BitOutputStream.java

package nayuki.arithcode;

import java.io.IOException;
import java.io.OutputStream;


/**
 * A stream where bits can be written to. Because they are written to an underlying byte stream, the end of the stream is padded with 0's up to a multiple of 8 bits. The bits are written in big endian.
 */
public final class BitOutputStream {
	
	// Underlying byte stream to write to.
	private OutputStream output;
	
	// The accumulated bits for the current byte. Always in the range 0x00 to 0xFF.
	private byte currentByte;
	
	// The number of accumulated bits in the current byte. Always between 0 and 7 (inclusive).
	private int numBitsInCurrentByte;
	
	
	
	// Creates a bit output stream based on the given byte output stream.
	public BitOutputStream(OutputStream out) {
		if (out == null)
			throw new NullPointerException("Argument is null");
		output = out;
		currentByte = (byte) 0;
		numBitsInCurrentByte = 0;
	}
	
	
	
	// Writes a bit to the stream. The specified bit must be 0 or 1.
	public void write(byte b) throws IOException {
		if (!(b == 0 || b == 1))
			throw new IllegalArgumentException("Argument must be 0 or 1");
		currentByte = (byte)(currentByte << 1 | b);  // shift will implicitly convert byte to int!!!
		numBitsInCurrentByte++;
		if (numBitsInCurrentByte == 8) {
			output.write(currentByte);
			numBitsInCurrentByte = 0;
			currentByte = (byte) 0;
		}
	}
	
	
	// Closes this stream and the underlying OutputStream. If called when this bit stream is not at a byte boundary,
	// then the minimum number of "0" bits (between 0 and 7 of them) are written as padding to reach the next byte boundary.
	public void close() throws IOException {
		while (numBitsInCurrentByte != 0)
			write((byte)0);
		output.close();
	}
	
}