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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.utils;

import java.io.IOException;
import java.io.InputStream;
import java.util.zip.CheckedInputStream;
import java.util.zip.Checksum;

/**
 * Verifies the checksum of the data read once the stream is exhausted.
 *
 * @NotThreadSafe
 * @since 1.7
 */
public class ChecksumVerifyingInputStream extends CheckedInputStream {
    // TODO Use Commons IO 2.16.0 ChecksumInputStream

    private long remaining;
    private final long expected;

    /**
     * Constructs a new instance.
     *
     * @param checksum         Checksum implementation.
     * @param in               the stream to wrap
     * @param size             the of the stream's content
     * @param expectedChecksum the expected checksum
     */
    public ChecksumVerifyingInputStream(final Checksum checksum, final InputStream in, final long size, final long expectedChecksum) {
        super(in, checksum);
        this.expected = expectedChecksum;
        this.remaining = size;
    }

    /**
     * Gets the byte count remaining to read.
     *
     * @return bytes remaining to read.
     * @since 1.21
     */
    public long getBytesRemaining() {
        return remaining;
    }

    /**
     * Reads a single byte from the stream
     *
     * @throws IOException if the underlying stream throws or the stream is exhausted and the Checksum doesn't match the expected value
     */
    @Override
    public int read() throws IOException {
        if (remaining <= 0) {
            return -1;
        }
        final int data = super.read();
        if (data >= 0) {
            --remaining;
        }
        verify();
        return data;
    }

    /**
     * Reads from the stream into a byte array.
     *
     * @throws IOException if the underlying stream throws or the stream is exhausted and the Checksum doesn't match the expected value
     */
    @Override
    public int read(final byte[] b, final int off, final int len) throws IOException {
        if (len == 0) {
            return 0;
        }
        final int readCount = super.read(b, off, len);
        if (readCount >= 0) {
            remaining -= readCount;
        }
        verify();
        return readCount;
    }

    private void verify() throws IOException {
        if (remaining <= 0 && expected != getChecksum().getValue()) {
            throw new IOException("Checksum verification failed");
        }
    }
}