDv7 (Time-Sortable Identifier).
 * @param {string} id
 * @returns {number}
 * The Unix timestamp encoded in the UUIDv7.
 */
export function validateId(id) {
    return parseInt(id.match(re).slice(1, 3).join(''), 16);
}