UnshrinkingInputStream.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.
 */
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
 */
final 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;

    /**
     * IOException is not actually thrown!
     *
     * @param inputStream
     */
    UnshrinkingInputStream(final InputStream inputStream) {
        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(final int previousCode, final byte character) throws IOException {
        int tableSize = getTableSize();
        while (tableSize < MAX_TABLE_SIZE && isUsed[tableSize]) {
            tableSize++;
        }
        setTableSize(tableSize);
        final int idx = addEntry(previousCode, character, MAX_TABLE_SIZE);
        if (idx >= 0) {
            isUsed[idx] = true;
        }
        return idx;
    }

    @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;
        }
        if (code != getClearCode()) {
            boolean addedUnfinishedEntry = false;
            int effectiveCode = code;
            if (!isUsed[code]) {
                effectiveCode = addRepeatOfPreviousCode();
                addedUnfinishedEntry = true;
            }
            return expandCodeToOutputStack(effectiveCode, addedUnfinishedEntry);
        }
        final int subCode = readNextCode();
        if (subCode < 0) {
            throw new IOException("Unexpected EOF;");
        }
        if (subCode == 1) {
            if (getCodeSize() >= MAX_CODE_SIZE) {
                throw new IOException("Attempt to increase code size beyond maximum");
            }
            incrementCodeSize();
        } else if (subCode == 2) {
            partialClear();
            setTableSize(getClearCode() + 1);
        } else {
            throw new IOException("Invalid clear code subcode " + subCode);
        }
        return 0;
    }

    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);
            }
        }
    }
}