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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * 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 jdk.dns.client.internal;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.ProtocolFamily;
import java.net.SocketException;
import java.nio.channels.DatagramChannel;
import java.util.Objects;
import java.util.Random;
import java.util.concurrent.locks.ReentrantLock;

class DnsDatagramChannelFactory {
    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.
     */
    static final class EphemeralPortRange {
        private EphemeralPortRange() {
        }

        static final int LOWER = PortConfig.getLower();
        static final int UPPER = 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 ProtocolFamily family; // null (default) means dual stack
    final int thresholdCount; // decision point
    final int deviation;
    final Random random;
    final PortHistory history;
    final ReentrantLock factoryLock = new ReentrantLock();

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

    DnsDatagramChannelFactory(Random random) {
        this(Objects.requireNonNull(random), null, DEVIATION, THRESHOLD);
    }

    DnsDatagramChannelFactory(Random random,
                              ProtocolFamily family,
                              int deviation,
                              int threshold) {
        this.random = Objects.requireNonNull(random);
        this.history = new PortHistory(HISTORY, random);
        this.family = family;
        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.
     *
     * @return A new DatagramChannel bound to a random port.
     * @throws SocketException if the socket cannot be created.
     */
    public DatagramChannel open() throws SocketException {
        factoryLock.lock();
        try {
            int lastseen = lastport;
            DatagramChannel dc;

            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
                dc = openRandom();
                if (dc != null) return dc;

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

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

            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 dc;
            }

            // Undecided... the new port was too close. Let's allocate a random
            // port using our own algorithm
            assert !thresholdCrossed;
            try {
                dc.close();
            } catch (IOException ioe) {
                throw new SocketException(ioe.getMessage());
            }
            dc = openRandom();
            unsuitablePortCount++;
            return dc;
        } finally {
            factoryLock.unlock();
        }
    }

    private DatagramChannel openDefault() throws SocketException {
        if (family != null) {
            try {
                DatagramChannel dc = DatagramChannel.open(family);
                try {
                    dc.bind(null);
                    return dc;
                } catch (Throwable x) {
                    dc.close();
                    throw x;
                }
            } catch (SocketException x) {
                throw x;
            } catch (IOException x) {
                SocketException e = new SocketException(x.getMessage());
                e.initCause(x);
                throw e;
            }
        }
        try {
            return DatagramChannel.open();
        } catch (IOException ioe) {
            throw new SocketException(ioe.getMessage());
        }
    }

    boolean isUsingNativePortRandomization() {
        factoryLock.lock();
        try {
            return unsuitablePortCount <= thresholdCount
                    && suitablePortCount > thresholdCount;
        } finally {
            factoryLock.unlock();
        }
    }

    boolean isUsingJavaPortRandomization() {
        factoryLock.lock();
        try {
            return unsuitablePortCount > thresholdCount;
        } finally {
            factoryLock.unlock();
        }
    }

    boolean isUndecided() {
        factoryLock.lock();
        try {
            return !isUsingJavaPortRandomization()
                    && !isUsingNativePortRandomization();
        } finally {
            factoryLock.unlock();
        }
    }

    private DatagramChannel openRandom() {
        int maxtries = MAX_RANDOM_TRIES;
        while (maxtries-- > 0) {
            int port = EphemeralPortRange.LOWER
                    + random.nextInt(EphemeralPortRange.RANGE);
            try {
                if (family != null) {
                    DatagramChannel dc = DatagramChannel.open(family);
                    try {
                        dc.bind(new InetSocketAddress(port));
                        return dc;
                    } catch (Throwable x) {
                        dc.close();
                        throw x;
                    }
                } else {

                }
                DatagramChannel dc = DatagramChannel.open(family);
                return dc.bind(new InetSocketAddress(port));
            } catch (IOException x) {
                // try again until maxtries == 0;
            }
        }
        return null;
    }

}