Shifting a Java BitSet

Question

I am using a java.util.BitSet to store a dense vector of bits.

I want to implement an operation that shifts the bits right by 1, analogous to >

Answer 1

In order to achieve better performance you can extend java.util.BitSet implementation and avoid unnecessary array copying. Here is implementation (I've basically reused Jeff Piersol implementation):

package first.specific.structure;

import java.lang.reflect.Field;
import java.util.BitSet;

public class BitSetMut extends BitSet {

    private long[] words;
    private static Field wordsField;

    static {
        try {
            wordsField = BitSet.class.getDeclaredField("words");
            wordsField.setAccessible(true);
        } catch (NoSuchFieldException e) {
            throw new IllegalStateException(e);
        }
    }

    public BitSetMut(final int regLength) {
        super(regLength);
        try {
            words = (long[]) wordsField.get(this);
        } catch (IllegalAccessException e) {
            throw new IllegalStateException(e);
        }
    }

    public void shiftRight(int n) {
        if (n < 0)
            throw new IllegalArgumentException("'n' must be >= 0");
        if (n >= 64)
            throw new IllegalArgumentException("'n' must be < 64");

        if (words.length > 0) {
            ensureCapacity(n);

            // Do the shift
            for (int i = words.length - 1; i > 0; i--) {
                words[i] <<= n; // Shift current word
                words[i] |= words[i - 1] >>> (64 - n); // Do the carry
            }
            words[0] <<= n; // shift [0] separately, since no carry
            // recalculateWordInUse() is unnecessary
        }
    }

    private void ensureCapacity(final int n) {
        if (words[words.length - 1] >>> n > 0) {
            long[] tmp = new long[words.length + 3];
            System.arraycopy(words, 0, tmp, 0, words.length);
            words = tmp;
            try {
                wordsField.set(this, tmp);
            } catch (IllegalAccessException e) {
                throw new IllegalStateException(e);
            }
        }
    }
}