Question

What is wrong with my compact boolean array implementation? It is not working. The tests are failing so I am doing some bitwise operation incorrectly. 🙁

public class CompactBooleanArray {

    private final int size;
    private final long[] bitmap;

    public CompactBooleanArray(int size) {
        this.size = size;
        int numberOfLongs = (size + 63) / 64;
        bitmap = new long[numberOfLongs];
    }

    public final void set(int index, boolean value) {
        int longIndex = index >> 6;
        int bitPosition = index & 63;

        if (value) {
            bitmap[longIndex] |= (1 << bitPosition);
        } else {
            bitmap[longIndex] &= ~(1 << bitPosition);
        }
    }

    public final boolean get(int index) {
        int longIndex = index >> 6;
        int bitPosition = index & 63;

        return (bitmap[longIndex] & (1 << bitPosition)) != 0;

    }

    public final int length() {
        return size;
    }
}

My failing test:

public class CompactBooleanArrayTest {

    @Test
    public void testSimple() {

        int[] x = { 4, 56, 60 };

        CompactBooleanArray array = new CompactBooleanArray(100);

        for(int i : x) array.set(i, true);

        for(int i = 0; i < array.length(); i++) {
            System.out.println("I: " + i + " -> " + array.get(i));
            if (check(i, x)) {
                Assert.assertTrue(array.get(i));
            } else {
                Assert.assertFalse(array.get(i));
            }
        }
    }

    private boolean check(int value, int[] array) {
        for(int i : array) {
            if (value == i) return true;
        }
        return false;
    }
}

Answer 1

Without trying it, I’d say your shifts of 1 << bitPosition, etc. are failing because you’re not using 1L. i.e. 1 << 50 = 0, 1L << 50 = 2^50th