nextClearBit returns incorrect values in certain cases

[heikitu]

We ran in to an issue that was caused by SparseBitSet#nextClearBit returning incorrect values, illustrated by the following JUnit test

    @Test
    public void testNextClearBit() {
	final SparseBitSet set = new SparseBitSet();
        // The last word in the first level3 block covers bits 1984-2047, set those + the last bit from second
        // to last word
        for (int i = 1983; i < 2047; i++) {
            set.set(i);
        }

        // Test nextClearBit from the end of the previous word
        Assert.assertEquals(2047, set.nextClearBit(1983));

        // set the last bit, after this all bits in bits[0][0][31] are set
        set.set(2047);
        Assert.assertEquals(2048, set.nextClearBit(1983));
    }

The above test fails in the last assert with Expected 2048, Actual 2112

The conditions needed for this to happen are

• Last word in the set is the last word of a level3 block
• All bits in the last word are set
• Search for nextClearBit starts from some other word, and there are no clear bits after the start index.