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The Apache Log4j Implementation
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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.logging.log4j.core.util;

import java.net.InetAddress;
import java.net.NetworkInterface;
import java.net.SocketException;
import java.net.UnknownHostException;
import java.nio.ByteBuffer;
import java.security.SecureRandom;
import java.util.Enumeration;
import java.util.Random;
import java.util.UUID;
import java.util.concurrent.atomic.AtomicInteger;

import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.status.StatusLogger;
import org.apache.logging.log4j.util.PropertiesUtil;

/**
 * Generates a unique ID. The generated UUID will be unique for approximately 8,925 years so long as
 * less than 10,000 IDs are generated per millisecond on the same device (as identified by its MAC address).
 */
public final class UuidUtil {
    /**
     * System property that may be used to seed the UUID generation with an integer value.
     */
    public static final String UUID_SEQUENCE = "org.apache.logging.log4j.uuidSequence";

    private static final Logger LOGGER = StatusLogger.getLogger();

    private static final String ASSIGNED_SEQUENCES = "org.apache.logging.log4j.assignedSequences";

    private static final AtomicInteger COUNT = new AtomicInteger(0);
    private static final long TYPE1 = 0x1000L;
    private static final byte VARIANT = (byte) 0x80;
    private static final int SEQUENCE_MASK = 0x3FFF;
    private static final long NUM_100NS_INTERVALS_SINCE_UUID_EPOCH = 0x01b21dd213814000L;
    private static final long INITIAL_UUID_SEQNO = PropertiesUtil.getProperties().getLongProperty(UUID_SEQUENCE, 0);

    private static final long LEAST;

    private static final long LOW_MASK = 0xffffffffL;
    private static final long MID_MASK = 0xffff00000000L;
    private static final long HIGH_MASK = 0xfff000000000000L;
    private static final int NODE_SIZE = 8;
    private static final int SHIFT_2 = 16;
    private static final int SHIFT_4 = 32;
    private static final int SHIFT_6 = 48;
    private static final int HUNDRED_NANOS_PER_MILLI = 10000;

    static {
        byte[] mac = getLocalMacAddress();
        final Random randomGenerator = new SecureRandom();
        if (mac == null || mac.length == 0) {
            mac = new byte[6];
            randomGenerator.nextBytes(mac);
        }
        final int length = mac.length >= 6 ? 6 : mac.length;
        final int index = mac.length >= 6 ? mac.length - 6 : 0;
        final byte[] node = new byte[NODE_SIZE];
        node[0] = VARIANT;
        node[1] = 0;
        for (int i = 2; i < NODE_SIZE; ++i) {
            node[i] = 0;
        }
        System.arraycopy(mac, index, node, index + 2, length);
        final ByteBuffer buf = ByteBuffer.wrap(node);
        long rand = INITIAL_UUID_SEQNO;
        String assigned = PropertiesUtil.getProperties().getStringProperty(ASSIGNED_SEQUENCES);
        long[] sequences;
        if (assigned == null) {
            sequences = new long[0];
        } else {
            final String[] array = assigned.split(Patterns.COMMA_SEPARATOR);
            sequences = new long[array.length];
            int i = 0;
            for (final String value : array) {
                sequences[i] = Long.parseLong(value);
                ++i;
            }
        }
        if (rand == 0) {
            rand = randomGenerator.nextLong();
        }
        rand &= SEQUENCE_MASK;
        boolean duplicate;
        do {
            duplicate = false;
            for (final long sequence : sequences) {
                if (sequence == rand) {
                    duplicate = true;
                    break;
                }
            }
            if (duplicate) {
                rand = (rand + 1) & SEQUENCE_MASK;
            }
        } while (duplicate);
        assigned = assigned == null ? Long.toString(rand) : assigned + ',' + Long.toString(rand);
        System.setProperty(ASSIGNED_SEQUENCES, assigned);

        LEAST = buf.getLong() | rand << SHIFT_6;
    }


    /* This class cannot be instantiated */
    private UuidUtil() {
    }

    /**
     * Generates Type 1 UUID. The time contains the number of 100NS intervals that have occurred
     * since 00:00:00.00 UTC, 10 October 1582. Each UUID on a particular machine is unique to the 100NS interval
     * until they rollover around 3400 A.D.
     * 
     * Digits 1-12 are the lower 48 bits of the number of 100 ns increments since the start of the UUID
     * epoch.
     * Digit 13 is the version (with a value of 1).
     * Digits 14-16 are a sequence number that is incremented each time a UUID is generated.
     * Digit 17 is the variant (with a value of binary 10) and 10 bits of the sequence number
     * Digit 18 is final 16 bits of the sequence number.
     * Digits 19-32 represent the system the application is running on.
     * 
     *
     * @return universally unique identifiers (UUID)
     */
    public static UUID getTimeBasedUuid() {

        final long time = ((System.currentTimeMillis() * HUNDRED_NANOS_PER_MILLI) +
            NUM_100NS_INTERVALS_SINCE_UUID_EPOCH) + (COUNT.incrementAndGet() % HUNDRED_NANOS_PER_MILLI);
        final long timeLow = (time & LOW_MASK) << SHIFT_4;
        final long timeMid = (time & MID_MASK) >> SHIFT_2;
        final long timeHi = (time & HIGH_MASK) >> SHIFT_6;
        final long most = timeLow | timeMid | TYPE1 | timeHi;
        return new UUID(most, LEAST);
    }

    /**
     * Returns the local network interface's MAC address if possible. The local network interface is defined here as
     * the {@link java.net.NetworkInterface} that is both up and not a loopback interface.
     *
     * @return the MAC address of the local network interface or {@code null} if no MAC address could be determined.
     * @since 2.1
     */
    private static byte[] getLocalMacAddress() {
        byte[] mac = null;
        try {
            final InetAddress localHost = InetAddress.getLocalHost();
            try {
                final NetworkInterface localInterface = NetworkInterface.getByInetAddress(localHost);
                if (isUpAndNotLoopback(localInterface)) {
                    mac = localInterface.getHardwareAddress();
                }
                if (mac == null) {
                    final Enumeration networkInterfaces = NetworkInterface.getNetworkInterfaces();
                    while (networkInterfaces.hasMoreElements() && mac == null) {
                        final NetworkInterface nic = networkInterfaces.nextElement();
                        if (isUpAndNotLoopback(nic)) {
                            mac = nic.getHardwareAddress();
                        }
                    }
                }
            } catch (final SocketException e) {
                LOGGER.catching(e);
            }
            if (mac == null || mac.length == 0) {
                mac = localHost.getAddress();
            }
        } catch (final UnknownHostException ignored) {
            // ignored
        }
        return mac;
    }

    private static boolean isUpAndNotLoopback(final NetworkInterface ni) throws SocketException {
        return ni != null && !ni.isLoopback() && ni.isUp();
    }
}