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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.dump;

import java.io.IOException;
import java.util.Arrays;
import org.apache.commons.compress.archivers.zip.ZipEncoding;
import org.apache.commons.compress.utils.ByteUtils;

/**
 * Various utilities for dump archives.
 */
class DumpArchiveUtil {
    /**
     * Private constructor to prevent instantiation.
     */
    private DumpArchiveUtil() {
    }

    /**
     * Calculate checksum for buffer.
     *
     * @param buffer buffer containing tape segment header
     * @returns checksum
     */
    public static int calculateChecksum(final byte[] buffer) {
        int calc = 0;

        for (int i = 0; i < 256; i++) {
            calc += DumpArchiveUtil.convert32(buffer, 4 * i);
        }

        return DumpArchiveConstants.CHECKSUM -
        (calc - DumpArchiveUtil.convert32(buffer, 28));
    }

    /**
     * Verify that the buffer contains a tape segment header.
     *
     * @param buffer
     */
    public static final boolean verify(final byte[] buffer) {
        // verify magic. for now only accept NFS_MAGIC.
        final int magic = convert32(buffer, 24);

        if (magic != DumpArchiveConstants.NFS_MAGIC) {
            return false;
        }

        //verify checksum...
        final int checksum = convert32(buffer, 28);

        return checksum == calculateChecksum(buffer);
    }

    /**
     * Get the ino associated with this buffer.
     *
     * @param buffer
     */
    public static final int getIno(final byte[] buffer) {
        return convert32(buffer, 20);
    }

    /**
     * Read 8-byte integer from buffer.
     *
     * @param buffer
     * @param offset
     * @return the 8-byte entry as a long
     */
    public static final long convert64(final byte[] buffer, final int offset) {
        return ByteUtils.fromLittleEndian(buffer, offset, 8);
    }

    /**
     * Read 4-byte integer from buffer.
     *
     * @param buffer
     * @param offset
     * @return the 4-byte entry as an int
     */
    public static final int convert32(final byte[] buffer, final int offset) {
        return (int) ByteUtils.fromLittleEndian(buffer, offset, 4);
    }

    /**
     * Read 2-byte integer from buffer.
     *
     * @param buffer
     * @param offset
     * @return the 2-byte entry as an int
     */
    public static final int convert16(final byte[] buffer, final int offset) {
        return (int) ByteUtils.fromLittleEndian(buffer, offset, 2);
    }

    /**
     * Decodes a byte array to a string.
     */
    static String decode(final ZipEncoding encoding, final byte[] b, final int offset, final int len)
        throws IOException {
        return encoding.decode(Arrays.copyOfRange(b, offset, offset + len));
    }
}