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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   * This code is free software; you can redistribute it and/or modify it
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   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
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   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
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   * You should have received a copy of the GNU General Public License version
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   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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  package java.util;
  
  import java.security.*;
  
  /**
   * A class that represents an immutable universally unique identifier (UUID).
   * A UUID represents a 128-bit value.
   *
   * <p> There exist different variants of these global identifiers.  The methods
   * of this class are for manipulating the Leach-Salz variant, although the
   * constructors allow the creation of any variant of UUID (described below).
   *
   * <p> The layout of a variant 2 (Leach-Salz) UUID is as follows:
   *
   * The most significant long consists of the following unsigned fields:
   * <pre>
   * 0xFFFFFFFF00000000 time_low
   * 0x00000000FFFF0000 time_mid
   * 0x000000000000F000 version
   * 0x0000000000000FFF time_hi
   * </pre>
   * The least significant long consists of the following unsigned fields:
   * <pre>
   * 0xC000000000000000 variant
   * 0x3FFF000000000000 clock_seq
   * 0x0000FFFFFFFFFFFF node
   * </pre>
   *
   * <p> The variant field contains a value which identifies the layout of the
   * {@code UUID}.  The bit layout described above is valid only for a {@code
   * UUID} with a variant value of 2, which indicates the Leach-Salz variant.
   *
   * <p> The version field holds a value that describes the type of this {@code
   * UUID}.  There are four different basic types of UUIDs: time-based, DCE
   * security, name-based, and randomly generated UUIDs.  These types have a
   * version value of 1, 2, 3 and 4, respectively.
   *
   * <p> For more information including algorithms used to create {@code UUID}s,
   * see <a href="http://www.ietf.org/rfc/rfc4122.txt"> <i>RFC&nbsp;4122: A
   * Universally Unique IDentifier (UUID) URN Namespace</i></a>, section 4.2
   * &quot;Algorithms for Creating a Time-Based UUID&quot;.
   *
   * @since   1.5
   */
  public final class UUID implements java.io.Serializable, Comparable<UUID> {
  
      /**
       * Explicit serialVersionUID for interoperability.
       */
      private static final long serialVersionUID = -4856846361193249489L;
  
      /*
       * The most significant 64 bits of this UUID.
       *
       * @serial
       */
      private final long mostSigBits;
  
      /*
       * The least significant 64 bits of this UUID.
       *
       * @serial
       */
      private final long leastSigBits;
  
      /*
       * The random number generator used by this class to create random
       * based UUIDs. In a holder class to defer initialization until needed.
       */
      private static class Holder {
          static final SecureRandom numberGenerator = new SecureRandom();
      }
  
      // Constructors and Factories
  
      /*
       * Private constructor which uses a byte array to construct the new UUID.
       */
      private UUID(byte[] data) {
          long msb = 0;
          long lsb = 0;
          assert data.length == 16 : "data must be 16 bytes in length";
          for (int i=0; i<8; i++)
              msb = (msb << 8) | (data[i] & 0xff);
          for (int i=8; i<16; i++)
              lsb = (lsb << 8) | (data[i] & 0xff);
          this.mostSigBits = msb;
          this.leastSigBits = lsb;
      }
  
      /**
       * Constructs a new {@code UUID} using the specified data.  {@code
       * mostSigBits} is used for the most significant 64 bits of the {@code
       * UUID} and {@code leastSigBits} becomes the least significant 64 bits of
       * the {@code UUID}.
       *
       * @param  mostSigBits
       *         The most significant bits of the {@code UUID}
       *
       * @param  leastSigBits
       *         The least significant bits of the {@code UUID}
       */
      public UUID(long mostSigBits, long leastSigBits) {
          this.mostSigBits = mostSigBits;
          this.leastSigBits = leastSigBits;
      }
  
      /**
       * Static factory to retrieve a type 4 (pseudo randomly generated) UUID.
       *
       * The {@code UUID} is generated using a cryptographically strong pseudo
       * random number generator.
       *
       * @return  A randomly generated {@code UUID}
       */
      public static UUID randomUUID() {
          SecureRandom ng = Holder.numberGenerator;
  
          byte[] randomBytes = new byte[16];
          ng.nextBytes(randomBytes);
          randomBytes[6]  &= 0x0f;  /* clear version        */
          randomBytes[6]  |= 0x40;  /* set to version 4     */
          randomBytes[8]  &= 0x3f;  /* clear variant        */
          randomBytes[8]  |= 0x80;  /* set to IETF variant  */
          return new UUID(randomBytes);
      }
  
      /**
       * Static factory to retrieve a type 3 (name based) {@code UUID} based on
       * the specified byte array.
       *
       * @param  name
       *         A byte array to be used to construct a {@code UUID}
       *
       * @return  A {@code UUID} generated from the specified array
       */
      public static UUID nameUUIDFromBytes(byte[] name) {
          MessageDigest md;
          try {
              md = MessageDigest.getInstance("MD5");
          } catch (NoSuchAlgorithmException nsae) {
              throw new InternalError("MD5 not supported", nsae);
          }
          byte[] md5Bytes = md.digest(name);
          md5Bytes[6]  &= 0x0f;  /* clear version        */
          md5Bytes[6]  |= 0x30;  /* set to version 3     */
          md5Bytes[8]  &= 0x3f;  /* clear variant        */
          md5Bytes[8]  |= 0x80;  /* set to IETF variant  */
          return new UUID(md5Bytes);
      }
  
      /**
       * Creates a {@code UUID} from the string standard representation as
       * described in the {@link #toString} method.
       *
       * @param  name
       *         A string that specifies a {@code UUID}
       *
       * @return  A {@code UUID} with the specified value
       *
       * @throws  IllegalArgumentException
       *          If name does not conform to the string representation as
       *          described in {@link #toString}
       *
       */
      public static UUID fromString(String name) {
          String[] components = name.split("-");
          if (components.length != 5)
              throw new IllegalArgumentException("Invalid UUID string: "+name);
          for (int i=0; i<5; i++)
              components[i] = "0x"+components[i];
  
          long mostSigBits = Long.decode(components[0]).longValue();
          mostSigBits <<= 16;
          mostSigBits |= Long.decode(components[1]).longValue();
          mostSigBits <<= 16;
          mostSigBits |= Long.decode(components[2]).longValue();
  
          long leastSigBits = Long.decode(components[3]).longValue();
          leastSigBits <<= 48;
          leastSigBits |= Long.decode(components[4]).longValue();
  
          return new UUID(mostSigBits, leastSigBits);
      }
  
      // Field Accessor Methods
  
      /**
       * Returns the least significant 64 bits of this UUID's 128 bit value.
       *
       * @return  The least significant 64 bits of this UUID's 128 bit value
       */
      public long getLeastSignificantBits() {
          return leastSigBits;
      }
  
      /**
       * Returns the most significant 64 bits of this UUID's 128 bit value.
       *
       * @return  The most significant 64 bits of this UUID's 128 bit value
       */
      public long getMostSignificantBits() {
          return mostSigBits;
      }
  
      /**
       * The version number associated with this {@code UUID}.  The version
       * number describes how this {@code UUID} was generated.
       *
       * The version number has the following meaning:
       * <ul>
       * <li>1    Time-based UUID
       * <li>2    DCE security UUID
       * <li>3    Name-based UUID
       * <li>4    Randomly generated UUID
       * </ul>
       *
       * @return  The version number of this {@code UUID}
       */
      public int version() {
          // Version is bits masked by 0x000000000000F000 in MS long
          return (int)((mostSigBits >> 12) & 0x0f);
      }
  
      /**
       * The variant number associated with this {@code UUID}.  The variant
       * number describes the layout of the {@code UUID}.
       *
       * The variant number has the following meaning:
       * <ul>
       * <li>0    Reserved for NCS backward compatibility
       * <li>2    <a href="http://www.ietf.org/rfc/rfc4122.txt">IETF&nbsp;RFC&nbsp;4122</a>
       * (Leach-Salz), used by this class
       * <li>6    Reserved, Microsoft Corporation backward compatibility
       * <li>7    Reserved for future definition
       * </ul>
       *
       * @return  The variant number of this {@code UUID}
       */
      public int variant() {
          // This field is composed of a varying number of bits.
          // 0    -    -    Reserved for NCS backward compatibility
          // 1    0    -    The IETF aka Leach-Salz variant (used by this class)
          // 1    1    0    Reserved, Microsoft backward compatibility
          // 1    1    1    Reserved for future definition.
          return (int) ((leastSigBits >>> (64 - (leastSigBits >>> 62)))
                        & (leastSigBits >> 63));
      }
  
      /**
       * The timestamp value associated with this UUID.
       *
       * <p> The 60 bit timestamp value is constructed from the time_low,
       * time_mid, and time_hi fields of this {@code UUID}.  The resulting
       * timestamp is measured in 100-nanosecond units since midnight,
       * October 15, 1582 UTC.
       *
       * <p> The timestamp value is only meaningful in a time-based UUID, which
       * has version type 1.  If this {@code UUID} is not a time-based UUID then
       * this method throws UnsupportedOperationException.
       *
       * @throws UnsupportedOperationException
       *         If this UUID is not a version 1 UUID
       * @return The timestamp of this {@code UUID}.
       */
      public long timestamp() {
          if (version() != 1) {
              throw new UnsupportedOperationException("Not a time-based UUID");
          }
  
          return (mostSigBits & 0x0FFFL) << 48
               | ((mostSigBits >> 16) & 0x0FFFFL) << 32
               | mostSigBits >>> 32;
      }
  
      /**
       * The clock sequence value associated with this UUID.
       *
       * <p> The 14 bit clock sequence value is constructed from the clock
       * sequence field of this UUID.  The clock sequence field is used to
       * guarantee temporal uniqueness in a time-based UUID.
       *
       * <p> The {@code clockSequence} value is only meaningful in a time-based
       * UUID, which has version type 1.  If this UUID is not a time-based UUID
       * then this method throws UnsupportedOperationException.
       *
       * @return  The clock sequence of this {@code UUID}
       *
       * @throws  UnsupportedOperationException
       *          If this UUID is not a version 1 UUID
       */
      public int clockSequence() {
          if (version() != 1) {
              throw new UnsupportedOperationException("Not a time-based UUID");
          }
  
          return (int)((leastSigBits & 0x3FFF000000000000L) >>> 48);
      }
  
      /**
       * The node value associated with this UUID.
       *
       * <p> The 48 bit node value is constructed from the node field of this
       * UUID.  This field is intended to hold the IEEE 802 address of the machine
       * that generated this UUID to guarantee spatial uniqueness.
       *
       * <p> The node value is only meaningful in a time-based UUID, which has
       * version type 1.  If this UUID is not a time-based UUID then this method
       * throws UnsupportedOperationException.
       *
       * @return  The node value of this {@code UUID}
       *
       * @throws  UnsupportedOperationException
       *          If this UUID is not a version 1 UUID
       */
      public long node() {
          if (version() != 1) {
              throw new UnsupportedOperationException("Not a time-based UUID");
          }
  
          return leastSigBits & 0x0000FFFFFFFFFFFFL;
      }
  
      // Object Inherited Methods
  
      /**
       * Returns a {@code String} object representing this {@code UUID}.
       *
       * <p> The UUID string representation is as described by this BNF:
       * <blockquote><pre>
       * {@code
       * UUID                   = <time_low> "-" <time_mid> "-"
       *                          <time_high_and_version> "-"
       *                          <variant_and_sequence> "-"
       *                          <node>
       * time_low               = 4*<hexOctet>
       * time_mid               = 2*<hexOctet>
       * time_high_and_version  = 2*<hexOctet>
       * variant_and_sequence   = 2*<hexOctet>
       * node                   = 6*<hexOctet>
       * hexOctet               = <hexDigit><hexDigit>
       * hexDigit               =
       *       "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"
       *       | "a" | "b" | "c" | "d" | "e" | "f"
       *       | "A" | "B" | "C" | "D" | "E" | "F"
       * }</pre></blockquote>
       *
       * @return  A string representation of this {@code UUID}
       */
      public String toString() {
          return (digits(mostSigBits >> 32, 8) + "-" +
                  digits(mostSigBits >> 16, 4) + "-" +
                  digits(mostSigBits, 4) + "-" +
                  digits(leastSigBits >> 48, 4) + "-" +
                  digits(leastSigBits, 12));
      }
  
      /** Returns val represented by the specified number of hex digits. */
      private static String digits(long val, int digits) {
          long hi = 1L << (digits * 4);
          return Long.toHexString(hi | (val & (hi - 1))).substring(1);
      }
  
      /**
       * Returns a hash code for this {@code UUID}.
       *
       * @return  A hash code value for this {@code UUID}
       */
      public int hashCode() {
          long hilo = mostSigBits ^ leastSigBits;
          return ((int)(hilo >> 32)) ^ (int) hilo;
      }
  
      /**
       * Compares this object to the specified object.  The result is {@code
       * true} if and only if the argument is not {@code null}, is a {@code UUID}
       * object, has the same variant, and contains the same value, bit for bit,
       * as this {@code UUID}.
       *
       * @param  obj
       *         The object to be compared
       *
       * @return  {@code true} if the objects are the same; {@code false}
       *          otherwise
       */
      public boolean equals(Object obj) {
          if ((null == obj) || (obj.getClass() != UUID.class))
              return false;
          UUID id = (UUID)obj;
          return (mostSigBits == id.mostSigBits &&
                  leastSigBits == id.leastSigBits);
      }
  
      // Comparison Operations
  
      /**
       * Compares this UUID with the specified UUID.
       *
       * <p> The first of two UUIDs is greater than the second if the most
       * significant field in which the UUIDs differ is greater for the first
       * UUID.
       *
       * @param  val
       *         {@code UUID} to which this {@code UUID} is to be compared
       *
       * @return  -1, 0 or 1 as this {@code UUID} is less than, equal to, or
       *          greater than {@code val}
       *
       */
      public int compareTo(UUID val) {
          // The ordering is intentionally set up so that the UUIDs
          // can simply be numerically compared as two numbers
          return (this.mostSigBits < val.mostSigBits ? -1 :
                  (this.mostSigBits > val.mostSigBits ? 1 :
                   (this.leastSigBits < val.leastSigBits ? -1 :
                    (this.leastSigBits > val.leastSigBits ? 1 :
                     0))));
      }
  }
  