layout(local_size_x = 256) in;
const uint WORKGROUP_SIZE = 256;

struct GlobalCounts {
    atomicUint firstHalfCount;
    atomicUint secondHalfCount;
};
layout(metal, binding = 0) buffer ssbo {
    GlobalCounts globalCounts;
};

workgroup atomicUint localCounts[2];

void main() {
    // Initialize the local counts.
    if (sk_LocalInvocationID.x == 0) {
        atomicStore(localCounts[0], 0);
        atomicStore(localCounts[1], 0);
    }

    // Synchronize the threads in the workgroup so they all see the initial value.
    workgroupBarrier();

    // Each thread increments one of the local counters based on its invocation index.
    uint idx = sk_LocalInvocationID.x < (WORKGROUP_SIZE / 2) ? 0 : 1;
    atomicAdd(localCounts[idx], 1);

    // Synchronize the threads again to ensure they have all executed the increments
    // and the following load reads the same value across all threads in the
    // workgroup.
    workgroupBarrier();

    // Add the workgroup-only tally to the global counter.
    if (sk_LocalInvocationID.x == 0) {
        atomicAdd(globalCounts.firstHalfCount, atomicLoad(localCounts[0]));
        atomicAdd(globalCounts.secondHalfCount, atomicLoad(localCounts[1]));
    }
}