httpcore5-h2/src/main/java/org/apache/hc/core5/http2/hpack/HuffmanDecoder.java

/*
 * ====================================================================
 * 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.
 * ====================================================================
 *
 * This software consists of voluntary contributions made by many
 * individuals on behalf of the Apache Software Foundation.  For more
 * information on the Apache Software Foundation, please see
 * <http://www.apache.org/>.
 *
 */

package org.apache.hc.core5.http2.hpack;

import java.nio.ByteBuffer;

import org.apache.hc.core5.util.ByteArrayBuffer;

/**
 * This Huffman codec implementation has been derived from Twitter HPack project
 * (https://github.com/twitter/hpack)
 */
final class HuffmanDecoder {

    private final HuffmanNode root;

    HuffmanDecoder(final int[] codes, final byte[] lengths) {
        root = buildTree(codes, lengths);
    }

    void decode(final ByteArrayBuffer out, final ByteBuffer src) throws HPackException {
        HuffmanNode node = this.root;
        int current = 0;
        int bits = 0;
        while (src.hasRemaining()) {
            final int b = src.get() & 0xFF;
            current = (current << 8) | b;
            bits += 8;
            while (bits >= 8) {
                final int c = (current >>> (bits - 8)) & 0xFF;
                node = node.getChild(c);
                bits -= node.getBits();
                if (node.isTerminal()) {
                    if (node.getSymbol() == Huffman.EOS) {
                        throw new HPackException("EOS decoded");
                    }
                    out.append(node.getSymbol());
                    node = root;
                }
            }
        }

        while (bits > 0) {
            final int c = (current << (8 - bits)) & 0xFF;
            node = node.getChild(c);
            if (node.isTerminal() && node.getBits() <= bits) {
                bits -= node.getBits();
                out.append(node.getSymbol());
                node = this.root;
            } else {
                break;
            }
        }

        // Section 5.2. String Literal Representation
        // Padding not corresponding to the most significant bits of the code
        // for the EOS symbol (0xFF) MUST be treated as a decoding error.
        final int mask = (1 << bits) - 1;
        if ((current & mask) != mask) {
            throw new HPackException("Invalid padding");
        }
    }

    private static HuffmanNode buildTree(final int[] codes, final byte[] lengths) {
        final HuffmanNode root = new HuffmanNode();
        for (int symbol = 0; symbol < codes.length; symbol++) {

            final int code = codes[symbol];
            int length = lengths[symbol];

            HuffmanNode current = root;
            while (length > 8) {
                if (current.isTerminal()) {
                    throw new IllegalStateException("Invalid Huffman code: prefix not unique");
                }
                length -= 8;
                final int i = (code >>> length) & 0xFF;
                if (!current.hasChild(i)) {
                    current.setChild(i, new HuffmanNode());
                }
                current = current.getChild(i);
            }

            final HuffmanNode terminal = new HuffmanNode(symbol, length);
            final int shift = 8 - length;
            final int start = (code << shift) & 0xFF;
            final int end = 1 << shift;
            for (int i = start; i < start + end; i++) {
                current.setChild(i, terminal);
            }
        }
        return root;
    }

}