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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.nio.ByteBuffer;
import java.util.UUID;

/**
 * The goods are here: www.ietf.org/rfc/rfc4122.txt.
 */
public class UUIDGen
{
    public static final int UUID_LEN = 16;

    /** creates a type 1 uuid from raw bytes. */
    public static UUID getUUID(ByteBuffer raw)
    {
        return new UUID(raw.getLong(raw.position()), raw.getLong(raw.position() + 8));
    }

    public static ByteBuffer toByteBuffer(UUID uuid)
    {
        ByteBuffer buffer = ByteBuffer.allocate(UUID_LEN);
        buffer.putLong(uuid.getMostSignificantBits());
        buffer.putLong(uuid.getLeastSignificantBits());
        buffer.flip();
        return buffer;
    }

    /** decomposes a uuid into raw bytes. */
    public static byte[] decompose(UUID uuid)
    {
        long most = uuid.getMostSignificantBits();
        long least = uuid.getLeastSignificantBits();
        byte[] b = new byte[16];
        for (int i = 0; i < 8; i++)
        {
            b[i] = (byte)(most >>> ((7-i) * 8));
            b[8+i] = (byte)(least >>> ((7-i) * 8));
        }
        return b;
    }

    /**
     * Returns a milliseconds-since-epoch value for a type-1 UUID.
     *
     * @param uuid a type-1 (time-based) UUID
     * @return the number of milliseconds since the unix epoch
     * @throws IllegalArgumentException if the UUID is not version 1
     */
    public static long getAdjustedTimestamp(UUID uuid)
    {
        if (uuid.version() != 1)
            throw new IllegalArgumentException("incompatible with uuid version: "+uuid.version());
        return (uuid.timestamp() / 10000) + TimeUUID.UUID_EPOCH_UNIX_MILLIS;
    }

}

// for the curious, here is how I generated START_EPOCH
//        Calendar c = Calendar.getInstance(TimeZone.getTimeZone("GMT-0"));
//        c.set(Calendar.YEAR, 1582);
//        c.set(Calendar.MONTH, Calendar.OCTOBER);
//        c.set(Calendar.DAY_OF_MONTH, 15);
//        c.set(Calendar.HOUR_OF_DAY, 0);
//        c.set(Calendar.MINUTE, 0);
//        c.set(Calendar.SECOND, 0);
//        c.set(Calendar.MILLISECOND, 0);
//        long START_EPOCH = c.getTimeInMillis();