netty5/codec/src/main/java/io/netty/handler/codec/compression/Bzip2HuffmanAllocator.java

/*
 * Copyright 2014 The Netty Project
 *
 * The Netty Project 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 io.netty.handler.codec.compression;

/**
 * An in-place, length restricted Canonical Huffman code length allocator.<br>
 * Based on the algorithm proposed by R. L. Milidi'u, A. A. Pessoa and E. S. Laber in
 * <a href="http://www-di.inf.puc-rio.br/~laber/public/spire98.ps">In-place Length-Restricted Prefix Coding</a>
 * and incorporating additional ideas from the implementation of
 * <a href="http://entropyware.info/shcodec/index.html">shcodec</a> by Simakov Alexander.
 */
final class Bzip2HuffmanAllocator {
    /**
     * @param array The code length array
     * @param i The input position
     * @param nodesToMove The number of internal nodes to be relocated
     * @return The smallest {@code k} such that {@code nodesToMove <= k <= i} and
     *         {@code i <= (array[k] % array.length)}
     */
    private static int first(final int[] array, int i, final int nodesToMove) {
        final int length = array.length;
        final int limit = i;
        int k = array.length - 2;

        while (i >= nodesToMove && array[i] % length > limit) {
            k = i;
            i -= limit - i + 1;
        }
        i = Math.max(nodesToMove - 1, i);

        while (k > i + 1) {
            int temp = i + k >> 1;
            if (array[temp] % length > limit) {
                k = temp;
            } else {
                i = temp;
            }
        }
        return k;
    }

    /**
     * Fills the code array with extended parent pointers.
     * @param array The code length array
     */
    private static void setExtendedParentPointers(final int[] array) {
        final int length = array.length;
        array[0] += array[1];

        for (int headNode = 0, tailNode = 1, topNode = 2; tailNode < length - 1; tailNode++) {
            int temp;
            if (topNode >= length || array[headNode] < array[topNode]) {
                temp = array[headNode];
                array[headNode++] = tailNode;
            } else {
                temp = array[topNode++];
            }

            if (topNode >= length || (headNode < tailNode && array[headNode] < array[topNode])) {
                temp += array[headNode];
                array[headNode++] = tailNode + length;
            } else {
                temp += array[topNode++];
            }
            array[tailNode] = temp;
        }
    }

    /**
     * Finds the number of nodes to relocate in order to achieve a given code length limit.
     * @param array The code length array
     * @param maximumLength The maximum bit length for the generated codes
     * @return The number of nodes to relocate
     */
    private static int findNodesToRelocate(final int[] array, final int maximumLength) {
        int currentNode = array.length - 2;
        for (int currentDepth = 1; currentDepth < maximumLength - 1 && currentNode > 1; currentDepth++) {
            currentNode =  first(array, currentNode - 1, 0);
        }
        return currentNode;
    }

    /**
     * A final allocation pass with no code length limit.
     * @param array The code length array
     */
    private static void allocateNodeLengths(final int[] array) {
        int firstNode = array.length - 2;
        int nextNode = array.length - 1;

        for (int currentDepth = 1, availableNodes = 2; availableNodes > 0; currentDepth++) {
            final int lastNode = firstNode;
            firstNode = first(array, lastNode - 1, 0);

            for (int i = availableNodes - (lastNode - firstNode); i > 0; i--) {
                array[nextNode--] = currentDepth;
            }

            availableNodes = (lastNode - firstNode) << 1;
        }
    }

    /**
     * A final allocation pass that relocates nodes in order to achieve a maximum code length limit.
     * @param array The code length array
     * @param nodesToMove The number of internal nodes to be relocated
     * @param insertDepth The depth at which to insert relocated nodes
     */
    private static void allocateNodeLengthsWithRelocation(final int[] array,
                                                           final int nodesToMove, final int insertDepth) {
        int firstNode = array.length - 2;
        int nextNode = array.length - 1;
        int currentDepth = insertDepth == 1 ? 2 : 1;
        int nodesLeftToMove = insertDepth == 1 ? nodesToMove - 2 : nodesToMove;

        for (int availableNodes = currentDepth << 1; availableNodes > 0; currentDepth++) {
            final int lastNode = firstNode;
            firstNode = firstNode <= nodesToMove ? firstNode : first(array, lastNode - 1, nodesToMove);

            int offset = 0;
            if (currentDepth >= insertDepth) {
                offset = Math.min(nodesLeftToMove, 1 << (currentDepth - insertDepth));
            } else if (currentDepth == insertDepth - 1) {
                offset = 1;
                if (array[firstNode] == lastNode) {
                    firstNode++;
                }
            }

            for (int i = availableNodes - (lastNode - firstNode + offset); i > 0; i--) {
                array[nextNode--] = currentDepth;
            }

            nodesLeftToMove -= offset;
            availableNodes = (lastNode - firstNode + offset) << 1;
        }
    }

    /**
     * Allocates Canonical Huffman code lengths in place based on a sorted frequency array.
     * @param array On input, a sorted array of symbol frequencies; On output, an array of Canonical
     *              Huffman code lengths
     * @param maximumLength The maximum code length. Must be at least {@code ceil(log2(array.length))}
     */
    static void allocateHuffmanCodeLengths(final int[] array, final int maximumLength) {
        switch (array.length) {
            case 2:
                array[1] = 1;
            case 1:
                array[0] = 1;
                return;
        }

        /* Pass 1 : Set extended parent pointers */
        setExtendedParentPointers(array);

        /* Pass 2 : Find number of nodes to relocate in order to achieve maximum code length */
        int nodesToRelocate = findNodesToRelocate(array, maximumLength);

        /* Pass 3 : Generate code lengths */
        if (array[0] % array.length >= nodesToRelocate) {
            allocateNodeLengths(array);
        } else {
            int insertDepth = maximumLength - (32 - Integer.numberOfLeadingZeros(nodesToRelocate - 1));
            allocateNodeLengthsWithRelocation(array, nodesToRelocate, insertDepth);
        }
    }

    private Bzip2HuffmanAllocator() { }
}