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

/**
 * Reads bits from an InputStream.
 * @since 1.10
 * @NotThreadSafe
 */
public class BitInputStream implements Closeable {
    private static final int MAXIMUM_CACHE_SIZE = 63; // bits in long minus sign bit
    private static final long[] MASKS = new long[MAXIMUM_CACHE_SIZE + 1];

    static {
        for (int i = 1; i <= MAXIMUM_CACHE_SIZE; i++) {
            MASKS[i] = (MASKS[i - 1] << 1) + 1;
        }
    }

    private final InputStream in;
    private final ByteOrder byteOrder;
    private long bitsCached = 0;
    private int bitsCachedSize = 0;

    /**
     * Constructor taking an InputStream and its bit arrangement. 
     * @param in the InputStream
     * @param byteOrder the bit arrangement across byte boundaries,
     *      either BIG_ENDIAN (aaaaabbb bb000000) or LITTLE_ENDIAN (bbbaaaaa 000000bb)
     */
    public BitInputStream(final InputStream in, final ByteOrder byteOrder) {
        this.in = in;
        this.byteOrder = byteOrder;
    }
    
    public void close() throws IOException {
        in.close();
    }
    
    /**
     * Clears the cache of bits that have been read from the
     * underlying stream but not yet provided via {@link #readBits}.
     */
    public void clearBitCache() {
        bitsCached = 0;
        bitsCachedSize = 0;
    }
    
    /**
     * Returns at most 63 bits read from the underlying stream.
     *
     * @param count the number of bits to read, must be a positive
     * number not bigger than 63.
     * @return the bits concatenated as a long using the stream's byte order.
     *         -1 if the end of the underlying stream has been reached before reading
     *         the requested number of bits
     * @throws IOException on error
     */
    public long readBits(final int count) throws IOException {
        if (count < 0 || count > MAXIMUM_CACHE_SIZE) {
            throw new IllegalArgumentException("count must not be negative or greater than " + MAXIMUM_CACHE_SIZE);
        }
        while (bitsCachedSize < count) {
            final long nextByte = in.read();
            if (nextByte < 0) {
                return nextByte;
            }
            if (byteOrder == ByteOrder.LITTLE_ENDIAN) {
                bitsCached |= (nextByte << bitsCachedSize);
            } else {
                bitsCached <<= 8;
                bitsCached |= nextByte;
            }
            bitsCachedSize += 8;
        }
        
        final long bitsOut;
        if (byteOrder == ByteOrder.LITTLE_ENDIAN) {
            bitsOut = (bitsCached & MASKS[count]);
            bitsCached >>>= count;
        } else {
            bitsOut = (bitsCached >> (bitsCachedSize - count)) & MASKS[count];
        }
        bitsCachedSize -= count;
        return bitsOut;
    }
}