Bit manipulation and output in Java

Question

If you have binary strings (literally String objects that contain only 1's and 0's), how would you output them as bits into a file?

This is for a text compressor I was working on; it's still bugging me, and it'd be nice to finally get it working. Thanks!

Answer 1

Easiest is to simply take 8 consecutive characters, turn them into a byte and output that byte. Pad with zeros at the end if you can recognize the end-of-stream, or add a header with length (in bits) at the beginning of the file.

The inner loop would look something like:

byte[] buffer = new byte[ ( string.length + 7 ) / 8 ];
for ( int i = 0; i < buffer.length; ++i ) {
   byte current = 0;
   for ( int j = 7; j >= 0; --j )
       if ( string[ i * 8 + j ] == '1' )
           current |= 1 << j;
   output( current );
}

You'll need to make some adjustments, but that's the general idea.

Answer 2

If you're lucky, java.math.BigInteger may do everything for you.

String s = "11001010001010101110101001001110";
byte[] bytes = (new java.math.BigInteger(s, 2)).toByteArray();

This does depend on the byte order (big-endian) and right-aligning (if the number of bits is not a multiple of 8) being what you want but it may be simpler to modify the array afterwards than to do the character conversion yourself.

Answer 3

public class BitOutputStream extends FilterOutputStream
{
    private int buffer   = 0;
    private int bitCount = 0;

    public BitOutputStream(OutputStream out)
    {
        super(out);
    }

    public void writeBits(int value, int numBits) throws IOException
    {
        while(numBits>0)
        {
            numBits--;
            int mix = ((value&1)<<bitCount++);
            buffer|=mix;
            value>>=1;
            if(bitCount==8)
                align8();
        }
    }

    @Override
    public void close() throws IOException
    {
        align8(); /* Flush any remaining partial bytes */
        super.close();
    }

    public void align8() throws IOException
    {
        if(bitCount > 0)
        {
            bitCount=0;
            write(buffer);
            buffer=0;
        }
    }
}

And then...

if (nextChar == '0')
{
    bos.writeBits(0, 1);
}
else
{
    bos.writeBits(1, 1);
}

Answer 4

Assuming the String has a multiple of eight bits, (you can pad it otherwise), take advantage of Java's built in parsing in the Integer.valueOf method to do something like this:

String s = "11001010001010101110101001001110";
byte[] data = new byte[s.length() / 8];
for (int i = 0; i < data.length; i++) {
    data[i] = (byte) Integer.parseInt(s.substring(i * 8, (i + 1) * 8), 2);
}

Then you should be able to write the bytes to a FileOutputStream pretty simply.

On the other hand, if you looking for effeciency, you should consider not using a String to store the bits to begin with, but build up the bytes directly in your compressor.