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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.util.zip.ZipException;

/**
 * An extra field who's sole purpose is to align and pad the local file header
 * so that the entry's data starts at a certain position.
 *
 * The padding content of the padding is ignored and not retained
 * when reading a padding field.
 *
 * This enables Commons Compress to create "aligned" archives
 * similar to Android's zipalign command line tool.
 *
 * @since 1.14
 * @see "https://developer.android.com/studio/command-line/zipalign.html"
 * @see ZipArchiveEntry#setAlignment
 */
public class ResourceAlignmentExtraField implements ZipExtraField {

    /**
     * Extra field id used for storing alignment and padding.
     */
    public static final ZipShort ID = new ZipShort(0xa11e);

    public static final int BASE_SIZE = 2;

    private static final int ALLOW_METHOD_MESSAGE_CHANGE_FLAG = 0x8000;

    private short alignment;

    private boolean allowMethodChange;

    private int padding = 0;

    public ResourceAlignmentExtraField() {
    }

    public ResourceAlignmentExtraField(int alignment) {
        this(alignment, false);
    }

    public ResourceAlignmentExtraField(int alignment, boolean allowMethodChange) {
        this(alignment, allowMethodChange, 0);
    }

    public ResourceAlignmentExtraField(int alignment, boolean allowMethodChange, int padding) {
        if (alignment < 0 || alignment > 0x7fff) {
            throw new IllegalArgumentException("Alignment must be between 0 and 0x7fff, was: " + alignment);
        }
        this.alignment = (short) alignment;
        this.allowMethodChange = allowMethodChange;
        this.padding = padding;
    }

    /**
     * Gets requested alignment.
     *
     * @return
     *      requested alignment.
     */
    public short getAlignment() {
        return alignment;
    }

    /**
     * Indicates whether method change is allowed when re-compressing the zip file.
     *
     * @return
     *      true if method change is allowed, false otherwise.
     */
    public boolean allowMethodChange() {
        return allowMethodChange;
    }

    @Override
    public ZipShort getHeaderId() {
        return ID;
    }

    @Override
    public ZipShort getLocalFileDataLength() {
        return new ZipShort(BASE_SIZE + padding);
    }

    @Override
    public ZipShort getCentralDirectoryLength() {
        return new ZipShort(BASE_SIZE);
    }

    @Override
    public byte[] getLocalFileDataData() {
        byte[] content = new byte[BASE_SIZE + padding];
        ZipShort.putShort(alignment | (allowMethodChange ? ALLOW_METHOD_MESSAGE_CHANGE_FLAG : 0),
                          content, 0);
        return content;
    }

    @Override
    public byte[] getCentralDirectoryData() {
        return ZipShort.getBytes(alignment | (allowMethodChange ? ALLOW_METHOD_MESSAGE_CHANGE_FLAG : 0));
    }

    @Override
    public void parseFromLocalFileData(byte[] buffer, int offset, int length) throws ZipException {
        parseFromCentralDirectoryData(buffer, offset, length);
        this.padding = length - BASE_SIZE;
    }

    @Override
    public void parseFromCentralDirectoryData(byte[] buffer, int offset, int length) throws ZipException {
        if (length < BASE_SIZE) {
            throw new ZipException("Too short content for ResourceAlignmentExtraField (0xa11e): " + length);
        }
        int alignmentValue = ZipShort.getValue(buffer, offset);
        this.alignment = (short) (alignmentValue & (ALLOW_METHOD_MESSAGE_CHANGE_FLAG - 1));
        this.allowMethodChange = (alignmentValue & ALLOW_METHOD_MESSAGE_CHANGE_FLAG) != 0;
    }
}