WaitGroup

const WaitGroup = struct {
    state: std.atomic.Value(usize),
    event: Io.Event,

    const init: WaitGroup = .{ .state = .{ .raw = 0 }, .event = .unset };

    const is_waiting: usize = 1 << 0;
    const one_pending: usize = 1 << 1;

    fn start(wg: *WaitGroup) void {
        const prev_state = wg.state.fetchAdd(one_pending, .monotonic);
        assert((prev_state / one_pending) < (std.math.maxInt(usize) / one_pending));
    }

    fn value(wg: *WaitGroup) usize {
        return wg.state.load(.monotonic) / one_pending;
    }

    fn wait(wg: *WaitGroup) void {
        const prev_state = wg.state.fetchAdd(is_waiting, .acquire);
        assert(prev_state & is_waiting == 0);
        if ((prev_state / one_pending) > 0) eventWait(&wg.event);
    }

    fn finish(wg: *WaitGroup) void {
        const state = wg.state.fetchSub(one_pending, .acq_rel);
        assert((state / one_pending) > 0);

        if (state == (one_pending | is_waiting)) {
            eventSet(&wg.event);
        }
    }
}