/**
 * A UUID generator that can construct unique IDs in a variety of formats.
 *
 * A UUID is generated immediately upon instantiation of this class. The UUID can be retrieved in its normal form
 * (e.g. f111b8c5-ca2f-4a1a-8d0d-a8dd5f37c05f) or as a shortened web-safe form (e.g. jp64hwPZ-Lh7vY8INQA7ImQPbQE), which
 * is constructed by converting the UUID to Base64 and replacing '/' and '+' with '-' and '_', respectively.
 */
public class Uuid {

	private static final String HEX_PREFIX = '0x';
	private static final String HEX_ALPHABET = '0123456789abcdef';
	private static final String[] HEX_CHARACTERS = HEX_ALPHABET.split('');

	public static final Integer UUID_V4_LENGTH = 36;
	public static final String UUID_V4_REGEX = '[0-9a-f]{8}-[0-9a-f]{4}-4[0-9a-f]{3}-[89ab][0-9a-f]{3}-[0-9a-f]{12}';
	public static final String UUID_SHORT_REGEX = '[0-9a-zA-Z-_]{27}';


	// UUID value in its normal form
	public final String value { get; private set; }

	// UUID value in its web-safe short-form
	public final String shortValue { get; private set; }

	/*
	 * Constructs a new UUID.
	 */
	public Uuid() {
		this.value = generate();
		this.shortValue = getShortValue(this);
	}

	/*
	 * Overrides the toString() method to return the formatted UUID value.
	 * @return Formatted UUID value
	 */
	public override String toString() {
		return value;
	}


	//==================================================================================================================
	// Private
	//==================================================================================================================

	/*
	 * Generates a UUID string according to the UUID v4 spec.
	 * @return A newly generated UUID
	 */
	private String generate() {
		String hexValue = EncodingUtil.convertToHex(Crypto.generateAesKey(128));

		// Version Calculation: (i & 0x0f) | 0x40
		// Version Format: Always begins with 4
		String versionShiftedHexBits = getShiftedHexBits(
			hexValue.substring(14, 16),
			convertHexToInteger('0x0f'),
			convertHexToInteger('0x40')
		);

		// Variant Calculation: (i & 0x3f) | 0x80
		// Variant Format: Always begins with 8, 9, A or B
		String variantShiftedHexBits = getShiftedHexBits(
			hexValue.substring(18, 20),
			convertHexToInteger('0x3f'),
			convertHexToInteger('0x80')
		);

		String uuid = hexValue.substring(0, 8)                          // time-low
			+ '-' + hexValue.substring(8, 12)                           // time-mid
			+ '-' + versionShiftedHexBits + hexValue.substring(14, 16)  // time-high-and-version
			+ '-' + variantShiftedHexBits + hexValue.substring(18, 20)  // clock-seq-and-reserved + clock-seq-low
			+ '-' + hexValue.substring(20);                             // node

		return uuid;
	}

	private String getShiftedHexBits(String hexSubstring, Integer lowerThreshold, Integer upperThreshold) {
		Integer shiftedIntegerBits = (convertHexToInteger(hexSubstring) & lowerThreshold) | upperThreshold;
		return convertIntegerToHex(shiftedIntegerBits);
	}

	/*
	 * Converts a given hexadecimal string to an integer value.
	 * @param hexValue {String} Value to be converted
	 * @return Integer equivalent to the given string
	 */
	private Integer convertHexToInteger(String hexValue) {
		Integer hexBase = HEX_ALPHABET.length();
		hexValue = hexValue.toLowerCase();

		if (hexValue.startsWith(HEX_PREFIX)) {
			hexValue = hexValue.substringAfter(HEX_PREFIX);
		}

		Integer integerValue = 0;
		for (String hexCharacter : hexValue.split('')) {
			Integer hexCharacterIndex = HEX_CHARACTERS.indexOf(hexCharacter);

			integerValue = hexBase * integerValue + hexCharacterIndex;
		}
		return integerValue;
	}

	/*
	 * Converts a given integer to a hexadecimal string.
	 * @param integerValue {Integer} Value to be converted
	 * @return Hexadecimal equivalent to the given integer
	 */
	private String convertIntegerToHex(Integer integerValue) {
		Integer hexBase = HEX_ALPHABET.length();
		String hexValue = '';
		while (integerValue > 0) {
			Integer hexCharacterIndex = Math.mod(integerValue, hexBase);

			hexValue = HEX_CHARACTERS[hexCharacterIndex] + hexValue;
			integerValue = integerValue / hexBase;
		}
		return hexValue;
	}


	//==================================================================================================================
	// Static
	//==================================================================================================================

	/*
	 * Determines whether a given string is a valid UUID.
	 * @param uuid {String} A UUID string
	 * @return True if the given string represents a valid UUID; false otherwise
	 */
	public static Boolean validate(String uuid) {

		// Should not be an empty string
		if (String.isBlank(uuid)) {
			return false;
		}

		// Should be of appropriate length
		if (uuid.length() != UUID_V4_LENGTH) {
			return false;
		}

		// Should match UUID regex
		Pattern uuidPattern = Pattern.compile(UUID_V4_REGEX.toLowerCase());
		Matcher uuidMatcher = uuidPattern.matcher(uuid.toLowerCase());
		return !!uuidMatcher.matches();
	}

	/*
	 * Generates a hashed and base64-encoded representation of the UUID, with '/' and '+' characters will be replaced by '-'
	 * and '_', respectively, to avoid compatibility issues.
	 * @param A UUID to convert
	 * @return UUID value encoded as a base64 string
	 */
	public static String getShortValue(Uuid uuid) {

		// Remove hyphens
		String value = uuid.value.replace('-', '');

		// Hash the UUID and convert to a base-64 string and replace '/' and '+'
		Blob digest = Crypto.generateDigest('SHA1', Blob.valueOf(value));
		String encoded = EncodingUtil.base64Encode(digest);

		// Drop trailing '=' and replace '/' and '+'
		return encoded.substring(0, encoded.length() - 1)
			.replace('/', '-')
			.replace('+', '_');
	}
}