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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * 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
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package com.sun.jndi.dns;

import java.io.IOException;
import java.net.DatagramSocket;
import java.net.SocketException;
import java.util.Random;

class DNSDatagramSocketFactory {
    static final int DEVIATION = 3;
    static final int THRESHOLD = 6;
    static final int BIT_DEVIATION = 2;
    static final int HISTORY = 32;
    static final int MAX_RANDOM_TRIES = 5;
    
The dynamic allocation port range (aka ephemeral ports), as configured
on the system. Use nested class for lazy evaluation.
/**
     * The dynamic allocation port range (aka ephemeral ports), as configured
     * on the system. Use nested class for lazy evaluation.
     */
    static final class EphemeralPortRange {
        private EphemeralPortRange() {}
        static final int LOWER = sun.net.PortConfig.getLower();
        static final int UPPER = sun.net.PortConfig.getUpper();
        static final int RANGE = UPPER - LOWER + 1;
    }

    // Records a subset of max {@code capacity} previously used ports
    static final class PortHistory {
        final int capacity;
        final int[] ports;
        final Random random;
        int index;
        PortHistory(int capacity, Random random) {
            this.random = random;
            this.capacity = capacity;
            this.ports = new int[capacity];
        }
        // returns true if the history contains the specified port.
        public boolean contains(int port) {
            int p = 0;
            for (int i=0; i<capacity; i++) {
                if ((p = ports[i]) == 0 || p == port) break;
            }
            return p == port;
        }
        // Adds the port to the history - doesn't check whether the port
        // is already present. Always adds the port and always return true.
        public boolean add(int port) {
            if (ports[index] != 0) { // at max capacity
                // remove one port at random and store the new port there
                ports[random.nextInt(capacity)] = port;
            } else { // there's a free slot
                ports[index] = port;
            }
            if (++index == capacity) index = 0;
            return true;
        }
        // Adds the port to the history if not already present.
        // Return true if the port was added, false if the port was already
        // present.
        public boolean offer(int port) {
            if (contains(port)) return false;
            else return add(port);
        }
    }

    int lastport = 0;
    int suitablePortCount;
    int unsuitablePortCount;
    final int thresholdCount; // decision point
    final int deviation;
    final Random random;
    final PortHistory history;

    DNSDatagramSocketFactory() {
        this(new Random());
    }

    DNSDatagramSocketFactory(Random random) {
        this(random, DEVIATION, THRESHOLD);
    }
    DNSDatagramSocketFactory(Random random,
                             int deviation,
                             int threshold) {
        if (random == null) {
            throw new NullPointerException();
        }

        this.random = random;
        this.history = new PortHistory(HISTORY, random);
        this.deviation = Math.max(1, deviation);
        this.thresholdCount = Math.max(2, threshold);
    }

    
Opens a datagram socket listening to the wildcard address on a
random port. If the underlying OS supports UDP port randomization
out of the box (if binding a socket to port 0 binds it to a random
port) then the underlying OS implementation is used. Otherwise, this
method will allocate and bind a socket on a randomly selected ephemeral
port in the dynamic range.
Throws:
SocketException – if the socket cannot be created.
Returns:
A new DatagramSocket bound to a random port./**
     * Opens a datagram socket listening to the wildcard address on a
     * random port. If the underlying OS supports UDP port randomization
     * out of the box (if binding a socket to port 0 binds it to a random
     * port) then the underlying OS implementation is used. Otherwise, this
     * method will allocate and bind a socket on a randomly selected ephemeral
     * port in the dynamic range.
     * @return A new DatagramSocket bound to a random port.
     * @throws SocketException if the socket cannot be created.
     */
    public synchronized DatagramSocket open() throws SocketException {
        int lastseen = lastport;
        DatagramSocket s;

        boolean thresholdCrossed = unsuitablePortCount > thresholdCount;
        if (thresholdCrossed) {
            // Underlying stack does not support random UDP port out of the box.
            // Use our own algorithm to allocate a random UDP port
            s = openRandom();
            if (s != null) return s;

            // couldn't allocate a random port: reset all counters and fall
            // through.
            unsuitablePortCount = 0; suitablePortCount = 0; lastseen = 0;
        }

        // Allocate an ephemeral port (port 0)
        s = openDefault();
        lastport = s.getLocalPort();
        if (lastseen == 0) {
            history.offer(lastport);
            return s;
        }

        thresholdCrossed = suitablePortCount > thresholdCount;
        boolean farEnough = Integer.bitCount(lastseen ^ lastport) > BIT_DEVIATION
                            && Math.abs(lastport - lastseen) > deviation;
        boolean recycled = history.contains(lastport);
        boolean suitable = (thresholdCrossed || farEnough && !recycled);
        if (suitable && !recycled) history.add(lastport);

        if (suitable) {
            if (!thresholdCrossed) {
                suitablePortCount++;
            } else if (!farEnough || recycled) {
                unsuitablePortCount = 1;
                suitablePortCount = thresholdCount/2;
            }
            // Either the underlying stack supports random UDP port allocation,
            // or the new port is sufficiently distant from last port to make
            // it look like it is. Let's use it.
            return s;
        }

        // Undecided... the new port was too close. Let's allocate a random
        // port using our own algorithm
        assert !thresholdCrossed;
        DatagramSocket ss = openRandom();
        if (ss == null) return s;
        unsuitablePortCount++;
        s.close();
        return ss;
    }

    private DatagramSocket openDefault() throws SocketException {
        return new DatagramSocket();
    }

    synchronized boolean isUsingNativePortRandomization() {
        return  unsuitablePortCount <= thresholdCount
                && suitablePortCount > thresholdCount;
    }

    synchronized boolean isUsingJavaPortRandomization() {
        return unsuitablePortCount > thresholdCount ;
    }

    synchronized boolean isUndecided() {
        return !isUsingJavaPortRandomization()
                && !isUsingNativePortRandomization();
    }

    private DatagramSocket openRandom() {
        int maxtries = MAX_RANDOM_TRIES;
        while (maxtries-- > 0) {
            int port = EphemeralPortRange.LOWER
                    + random.nextInt(EphemeralPortRange.RANGE);
            try {
                return new DatagramSocket(port);
            } catch (IOException x) {
                // try again until maxtries == 0;
            }
        }
        return null;
    }

}