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/*

* Reference arithmetic coding

*

* Copyright (c) Project Nayuki

* MIT License. See readme file.

* https://www.nayuki.io/page/reference-arithmetic-coding

*/

import java.io.EOFException;

import java.io.IOException;

import java.io.InputStream;

import java.util.Objects;

/**

* A stream of bits that can be read. Because they come from an underlying byte stream,

* the total number of bits is always a multiple of 8. The bits are read in big endian.

* Mutable and not thread-safe.

* @see BitOutputStream

*/

public final class BitInputStream implements AutoCloseable {

/*---- Fields ----*/

// The underlying byte stream to read from (not null).

private InputStream input;

// Either in the range [0x00, 0xFF] if bits are available, or -1 if end of stream is reached.

private int currentByte;

// Number of remaining bits in the current byte, always between 0 and 7 (inclusive).

private int numBitsRemaining;

/*---- Constructor ----*/

/**

* Constructs a bit input stream based on the specified byte input stream.

* @param in the byte input stream

* @throws NullPointerException if the input stream is {@code null}

*/

public BitInputStream(InputStream in) {

input = Objects.requireNonNull(in);

currentByte = 0;

numBitsRemaining = 0;

}

/*---- Methods ----*/

/**

* Reads a bit from this stream. Returns 0 or 1 if a bit is available, or -1 if

* the end of stream is reached. The end of stream always occurs on a byte boundary.

* @return the next bit of 0 or 1, or -1 for the end of stream

* @throws IOException if an I/O exception occurred

*/

public int read() throws IOException {

if (currentByte == -1)

return -1;

if (numBitsRemaining == 0) {

currentByte = input.read();

if (currentByte == -1)

return -1;

numBitsRemaining = 8;

}

if (numBitsRemaining <= 0)

throw new AssertionError();

numBitsRemaining--;

return (currentByte >>> numBitsRemaining) & 1;

}

/**

* Reads a bit from this stream. Returns 0 or 1 if a bit is available, or throws an {@code EOFException}

* if the end of stream is reached. The end of stream always occurs on a byte boundary.

* @return the next bit of 0 or 1

* @throws IOException if an I/O exception occurred

* @throws EOFException if the end of stream is reached

*/

public int readNoEof() throws IOException {

int result = read();

if (result != -1)

return result;

else

throw new EOFException();

}

/**

* Closes this stream and the underlying input stream.

* @throws IOException if an I/O exception occurred

*/

public void close() throws IOException {

input.close();

currentByte = -1;

numBitsRemaining = 0;

}

}