org.apache.lucene.util.LSBRadixSorter.java Source code

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Here is the source code for org.apache.lucene.util.LSBRadixSorter.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.lucene.util;

import java.util.Arrays;

/**
 * A LSB Radix sorter for unsigned int values.
 * @lucene.internal
 */
public final class LSBRadixSorter {

    private static final int INSERTION_SORT_THRESHOLD = 30;
    private static final int HISTOGRAM_SIZE = 256;

    private final int[] histogram = new int[HISTOGRAM_SIZE];
    private int[] buffer = new int[0];

    private static void buildHistogram(int[] array, int len, int[] histogram, int shift) {
        for (int i = 0; i < len; ++i) {
            final int b = (array[i] >>> shift) & 0xFF;
            histogram[b] += 1;
        }
    }

    private static void sumHistogram(int[] histogram) {
        int accum = 0;
        for (int i = 0; i < HISTOGRAM_SIZE; ++i) {
            final int count = histogram[i];
            histogram[i] = accum;
            accum += count;
        }
    }

    private static void reorder(int[] array, int len, int[] histogram, int shift, int[] dest) {
        for (int i = 0; i < len; ++i) {
            final int v = array[i];
            final int b = (v >>> shift) & 0xFF;
            dest[histogram[b]++] = v;
        }
    }

    private static boolean sort(int[] array, int len, int[] histogram, int shift, int[] dest) {
        Arrays.fill(histogram, 0);
        buildHistogram(array, len, histogram, shift);
        if (histogram[0] == len) {
            return false;
        }
        sumHistogram(histogram);
        reorder(array, len, histogram, shift, dest);
        return true;
    }

    private static void insertionSort(int[] array, int off, int len) {
        for (int i = off + 1, end = off + len; i < end; ++i) {
            for (int j = i; j > off; --j) {
                if (array[j - 1] > array[j]) {
                    int tmp = array[j - 1];
                    array[j - 1] = array[j];
                    array[j] = tmp;
                } else {
                    break;
                }
            }
        }
    }

    /** Sort {@code array[0:len]} in place.
     * @param numBits how many bits are required to store any of the values in
     *                {@code array[0:len]}. Pass {@code 32} if unknown. */
    public void sort(int numBits, final int[] array, int len) {
        if (len < INSERTION_SORT_THRESHOLD) {
            insertionSort(array, 0, len);
            return;
        }

        buffer = ArrayUtil.grow(buffer, len);

        int[] arr = array;

        int[] buf = buffer;

        for (int shift = 0; shift < numBits; shift += 8) {
            if (sort(arr, len, histogram, shift, buf)) {
                // swap arrays
                int[] tmp = arr;
                arr = buf;
                buf = tmp;
            }
        }

        if (array == buf) {
            System.arraycopy(arr, 0, array, 0, len);
        }
    }

}