// 16 bits, 8 bits for "clk_seq_hi_res",.
				// 8 bits for "clk_seq_low",.
				// two most significant bits holds zero and one for variant DCE1.1.
				wp_rand( 0, 0x3fff ) | 0x8000,
				// 48 bits for "node".
				wp_rand( 0, 0xffff ),
				wp_rand( 0, 0xffff ),
				wp_rand( 0, 0xffff )
			);
		}

		return $this->client_id;
	}

	/**
	 * Check if the specified UUID is valid.
	 *
	 * @link http://php.net/preg_match
	 *
	 * @param string $uuid String.
	 * @return boolean True if value is a valid UUID, false otherwise.
	 */
	public static function is_uuid( $uuid ) {
		$result = preg_match( '#^[0-9A-F]{8}-[0-9A-F]{4}-4[0-9A-F]{3}-[89AB][0-9A-F]{3}-[0-9A-F]{12}$#i', $uuid );

		return 1 === $result;
	}

	/**
	 * Get cookie client ID.
	 *
	 * @return string|null
	 */
	public static function get_cookie_client_id() {
		$client_id = null;

		$ga_cookie = filter_input( INPUT_COOKIE, '_ga', FILTER_SANITIZE_STRING );

		if ( empty( $ga_cookie ) ) {
			// No `_ga` cookie available.
			return $client_id;
		}

		$ga = explode( '.', $ga_cookie );

		if ( isset( $ga[2] ) && self::is_uuid( $ga[2] ) ) {
			// Use UUID from cookie.
			$client_id = $ga[2];
		} elseif ( isset( $ga[2], $ga[3] ) ) {
			// Older Google Client ID.
			$client_id = sprintf( '%s.%s', $ga[2], $ga[3] );
		}

		return $client_id;
	}

	/**
	 * Payment redirect URL.
	 *
	 * @param string  $url     Redirect URL.
	 * @param Payment $payment Payment.
	 * @return string
	 */
	public function payment_redirect_url( $url, Payment $payment ) {
		/**
		 * Payment Gateway Referral Exclusions in Google Analytics.
		 *
		 * @link https://lmgtfy.com/?q=google+analytics+exclude+payment+providers
		 * @link https://webvalue.nl/blog/betaalproviders-uitsluiten-google-analytics
		 * @link https://seeders.nl/lijst-nederlandse-paymentproviders-om-sluiten-google-analytics/
		 * @link http://blog.analytics-toolkit.com/2015/payment-gateway-referrer-exclusions-google-analytics/
		 * @link https://andrescholten.nl/a-bank-ideal-ogone-paypal-or-other-payment-providers-getting-all-revenue-in-google-analytics/
		 * @link https://nicksnell.dev/log/2017/06/exclude-social-login-referrals/
		 * @link https://bluegg.co.uk/blog/tracking-referrals-when-using-an-external-payment-gatewa
		 * @link https://github.com/Adyen/adyen-magento2/search?q=utm_nooverride
		 */
		$url = \add_query_arg( 'utm_nooverride', '1', $url );

		return $url;
	}
}