assertStatsApproxEqual(TWO_VALUES_STATS, TWO_VALUES_PAIRED_STATS.xStats());
    assertStatsApproxEqual(MANY_VALUES_STATS_ITERABLE, MANY_VALUES_PAIRED_STATS.xStats());
  }

  public void testYStats() {
    assertStatsApproxEqual(EMPTY_STATS_ITERABLE, EMPTY_PAIRED_STATS.yStats());
    assertStatsApproxEqual(OTHER_ONE_VALUE_STATS, ONE_VALUE_PAIRED_STATS.yStats());

    assertStatsApproxEqual(TWO_VALUES_STATS, TWO_VALUES_PAIRED_STATS.xStats());
    assertStatsApproxEqual(MANY_VALUES_STATS_ITERABLE, MANY_VALUES_PAIRED_STATS.xStats());
  }

  public void testYStats() {
    assertStatsApproxEqual(EMPTY_STATS_ITERABLE, EMPTY_PAIRED_STATS.yStats());
    assertStatsApproxEqual(OTHER_ONE_VALUE_STATS, ONE_VALUE_PAIRED_STATS.yStats());

   *
   * <ul>
   *   <li>Both {@code xStats} and {@code yStats} must have the same {@code count}.
   *   <li>If that {@code count} is 1, {@code sumOfProductsOfDeltas} must be exactly 0.0.
   *   <li>If that {@code count} is more than 1, {@code sumOfProductsOfDeltas} must be finite.
   * </ul>
   */
  PairedStats(Stats xStats, Stats yStats, double sumOfProductsOfDeltas) {
    this.xStats = xStats;
    this.yStats = yStats;

    assertStatsApproxEqual(OTHER_ONE_VALUE_STATS, oneValueAccumulator.yStats());
    assertStatsApproxEqual(
        OTHER_ONE_VALUE_STATS, oneValueAccumulatorByAddAllEmptyPairedStats.yStats());
    assertStatsApproxEqual(OTHER_TWO_VALUES_STATS, twoValuesAccumulator.yStats());
    assertStatsApproxEqual(
        OTHER_TWO_VALUES_STATS, twoValuesAccumulatorByAddAllPartitionedPairedStats.yStats());

    assertStatsApproxEqual(OTHER_ONE_VALUE_STATS, oneValueAccumulator.yStats());
    assertStatsApproxEqual(
        OTHER_ONE_VALUE_STATS, oneValueAccumulatorByAddAllEmptyPairedStats.yStats());
    assertStatsApproxEqual(OTHER_TWO_VALUES_STATS, twoValuesAccumulator.yStats());
    assertStatsApproxEqual(
        OTHER_TWO_VALUES_STATS, twoValuesAccumulatorByAddAllPartitionedPairedStats.yStats());

              + (values.xStats().mean() - xStats.mean())
                  * (values.yStats().mean() - yStats.mean())
                  * values.count();
    }
    yStats.addAll(values.yStats());
  }

  /** Returns an immutable snapshot of the current statistics. */
  public PairedStats snapshot() {
    return new PairedStats(xStats.snapshot(), yStats.snapshot(), sumOfProductsOfDeltas);
  }

    stats = cache.stats();
    assertEquals(2, stats.missCount());
    assertEquals(3, stats.loadSuccessCount());
    assertEquals(0, stats.loadExceptionCount());
    assertEquals(0, stats.hitCount());

    assertSame(key, cache.get(key));
    stats = cache.stats();
    assertEquals(2, stats.missCount());
    assertEquals(3, stats.loadSuccessCount());
    assertEquals(0, stats.loadExceptionCount());

    Stats stats = Stats.of(megaPrimitiveDoubleStream());
    assertThat(stats.count()).isEqualTo(MEGA_STREAM_COUNT);
    assertThat(stats.mean()).isWithin(ALLOWED_ERROR * MEGA_STREAM_COUNT).of(MEGA_STREAM_MEAN);
    assertThat(stats.populationVariance())
        .isWithin(ALLOWED_ERROR * MEGA_STREAM_COUNT)
        .of(MEGA_STREAM_POPULATION_VARIANCE);
    assertThat(stats.min()).isEqualTo(MEGA_STREAM_MIN);

  public void testEmpty() {
    CacheStats stats = new CacheStats(0, 0, 0, 0, 0, 0);
    assertEquals(0, stats.requestCount());
    assertEquals(0, stats.hitCount());
    assertEquals(1.0, stats.hitRate());
    assertEquals(0, stats.missCount());
    assertEquals(0.0, stats.missRate());
    assertEquals(0, stats.loadSuccessCount());
    assertEquals(0, stats.loadExceptionCount());
    assertEquals(0.0, stats.loadExceptionRate());

/**
 * Tests for {@link StatsAccumulator}. This tests the stats methods for instances built with {@link
 * StatsAccumulator#add} and {@link StatsAccumulator#addAll}, and various error cases of the {@link
 * StatsAccumulator#add} and {@link StatsAccumulator#addAll} methods. For tests of the {@link
 * StatsAccumulator#snapshot} method which returns {@link Stats} instances, see {@link StatsTest}.
 *
 * @author Pete Gillin
 */