/src/java/org/apache/cassandra/utils/UUIDGen.java

/*
 * 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.nio.ByteBuffer;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;
import java.util.UUID;


/**
 * The goods are here: www.ietf.org/rfc/rfc4122.txt.
 */
public class UUIDGen
{
    // A grand day! millis at 00:00:00.000 15 Oct 1582.
    private static final long START_EPOCH = -12219292800000L;
    private static final long clock = new Random(System.currentTimeMillis()).nextLong();

    // placement of this singleton is important.  It needs to be instantiated *AFTER* the other statics.
    private static final UUIDGen instance = new UUIDGen();

    private long lastNanos;
    private final Map<InetAddress, Long> nodeCache = new HashMap<InetAddress, Long>();

    private static final ThreadLocal<MessageDigest> localMD5Digest = new ThreadLocal<MessageDigest>()
    {
        @Override
        protected MessageDigest initialValue()
        {
            try
            {
                return MessageDigest.getInstance("MD5");
            }
            catch (NoSuchAlgorithmException nsae)
            {
                throw new RuntimeException("MD5 digest algorithm is not available", nsae);
            }
        }

        @Override
        public MessageDigest get()
        {
            MessageDigest digest = super.get();
            digest.reset();
            return digest;
        }
    };

    private UUIDGen()
    {
        // make sure someone didn't whack the clock by changing the order of instantiation.
        if (clock == 0) throw new RuntimeException("singleton instantiation is misplaced.");
    }

    /**
     * Creates a type 1 UUID (time-based UUID) that substitutes a hash of
     * an IP address in place of the MAC (unavailable to Java).
     *
     * @param addr the host address to use
     * @return a UUID instance
     */
    public static UUID makeType1UUIDFromHost(InetAddress addr)
    {
        return new UUID(instance.createTimeSafe(), instance.getClockSeqAndNode(addr));
    }

    /** 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));
    }

    /** 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 16 byte representation of a type 1 UUID (a time-based UUID),
     * based on the current system time.
     *
     * @return a type 1 UUID represented as a byte[]
     */
    public static byte[] getTimeUUIDBytes()
    {
        return createTimeUUIDBytes(instance.createTimeSafe());
    }

    /**
     * Converts a milliseconds-since-epoch timestamp into the 16 byte representation
     * of a type 1 UUID (a time-based UUID).
     *
     * <p><i><b>Warning:</b> This method is not guaranteed to return unique UUIDs; Multiple
     * invocations using identical timestamps will result in identical UUIDs.</i></p>
     *
     * @param timeMillis
     * @return a type 1 UUID represented as a byte[]
     */
    public static byte[] getTimeUUIDBytes(long timeMillis)
    {
        return createTimeUUIDBytes(instance.createTimeUnsafe(timeMillis));
    }

    /**
     * Converts a 100-nanoseconds precision timestamp into the 16 byte representation
     * of a type 1 UUID (a time-based UUID).
     *
     * To specify a 100-nanoseconds precision timestamp, one should provide a milliseconds timestamp and
     * a number 0 <= n < 10000 such that n*100 is the number of nanoseconds within that millisecond.
     *
     * <p><i><b>Warning:</b> This method is not guaranteed to return unique UUIDs; Multiple
     * invocations using identical timestamps will result in identical UUIDs.</i></p>
     *
     * @return a type 1 UUID represented as a byte[]
     */
    public static byte[] getTimeUUIDBytes(long timeMillis, int nanos)
    {
        if (nanos >= 10000)
            throw new IllegalArgumentException();
        return createTimeUUIDBytes(instance.createTimeUnsafe(timeMillis, nanos));
    }

    private static byte[] createTimeUUIDBytes(long msb)
    {
        long lsb = instance.getClockSeqAndNode(FBUtilities.getLocalAddress());
        byte[] uuidBytes = new byte[16];

        for (int i = 0; i < 8; i++)
            uuidBytes[i] = (byte) (msb >>> 8 * (7 - i));

        for (int i = 8; i < 16; i++)
            uuidBytes[i] = (byte) (lsb >>> 8 * (7 - i));

        return uuidBytes;
    }

    /**
     * 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) - START_EPOCH;
    }

    // todo: could cache value if we assume node doesn't change.
    private long getClockSeqAndNode(InetAddress addr)
    {
        long lsb = 0;
        lsb |= (clock & 0x3f00000000000000L) >>> 56; // was 58?
        lsb |= 0x0000000000000080;
        lsb |= (clock & 0x00ff000000000000L) >>> 48;
        lsb |= makeNode(addr);
        return lsb;
    }

    // needs to return two different values for the same when.
    // we can generate at most 10k UUIDs per ms.
    private synchronized long createTimeSafe()
    {
        long nanosSince = (System.currentTimeMillis() - START_EPOCH) * 10000;
        if (nanosSince > lastNanos)
            lastNanos = nanosSince;
        else
            nanosSince = ++lastNanos;

        return createTime(nanosSince);
    }

    private long createTimeUnsafe(long when)
    {
        return createTimeUnsafe(when, 0);
    }

    private long createTimeUnsafe(long when, int nanos)
    {
        long nanosSince = ((when - START_EPOCH) * 10000) + nanos;
        return createTime(nanosSince);
    }

    private long createTime(long nanosSince)
    {
        long msb = 0L;
        msb |= (0x00000000ffffffffL & nanosSince) << 32;
        msb |= (0x0000ffff00000000L & nanosSince) >>> 16;
        msb |= (0xffff000000000000L & nanosSince) >>> 48;
        msb |= 0x0000000000001000L; // sets the version to 1.
        return msb;
    }

    // Lazily create node hashes, and cache them for later
    private long makeNode(InetAddress addr)
    {
        if (nodeCache.containsKey(addr))
            return nodeCache.get(addr);

        // ideally, we'd use the MAC address, but java doesn't expose that.
        byte[] hash = hash(addr.toString());
        long node = 0;
        for (int i = 0; i < Math.min(6, hash.length); i++)
            node |= (0x00000000000000ff & (long)hash[i]) << (5-i)*8;
        assert (0xff00000000000000L & node) == 0;

        nodeCache.put(addr, node);

        return node;
    }

    private static byte[] hash(String... data)
    {
        MessageDigest messageDigest = localMD5Digest.get();
        for(String block : data)
            messageDigest.update(block.getBytes());

        return messageDigest.digest();
    }
}

// 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();