/*
 * Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * 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
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.nio;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.locks.StampedLock;

import jdk.internal.misc.JavaNioAccess;
import jdk.internal.misc.JavaLangRefAccess;
import jdk.internal.misc.SharedSecrets;
import jdk.internal.misc.Unsafe;
import jdk.internal.misc.VM;
import jdk.internal.ref.CleanerFactory;

/**
 * Access to bits, native and otherwise.
 */

class Bits {                            // package-private

    private Bits() { }


    // -- Swapping --

    static short swap(short x) {
        return Short.reverseBytes(x);
    }

    static char swap(char x) {
        return Character.reverseBytes(x);
    }

    static int swap(int x) {
        return Integer.reverseBytes(x);
    }

    static long swap(long x) {
        return Long.reverseBytes(x);
    }


    // -- get/put char --

    private static char makeChar(byte b1, byte b0) {
        return (char)((b1 << 8) | (b0 & 0xff));
    }

    static char getCharL(ByteBuffer bb, int bi) {
        return makeChar(bb._get(bi + 1),
                        bb._get(bi    ));
    }

    static char getCharL(long a) {
        return makeChar(_get(a + 1),
                        _get(a    ));
    }

    static char getCharB(ByteBuffer bb, int bi) {
        return makeChar(bb._get(bi    ),
                        bb._get(bi + 1));
    }

    static char getCharB(long a) {
        return makeChar(_get(a    ),
                        _get(a + 1));
    }

    static char getChar(ByteBuffer bb, int bi, boolean bigEndian) {
        return bigEndian ? getCharB(bb, bi) : getCharL(bb, bi);
    }

    static char getChar(long a, boolean bigEndian) {
        return bigEndian ? getCharB(a) : getCharL(a);
    }

    private static byte char1(char x) { return (byte)(x >> 8); }
    private static byte char0(char x) { return (byte)(x     ); }

    static void putCharL(ByteBuffer bb, int bi, char x) {
        bb._put(bi    , char0(x));
        bb._put(bi + 1, char1(x));
    }

    static void putCharL(long a, char x) {
        _put(a    , char0(x));
        _put(a + 1, char1(x));
    }

    static void putCharB(ByteBuffer bb, int bi, char x) {
        bb._put(bi    , char1(x));
        bb._put(bi + 1, char0(x));
    }

    static void putCharB(long a, char x) {
        _put(a    , char1(x));
        _put(a + 1, char0(x));
    }

    static void putChar(ByteBuffer bb, int bi, char x, boolean bigEndian) {
        if (bigEndian)
            putCharB(bb, bi, x);
        else
            putCharL(bb, bi, x);
    }

    static void putChar(long a, char x, boolean bigEndian) {
        if (bigEndian)
            putCharB(a, x);
        else
            putCharL(a, x);
    }


    // -- get/put short --

    private static short makeShort(byte b1, byte b0) {
        return (short)((b1 << 8) | (b0 & 0xff));
    }

    static short getShortL(ByteBuffer bb, int bi) {
        return makeShort(bb._get(bi + 1),
                         bb._get(bi    ));
    }

    static short getShortL(long a) {
        return makeShort(_get(a + 1),
                         _get(a    ));
    }

    static short getShortB(ByteBuffer bb, int bi) {
        return makeShort(bb._get(bi    ),
                         bb._get(bi + 1));
    }

    static short getShortB(long a) {
        return makeShort(_get(a    ),
                         _get(a + 1));
    }

    static short getShort(ByteBuffer bb, int bi, boolean bigEndian) {
        return bigEndian ? getShortB(bb, bi) : getShortL(bb, bi);
    }

    static short getShort(long a, boolean bigEndian) {
        return bigEndian ? getShortB(a) : getShortL(a);
    }

    private static byte short1(short x) { return (byte)(x >> 8); }
    private static byte short0(short x) { return (byte)(x     ); }

    static void putShortL(ByteBuffer bb, int bi, short x) {
        bb._put(bi    , short0(x));
        bb._put(bi + 1, short1(x));
    }

    static void putShortL(long a, short x) {
        _put(a    , short0(x));
        _put(a + 1, short1(x));
    }

    static void putShortB(ByteBuffer bb, int bi, short x) {
        bb._put(bi    , short1(x));
        bb._put(bi + 1, short0(x));
    }

    static void putShortB(long a, short x) {
        _put(a    , short1(x));
        _put(a + 1, short0(x));
    }

    static void putShort(ByteBuffer bb, int bi, short x, boolean bigEndian) {
        if (bigEndian)
            putShortB(bb, bi, x);
        else
            putShortL(bb, bi, x);
    }

    static void putShort(long a, short x, boolean bigEndian) {
        if (bigEndian)
            putShortB(a, x);
        else
            putShortL(a, x);
    }


    // -- get/put int --

    private static int makeInt(byte b3, byte b2, byte b1, byte b0) {
        return (((b3       ) << 24) |
                ((b2 & 0xff) << 16) |
                ((b1 & 0xff) <<  8) |
                ((b0 & 0xff)      ));
    }

    static int getIntL(ByteBuffer bb, int bi) {
        return makeInt(bb._get(bi + 3),
                       bb._get(bi + 2),
                       bb._get(bi + 1),
                       bb._get(bi    ));
    }

    static int getIntL(long a) {
        return makeInt(_get(a + 3),
                       _get(a + 2),
                       _get(a + 1),
                       _get(a    ));
    }

    static int getIntB(ByteBuffer bb, int bi) {
        return makeInt(bb._get(bi    ),
                       bb._get(bi + 1),
                       bb._get(bi + 2),
                       bb._get(bi + 3));
    }

    static int getIntB(long a) {
        return makeInt(_get(a    ),
                       _get(a + 1),
                       _get(a + 2),
                       _get(a + 3));
    }

    static int getInt(ByteBuffer bb, int bi, boolean bigEndian) {
        return bigEndian ? getIntB(bb, bi) : getIntL(bb, bi) ;
    }

    static int getInt(long a, boolean bigEndian) {
        return bigEndian ? getIntB(a) : getIntL(a) ;
    }

    private static byte int3(int x) { return (byte)(x >> 24); }
    private static byte int2(int x) { return (byte)(x >> 16); }
    private static byte int1(int x) { return (byte)(x >>  8); }
    private static byte int0(int x) { return (byte)(x      ); }

    static void putIntL(ByteBuffer bb, int bi, int x) {
        bb._put(bi + 3, int3(x));
        bb._put(bi + 2, int2(x));
        bb._put(bi + 1, int1(x));
        bb._put(bi    , int0(x));
    }

    static void putIntL(long a, int x) {
        _put(a + 3, int3(x));
        _put(a + 2, int2(x));
        _put(a + 1, int1(x));
        _put(a    , int0(x));
    }

    static void putIntB(ByteBuffer bb, int bi, int x) {
        bb._put(bi    , int3(x));
        bb._put(bi + 1, int2(x));
        bb._put(bi + 2, int1(x));
        bb._put(bi + 3, int0(x));
    }

    static void putIntB(long a, int x) {
        _put(a    , int3(x));
        _put(a + 1, int2(x));
        _put(a + 2, int1(x));
        _put(a + 3, int0(x));
    }

    static void putInt(ByteBuffer bb, int bi, int x, boolean bigEndian) {
        if (bigEndian)
            putIntB(bb, bi, x);
        else
            putIntL(bb, bi, x);
    }

    static void putInt(long a, int x, boolean bigEndian) {
        if (bigEndian)
            putIntB(a, x);
        else
            putIntL(a, x);
    }


    // -- get/put long --

    private static long makeLong(byte b7, byte b6, byte b5, byte b4,
                                 byte b3, byte b2, byte b1, byte b0)
    {
        return ((((long)b7       ) << 56) |
                (((long)b6 & 0xff) << 48) |
                (((long)b5 & 0xff) << 40) |
                (((long)b4 & 0xff) << 32) |
                (((long)b3 & 0xff) << 24) |
                (((long)b2 & 0xff) << 16) |
                (((long)b1 & 0xff) <<  8) |
                (((long)b0 & 0xff)      ));
    }

    static long getLongL(ByteBuffer bb, int bi) {
        return makeLong(bb._get(bi + 7),
                        bb._get(bi + 6),
                        bb._get(bi + 5),
                        bb._get(bi + 4),
                        bb._get(bi + 3),
                        bb._get(bi + 2),
                        bb._get(bi + 1),
                        bb._get(bi    ));
    }

    static long getLongL(long a) {
        return makeLong(_get(a + 7),
                        _get(a + 6),
                        _get(a + 5),
                        _get(a + 4),
                        _get(a + 3),
                        _get(a + 2),
                        _get(a + 1),
                        _get(a    ));
    }

    static long getLongB(ByteBuffer bb, int bi) {
        return makeLong(bb._get(bi    ),
                        bb._get(bi + 1),
                        bb._get(bi + 2),
                        bb._get(bi + 3),
                        bb._get(bi + 4),
                        bb._get(bi + 5),
                        bb._get(bi + 6),
                        bb._get(bi + 7));
    }

    static long getLongB(long a) {
        return makeLong(_get(a    ),
                        _get(a + 1),
                        _get(a + 2),
                        _get(a + 3),
                        _get(a + 4),
                        _get(a + 5),
                        _get(a + 6),
                        _get(a + 7));
    }

    static long getLong(ByteBuffer bb, int bi, boolean bigEndian) {
        return bigEndian ? getLongB(bb, bi) : getLongL(bb, bi);
    }

    static long getLong(long a, boolean bigEndian) {
        return bigEndian ? getLongB(a) : getLongL(a);
    }

    private static byte long7(long x) { return (byte)(x >> 56); }
    private static byte long6(long x) { return (byte)(x >> 48); }
    private static byte long5(long x) { return (byte)(x >> 40); }
    private static byte long4(long x) { return (byte)(x >> 32); }
    private static byte long3(long x) { return (byte)(x >> 24); }
    private static byte long2(long x) { return (byte)(x >> 16); }
    private static byte long1(long x) { return (byte)(x >>  8); }
    private static byte long0(long x) { return (byte)(x      ); }

    static void putLongL(ByteBuffer bb, int bi, long x) {
        bb._put(bi + 7, long7(x));
        bb._put(bi + 6, long6(x));
        bb._put(bi + 5, long5(x));
        bb._put(bi + 4, long4(x));
        bb._put(bi + 3, long3(x));
        bb._put(bi + 2, long2(x));
        bb._put(bi + 1, long1(x));
        bb._put(bi    , long0(x));
    }

    static void putLongL(long a, long x) {
        _put(a + 7, long7(x));
        _put(a + 6, long6(x));
        _put(a + 5, long5(x));
        _put(a + 4, long4(x));
        _put(a + 3, long3(x));
        _put(a + 2, long2(x));
        _put(a + 1, long1(x));
        _put(a    , long0(x));
    }

    static void putLongB(ByteBuffer bb, int bi, long x) {
        bb._put(bi    , long7(x));
        bb._put(bi + 1, long6(x));
        bb._put(bi + 2, long5(x));
        bb._put(bi + 3, long4(x));
        bb._put(bi + 4, long3(x));
        bb._put(bi + 5, long2(x));
        bb._put(bi + 6, long1(x));
        bb._put(bi + 7, long0(x));
    }

    static void putLongB(long a, long x) {
        _put(a    , long7(x));
        _put(a + 1, long6(x));
        _put(a + 2, long5(x));
        _put(a + 3, long4(x));
        _put(a + 4, long3(x));
        _put(a + 5, long2(x));
        _put(a + 6, long1(x));
        _put(a + 7, long0(x));
    }

    static void putLong(ByteBuffer bb, int bi, long x, boolean bigEndian) {
        if (bigEndian)
            putLongB(bb, bi, x);
        else
            putLongL(bb, bi, x);
    }

    static void putLong(long a, long x, boolean bigEndian) {
        if (bigEndian)
            putLongB(a, x);
        else
            putLongL(a, x);
    }


    // -- get/put float --

    static float getFloatL(ByteBuffer bb, int bi) {
        return Float.intBitsToFloat(getIntL(bb, bi));
    }

    static float getFloatL(long a) {
        return Float.intBitsToFloat(getIntL(a));
    }

    static float getFloatB(ByteBuffer bb, int bi) {
        return Float.intBitsToFloat(getIntB(bb, bi));
    }

    static float getFloatB(long a) {
        return Float.intBitsToFloat(getIntB(a));
    }

    static float getFloat(ByteBuffer bb, int bi, boolean bigEndian) {
        return bigEndian ? getFloatB(bb, bi) : getFloatL(bb, bi);
    }

    static float getFloat(long a, boolean bigEndian) {
        return bigEndian ? getFloatB(a) : getFloatL(a);
    }

    static void putFloatL(ByteBuffer bb, int bi, float x) {
        putIntL(bb, bi, Float.floatToRawIntBits(x));
    }

    static void putFloatL(long a, float x) {
        putIntL(a, Float.floatToRawIntBits(x));
    }

    static void putFloatB(ByteBuffer bb, int bi, float x) {
        putIntB(bb, bi, Float.floatToRawIntBits(x));
    }

    static void putFloatB(long a, float x) {
        putIntB(a, Float.floatToRawIntBits(x));
    }

    static void putFloat(ByteBuffer bb, int bi, float x, boolean bigEndian) {
        if (bigEndian)
            putFloatB(bb, bi, x);
        else
            putFloatL(bb, bi, x);
    }

    static void putFloat(long a, float x, boolean bigEndian) {
        if (bigEndian)
            putFloatB(a, x);
        else
            putFloatL(a, x);
    }


    // -- get/put double --

    static double getDoubleL(ByteBuffer bb, int bi) {
        return Double.longBitsToDouble(getLongL(bb, bi));
    }

    static double getDoubleL(long a) {
        return Double.longBitsToDouble(getLongL(a));
    }

    static double getDoubleB(ByteBuffer bb, int bi) {
        return Double.longBitsToDouble(getLongB(bb, bi));
    }

    static double getDoubleB(long a) {
        return Double.longBitsToDouble(getLongB(a));
    }

    static double getDouble(ByteBuffer bb, int bi, boolean bigEndian) {
        return bigEndian ? getDoubleB(bb, bi) : getDoubleL(bb, bi);
    }

    static double getDouble(long a, boolean bigEndian) {
        return bigEndian ? getDoubleB(a) : getDoubleL(a);
    }

    static void putDoubleL(ByteBuffer bb, int bi, double x) {
        putLongL(bb, bi, Double.doubleToRawLongBits(x));
    }

    static void putDoubleL(long a, double x) {
        putLongL(a, Double.doubleToRawLongBits(x));
    }

    static void putDoubleB(ByteBuffer bb, int bi, double x) {
        putLongB(bb, bi, Double.doubleToRawLongBits(x));
    }

    static void putDoubleB(long a, double x) {
        putLongB(a, Double.doubleToRawLongBits(x));
    }

    static void putDouble(ByteBuffer bb, int bi, double x, boolean bigEndian) {
        if (bigEndian)
            putDoubleB(bb, bi, x);
        else
            putDoubleL(bb, bi, x);
    }

    static void putDouble(long a, double x, boolean bigEndian) {
        if (bigEndian)
            putDoubleB(a, x);
        else
            putDoubleL(a, x);
    }


    // -- Unsafe access --

    private static final Unsafe unsafe = Unsafe.getUnsafe();

    private static byte _get(long a) {
        return unsafe.getByte(a);
    }

    private static void _put(long a, byte b) {
        unsafe.putByte(a, b);
    }

    static Unsafe unsafe() {
        return unsafe;
    }


    // -- Processor and memory-system properties --

    private static final ByteOrder byteOrder
        = unsafe.isBigEndian() ? ByteOrder.BIG_ENDIAN : ByteOrder.LITTLE_ENDIAN;

    static ByteOrder byteOrder() {
        return byteOrder;
    }

    private static int pageSize = -1;

    static int pageSize() {
        if (pageSize == -1)
            pageSize = unsafe().pageSize();
        return pageSize;
    }

    static int pageCount(long size) {
        return (int)(size + (long)pageSize() - 1L) / pageSize();
    }

    private static boolean unaligned = unsafe.unalignedAccess();

    static boolean unaligned() {
        return unaligned;
    }


    // -- Direct memory management --

    // A user-settable upper limit on the maximum amount of allocatable
    // direct buffer memory.  This value may be changed during VM
    // initialization if it is launched with "-XX:MaxDirectMemorySize=<size>".
    // A change to maxMemory is published via memoryLimitSet volatile boolean
    // flag changing state from false -> true, so maxMemory need not be volatile.
    private static long maxMemory = VM.maxDirectMemory();
    private static volatile boolean memoryLimitSet;

    private static final AtomicLong reservedMemory = new AtomicLong();
    private static final AtomicLong totalCapacity = new AtomicLong();
    private static final AtomicLong count = new AtomicLong();

    // A fair lock for direct memory allocator threads to queue after 1st optimistic
    // reservation fails so that only a single thread at a time is retrying
    // reservation while:
    //   - waiting for unreservation events until time out
    //   - followed by triggering reference discovery
    //   - followed by another round of waiting for unreservation events until time out
    // ... before finally giving up with OutOfMemoryError.
    private static final StampedLock reserveLock = new StampedLock();

    // A counter of unreservations incremented by unreservation threads,
    // guarded by unreserveLock, but also read by reservation threads without
    // holding unreserveLock, so it must be volatile.
    private static volatile int unreserveCount;

    // Last value of 'unreserveCount' observed by reservation threads,
    // guarded by reserveLock. If 'lastUnreserveCount' and 'unreserveCount'
    // differ, an unreservation event (or multiple events) is detected.
    private static int lastUnreserveCount;

    // the System.nanoTime() of the detection of last unreservation event
    // by the reservation thread(s), guarded by reserveLock.
    private static long lastUnreserveNanoTime;

    // flag indicating if 'lastUnreserveNanoTime' has already been set,
    // guarded by reserveLock.
    private static boolean lastUnreserveNanoTimeSet;

    // max. wait time for unreservation event before triggering GC and/or
    // failing with OOME.
    private static final long MAX_UNRESERVE_WAIT_NANOS =
        TimeUnit.SECONDS.toNanos(1L);

    // the duration of last successful waiting for unreservation
    // event by reservation thread. starts with MAX_UNRESERVE_WAIT_NANOS / 4
    // and is dynamically adjusted at each successful call to awaitUnreserveMemory()
    // that actually waited for the event.
    private static long lastSuccessfullUnreserveWaitNanos =
        MAX_UNRESERVE_WAIT_NANOS / 4;

    // monitor lock via which unreservation threads notify reservation threads
    // about the unreservation events.
    private static final Object unreserveLock = new Object();

    // These methods should be called whenever direct memory is allocated or
    // freed.  They allow the user to control the amount of direct memory
    // which a process may access.  All sizes are specified in bytes.
    static void reserveMemory(long size, int cap) {

        if (!memoryLimitSet && VM.initLevel() >= 1) {
            maxMemory = VM.maxDirectMemory();
            memoryLimitSet = true;
        }

        JavaLangRefAccess jlra = SharedSecrets.getJavaLangRefAccess();

        // optimist!
        long stamp = reserveLock.tryReadLock();
        if (stamp != 0L) try {
            // retry reservation while helping the Cleaner thread process enqueued
            // Cleanable(s) which includes the ones that free direct buffer memory
            // until the queue drains out
            do {
                if (tryReserveMemory(size, cap)) {
                    return;
                }
            } while (jlra.cleanNextEnqueuedCleanable(CleanerFactory.cleaner()));
        } finally {
            reserveLock.unlockRead(stamp);
        }

        // reservation threads that don't succeed at first must queue so that
        // only one of them at a time is triggering reference discovery and
        // retrials with waiting on un-reservation events...
        stamp = reserveLock.writeLock();
        try {
            // retry reservation until there are no unreservation events
            // detected for at least 4 times as much as we waited for last
            // successful unreservation event but no more than
            // MAX_UNRESERVE_WAIT_NANOS.
            do {
                if (tryReserveMemory(size, cap)) {
                    return;
                }
            } while (awaitUnreserveMemory(Math.min(MAX_UNRESERVE_WAIT_NANOS,
                                                   lastSuccessfullUnreserveWaitNanos * 4)));

            // trigger reference discovery
            System.gc();
            // GC stops the world and can last for a long time,
            // so restart timing from 0.
            lastUnreserveNanoTime = System.nanoTime();

            // retry reservation until there are no unreservation events
            // detected for at least MAX_UNRESERVE_WAIT_NANOS.
            do {
                if (tryReserveMemory(size, cap)) {
                    return;
                }
            } while (awaitUnreserveMemory(MAX_UNRESERVE_WAIT_NANOS));

            // no luck
            throw new OutOfMemoryError("Direct buffer memory");
        } finally {
            reserveLock.unlockWrite(stamp);
        }
    }

    private static boolean tryReserveMemory(long size, int cap) {

        // -XX:MaxDirectMemorySize limits the total capacity rather than the
        // actual memory usage, which will differ when buffers are page
        // aligned.
        long totalCap;
        while (cap <= maxMemory - (totalCap = totalCapacity.get())) {
            if (totalCapacity.compareAndSet(totalCap, totalCap + cap)) {
                reservedMemory.addAndGet(size);
                count.incrementAndGet();
                return true;
            }
        }

        return false;
    }

    private static boolean awaitUnreserveMemory(long timeoutNanos) {
        assert reserveLock.isWriteLocked();

        // optimistic 1st try without lock
        int c;
        if ((c = unreserveCount) != lastUnreserveCount) {
            lastUnreserveCount = c;
            lastUnreserveNanoTime = System.nanoTime();
            lastUnreserveNanoTimeSet = true;
            return true;
        }
        boolean interrupted = false;
        try {
            synchronized (unreserveLock) {
                long nt = System.nanoTime();
                if (!lastUnreserveNanoTimeSet) {
                    lastUnreserveNanoTime = nt;
                    lastUnreserveNanoTimeSet = true;
                }
                long deadline = lastUnreserveNanoTime + timeoutNanos;
                long waitUnreserve;
                while ((c = unreserveCount) == lastUnreserveCount &&
                       (waitUnreserve = deadline - nt) > 0L) {
                    interrupted |= waitNanos(unreserveLock, waitUnreserve);
                    nt = System.nanoTime();
                }
                if (c == lastUnreserveCount) {
                    // time out
                    return false;
                } else {
                    lastSuccessfullUnreserveWaitNanos = nt - lastUnreserveNanoTime;
                    lastUnreserveCount = c;
                    lastUnreserveNanoTime = nt;
                    return true;
                }
            }
        } finally {
            if (interrupted) {
                Thread.currentThread().interrupt();
            }
        }
    }

    private static boolean waitNanos(Object lock, long waitNanos) {
        try {
            long millis = TimeUnit.NANOSECONDS.toMillis(waitNanos);
            int nanos = (int) (waitNanos - TimeUnit.MILLISECONDS.toNanos(millis));
            lock.wait(millis, nanos);
            return false;
        } catch (InterruptedException e) {
            return true;
        }
    }

    static void unreserveMemory(long size, int cap) {
        synchronized (unreserveLock) {
            long cnt = count.decrementAndGet();
            long reservedMem = reservedMemory.addAndGet(-size);
            long totalCap = totalCapacity.addAndGet(-cap);
            assert cnt >= 0 && reservedMem >= 0 && totalCap >= 0;
            unreserveCount++;
            unreserveLock.notifyAll();
        }
    }

    // -- Monitoring of direct buffer usage --

    static {
        // setup access to this package in SharedSecrets
        SharedSecrets.setJavaNioAccess(
            new JavaNioAccess() {
                @Override
                public JavaNioAccess.BufferPool getDirectBufferPool() {
                    return new JavaNioAccess.BufferPool() {
                        @Override
                        public String getName() {
                            return "direct";
                        }
                        @Override
                        public long getCount() {
                            return Bits.count.get();
                        }
                        @Override
                        public long getTotalCapacity() {
                            return Bits.totalCapacity.get();
                        }
                        @Override
                        public long getMemoryUsed() {
                            return Bits.reservedMemory.get();
                        }
                    };
                }
                @Override
                public ByteBuffer newDirectByteBuffer(long addr, int cap, Object ob) {
                    return new DirectByteBuffer(addr, cap, ob);
                }
                @Override
                public void truncate(Buffer buf) {
                    buf.truncate();
                }
        });
    }

    // These numbers represent the point at which we have empirically
    // determined that the average cost of a JNI call exceeds the expense
    // of an element by element copy.  These numbers may change over time.
    static final int JNI_COPY_TO_ARRAY_THRESHOLD   = 6;
    static final int JNI_COPY_FROM_ARRAY_THRESHOLD = 6;
}