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Apache Commons Compress defines an API for working with compression and archive formats. These include bzip2, gzip, pack200, LZMA, XZ, Snappy, traditional Unix Compress, DEFLATE, DEFLATE64, LZ4, Brotli, Zstandard and ar, cpio, jar, tar, zip, dump, 7z, arj.

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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.commons.compress.archivers.zip;

import java.io.IOException;
import java.io.InputStream;
import java.nio.ByteOrder;

import org.apache.commons.compress.compressors.lzw.LZWInputStream;

/**
 * Input stream that decompresses ZIP method 1 (unshrinking). A variation of the LZW algorithm, with some twists.
 * @NotThreadSafe
 * @since 1.7
 */
class UnshrinkingInputStream extends LZWInputStream {
    private static final int MAX_CODE_SIZE = 13;
    private static final int MAX_TABLE_SIZE = 1 << MAX_CODE_SIZE;
    private final boolean[] isUsed;
    
    public UnshrinkingInputStream(InputStream inputStream) throws IOException {
        super(inputStream, ByteOrder.LITTLE_ENDIAN);
        setClearCode(DEFAULT_CODE_SIZE);
        initializeTables(MAX_CODE_SIZE);
        isUsed = new boolean[getPrefixesLength()];
        for (int i = 0; i < (1 << 8); i++) {
            isUsed[i] = true;
        }
        setTableSize(getClearCode() + 1);
    }

    @Override
    protected int addEntry(int previousCode, byte character) throws IOException {
        int tableSize = getTableSize();
        while ((tableSize < MAX_TABLE_SIZE) && isUsed[tableSize]) {
            tableSize++;
        }
        setTableSize(tableSize);
        int idx = addEntry(previousCode, character, MAX_TABLE_SIZE);
        if (idx >= 0) {
            isUsed[idx] = true;
        }
        return idx;
    }
    
    private void partialClear() {
        final boolean[] isParent = new boolean[MAX_TABLE_SIZE];
        for (int i = 0; i < isUsed.length; i++) {
            if (isUsed[i] && getPrefix(i) != UNUSED_PREFIX) {
                isParent[getPrefix(i)] = true;
            }
        }
        for (int i = getClearCode() + 1; i < isParent.length; i++) {
            if (!isParent[i]) {
                isUsed[i] = false;
                setPrefix(i, UNUSED_PREFIX);
            }
        }
    }

    @Override
    protected int decompressNextSymbol() throws IOException {
        //
        //                   table entry    table entry
        //                  _____________   _____
        //    table entry  /             \ /     \
        //    ____________/               \       \
        //   /           / \             / \       \
        //  +---+---+---+---+---+---+---+---+---+---+
        //  | . | . | . | . | . | . | . | . | . | . |
        //  +---+---+---+---+---+---+---+---+---+---+
        //  |<--------->|<------------->|<----->|<->|
        //     symbol        symbol      symbol  symbol
        //
        final int code = readNextCode();
        if (code < 0) {
            return -1;
        } else if (code == getClearCode()) {
            final int subCode = readNextCode();
            if (subCode < 0) {
                throw new IOException("Unexpected EOF;");
            } else if (subCode == 1) {
                if (getCodeSize() < MAX_CODE_SIZE) {
                    incrementCodeSize();
                } else {
                    throw new IOException("Attempt to increase code size beyond maximum");
                }
            } else if (subCode == 2) {
                partialClear();
                setTableSize(getClearCode() + 1);
            } else {
                throw new IOException("Invalid clear code subcode " + subCode);
            }
            return 0;
        } else {
            boolean addedUnfinishedEntry = false;
            int effectiveCode = code;
            if (!isUsed[code]) {
                effectiveCode = addRepeatOfPreviousCode();
                addedUnfinishedEntry = true;
            }
            return expandCodeToOutputStack(effectiveCode, addedUnfinishedEntry);
        }
    }
}