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The Apache Cassandra Project develops a highly scalable second-generation distributed database, bringing together Dynamo's fully distributed design and Bigtable's ColumnFamily-based data model.

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/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.cassandra.utils;

import java.net.InetAddress;
import java.net.UnknownHostException;
import java.nio.ByteBuffer;
import java.security.SecureRandom;
import java.util.Random;

public class GuidGenerator
{
    private static final Random myRand;
    private static final SecureRandom mySecureRand;
    private static final String s_id;

    static
    {
        if (System.getProperty("java.security.egd") == null)
        {
            System.setProperty("java.security.egd", "file:/dev/urandom");
        }
        mySecureRand = new SecureRandom();
        long secureInitializer = mySecureRand.nextLong();
        myRand = new Random(secureInitializer);
        try
        {
            s_id = InetAddress.getLocalHost().toString();
        }
        catch (UnknownHostException e)
        {
            throw new AssertionError(e);
        }
    }

    public static String guid()
    {
        ByteBuffer array = guidAsBytes();

        StringBuilder sb = new StringBuilder();
        for (int j = array.position(); j < array.limit(); ++j)
        {
            int b = array.get(j) & 0xFF;
            if (b < 0x10) sb.append('0');
            sb.append(Integer.toHexString(b));
        }

        return convertToStandardFormat( sb.toString() );
    }

    public static String guidToString(byte[] bytes)
    {
        StringBuilder sb = new StringBuilder();
        for (int j = 0; j < bytes.length; ++j)
        {
            int b = bytes[j] & 0xFF;
            if (b < 0x10) sb.append('0');
            sb.append(Integer.toHexString(b));
        }

        return convertToStandardFormat( sb.toString() );
    }

    public static ByteBuffer guidAsBytes(Random random, String hostId, long time)
    {
        StringBuilder sbValueBeforeMD5 = new StringBuilder();
        long rand = random.nextLong();
        sbValueBeforeMD5.append(hostId)
                        .append(":")
                        .append(Long.toString(time))
                        .append(":")
                        .append(Long.toString(rand));

        String valueBeforeMD5 = sbValueBeforeMD5.toString();
        return ByteBuffer.wrap(MD5Digest.threadLocalMD5Digest().digest(valueBeforeMD5.getBytes()));
    }

    public static ByteBuffer guidAsBytes()
    {
        return guidAsBytes(myRand, s_id, System.currentTimeMillis());
    }

    /*
        * Convert to the standard format for GUID
        * Example: C2FEEEAC-CFCD-11D1-8B05-00600806D9B6
    */

    private static String convertToStandardFormat(String valueAfterMD5)
    {
        String raw = valueAfterMD5.toUpperCase();
        StringBuilder sb = new StringBuilder();
        sb.append(raw.substring(0, 8))
          .append("-")
          .append(raw.substring(8, 12))
          .append("-")
          .append(raw.substring(12, 16))
          .append("-")
          .append(raw.substring(16, 20))
          .append("-")
          .append(raw.substring(20));
        return sb.toString();
    }
}