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 >>> on ints.

Is there a library function that shifts BitSets?

If not, is there a better way than the below?

public static void logicalRightShift(BitSet bs) {
  for (int i = 0; (i = bs.nextSetBit(i)) >= 0;) {
    // i is the first bit in a run of set bits.

    // Set any bit to the left of the run.
    if (i != 0) { bs.set(i - 1); }

    // Now i is the index of the bit after the end of the run.
    i = bs.nextClearBit(i);  // nextClearBit never returns -1.
    // Clear the last bit of the run.
    bs.clear(i - 1);

    // 0000111100000...
    //     a   b
    // i starts off the loop at a, and ends the loop at b.
    // The mutations change the run to
    // 0001111000000...
  }
}

Answer 1

That should do the trick:

BitSet shifted = bs.get(1, bs.length());

It will give you a bitset equal to the orginial one, but without the lower-most bit.

EDIT:

To generalize this to n bits,

BitSet shifted = bs.get(n, Math.max(n, bs.length()));