Question

The standard api does not include an AtomicBitSet implementation. I could roll my own on top of AtomicIntegerArray, but would prefer not too.

Is anyone aware of an existing implementation released under a licence compatible with Apache 2? I require only basic operations to set and check bits.

Edit:

The code is both performance and memory critical so I’d like to avoid synchronization or an integer per flag if possible.

Answer 1

I would use an AtomicIntegerArray and I would use 32 flags per integer which would give you the same density as BitSet but without needing locks for thread safety.

public class AtomicBitSet {
    private final AtomicIntegerArray array;

    public AtomicBitSet(int length) {
        int intLength = (length + 31) >>> 5; // unsigned / 32
        array = new AtomicIntegerArray(intLength);
    }

    public void set(long n) {
        int bit = 1 << n;
        int idx = (int) (n >>> 5);
        while (true) {
            int num = array.get(idx);
            int num2 = num | bit;
            if (num == num2 || array.compareAndSet(idx, num, num2))
                return;
        }
    }

    public boolean get(long n) {
        int bit = 1 << n;
        int idx = (int) (n >>> 5);
        int num = array.get(idx);
        return (num & bit) != 0;
    }
}