return xStats.count();
  }

  /** Returns an immutable snapshot of the statistics on the {@code x} values alone. */
  public Stats xStats() {
    return xStats.snapshot();
  }

  /** Returns an immutable snapshot of the statistics on the {@code y} values alone. */
  public Stats yStats() {
    return yStats.snapshot();
  }

  /**
   * Returns the population covariance of the values. The count must be non-zero.

### Replication of DeleteMarker and versioned Delete

        AggregateFutureState<?> state, @Nullable Set<Throwable> expect, Set<Throwable> update) {
      synchronized (state) {
        if (state.seenExceptionsField == expect) {
          state.seenExceptionsField = update;
        }
      }
    }

    @Override
    int decrementAndGetRemainingCount(AggregateFutureState<?> state) {
      synchronized (state) {
        return --state.remainingField;
      }

      startUpCalled++;
      assertEquals(State.STARTING, state());
    }

    @Override
    protected void shutDown() throws Exception {
      assertEquals(1, startUpCalled);
      assertEquals(0, shutDownCalled);
      shutDownCalled++;
      assertEquals(State.STOPPING, state());
    }

    @Override
    protected Executor executor() {
      transitionStates.add(state());
      return directExecutor();
    }

    service.startAsync().awaitRunning();
    assertEquals(State.RUNNING, service.state());

    service.stopAsync();
    service.stopAsync();
    assertEquals(State.TERMINATED, service.state());
    assertEquals(
        ImmutableList.of(State.STARTING, State.RUNNING, State.STOPPING, State.TERMINATED),
        listener.getStateHistory());
  }

    service.startAsync().awaitRunning();
    assertEquals(State.RUNNING, service.state());

    service.stopAsync();
    service.stopAsync();
    assertEquals(State.TERMINATED, service.state());
    assertEquals(
        ImmutableList.of(State.STARTING, State.RUNNING, State.STOPPING, State.TERMINATED),
        listener.getStateHistory());
  }

    assertEquals(Service.State.RUNNING, service.state());

    exitRun.countDown(); // the service will exit voluntarily
    executionThread.join();

    assertTrue(service.shutDownCalled);
    assertEquals(Service.State.TERMINATED, service.state());

    service.stopAsync().awaitTerminated(); // no-op
    assertEquals(Service.State.TERMINATED, service.state());
    assertTrue(service.shutDownCalled);
  }

    //            (__)\       )\/\
    //                ||----w |
    //                ||     ||
    Runnable state = get();
    Blocker blocker = null;
    while (state instanceof Blocker || state == PARKED) {
      if (state instanceof Blocker) {
        blocker = (Blocker) state;
      }
      spinCount++;
      if (spinCount > MAX_BUSY_WAIT_SPINS) {
        /*

    assertEquals(Service.State.RUNNING, service.state());

    exitRun.countDown(); // the service will exit voluntarily
    executionThread.join();

    assertTrue(service.shutDownCalled);
    assertEquals(Service.State.TERMINATED, service.state());

    service.stopAsync().awaitTerminated(); // no-op
    assertEquals(Service.State.TERMINATED, service.state());
    assertTrue(service.shutDownCalled);
  }

    checkState(state != State.FAILED);
    switch (state) {
      case DONE:
        return false;
      case READY:
        return true;
      default:
    }
    return tryToComputeNext();
  }

  private boolean tryToComputeNext() {
    state = State.FAILED; // temporary pessimism
    next = computeNext();
    if (state != State.DONE) {
      state = State.READY;
      return true;
    }