org.apache.tajo.util.UnsafeComparer.java Source code

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Here is the source code for org.apache.tajo.util.UnsafeComparer.java

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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.tajo.util;

import com.google.common.primitives.Longs;
import com.google.common.primitives.UnsignedBytes;
import com.google.common.primitives.UnsignedLongs;
import sun.misc.Unsafe;

import java.lang.reflect.Field;
import java.nio.ByteOrder;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.Comparator;

/**
 * This is borrowed from Guava's UnsignedBytes.
 */
public class UnsafeComparer implements Comparator<byte[]> {

    public static final UnsafeComparer INSTANCE;

    static {
        INSTANCE = new UnsafeComparer();
    }

    public Comparator<byte[]> get() {
        return INSTANCE;
    }

    private UnsafeComparer() {
    }

    static final boolean littleEndian = ByteOrder.nativeOrder().equals(ByteOrder.LITTLE_ENDIAN);

    /*
     * The following static final fields exist for performance reasons.
     *
     * In UnsignedBytesBenchmark, accessing the following objects via static
     * final fields is the fastest (more than twice as fast as the Java
     * implementation, vs ~1.5x with non-final static fields, on x86_32)
     * under the Hotspot server compiler. The reason is obviously that the
     * non-final fields need to be reloaded inside the loop.
     *
     * And, no, defining (final or not) local variables out of the loop still
     * isn't as good because the null check on the theUnsafe object remains
     * inside the loop and BYTE_ARRAY_BASE_OFFSET doesn't get
     * constant-folded.
     *
     * The compiler can treat static final fields as compile-time constants
     * and can constant-fold them while (final or not) local variables are
     * run time values.
     */

    static final Unsafe theUnsafe;

    /** The offset to the first element in a byte array. */
    static final int BYTE_ARRAY_BASE_OFFSET;

    static {
        theUnsafe = (Unsafe) AccessController.doPrivileged(new PrivilegedAction<Object>() {
            @Override
            public Object run() {
                try {
                    Field f = Unsafe.class.getDeclaredField("theUnsafe");
                    f.setAccessible(true);
                    return f.get(null);
                } catch (NoSuchFieldException e) {
                    // It doesn't matter what we throw;
                    // it's swallowed in getBestComparator().
                    throw new Error();
                } catch (IllegalAccessException e) {
                    throw new Error();
                }
            }
        });

        BYTE_ARRAY_BASE_OFFSET = theUnsafe.arrayBaseOffset(byte[].class);

        // sanity check - this should never fail
        if (theUnsafe.arrayIndexScale(byte[].class) != 1) {
            throw new AssertionError();
        }
    }

    @SuppressWarnings("unused")
    public static int compareStatic(byte[] left, byte[] right) {
        return INSTANCE.compare(left, right);
    }

    @Override
    public int compare(byte[] left, byte[] right) {
        int minLength = Math.min(left.length, right.length);
        int minWords = minLength / Longs.BYTES;

        /*
         * Compare 8 bytes at a time. Benchmarking shows comparing 8 bytes at a
         * time is no slower than comparing 4 bytes at a time even on 32-bit.
         * On the other hand, it is substantially faster on 64-bit.
         */
        for (int i = 0; i < minWords * Longs.BYTES; i += Longs.BYTES) {
            long lw = theUnsafe.getLong(left, BYTE_ARRAY_BASE_OFFSET + (long) i);
            long rw = theUnsafe.getLong(right, BYTE_ARRAY_BASE_OFFSET + (long) i);
            long diff = lw ^ rw;

            if (diff != 0) {
                if (!littleEndian) {
                    return UnsignedLongs.compare(lw, rw);
                }

                // Use binary search
                int n = 0;
                int y;
                int x = (int) diff;
                if (x == 0) {
                    x = (int) (diff >>> 32);
                    n = 32;
                }

                y = x << 16;
                if (y == 0) {
                    n += 16;
                } else {
                    x = y;
                }

                y = x << 8;
                if (y == 0) {
                    n += 8;
                }
                return (int) (((lw >>> n) & 0xFFL) - ((rw >>> n) & 0xFFL));
            }
        }

        // The epilogue to cover the last (minLength % 8) elements.
        for (int i = minWords * Longs.BYTES; i < minLength; i++) {
            int result = UnsignedBytes.compare(left[i], right[i]);
            if (result != 0) {
                return result;
            }
        }
        return left.length - right.length;
    }
}