Support computing percentiles on a rolling window of samples

cc/scheduler/rolling_sample_window_unittest.cc

+// Copyright 2013 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "cc/scheduler/rolling_sample_window.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace cc {
+namespace {
+
+TEST(RollingSampleWindowTest, EmptyWindow) {
+  RollingSampleWindow empty_window(0);
+
+  EXPECT_EQ(base::TimeDelta(), empty_window.Percentile(0.0));
+  EXPECT_EQ(base::TimeDelta(), empty_window.Percentile(50.0));
+  EXPECT_EQ(base::TimeDelta(), empty_window.Percentile(100.0));
+
+  empty_window.InsertSample(base::TimeDelta::FromMilliseconds(10));
+  empty_window.InsertSample(base::TimeDelta::FromMilliseconds(15));
+  empty_window.InsertSample(base::TimeDelta::FromMilliseconds(20));
+
+  EXPECT_EQ(base::TimeDelta(), empty_window.Percentile(0.0));
+  EXPECT_EQ(base::TimeDelta(), empty_window.Percentile(50.0));
+  EXPECT_EQ(base::TimeDelta(), empty_window.Percentile(100.0));
+
+  empty_window.Clear();
+  EXPECT_EQ(base::TimeDelta(), empty_window.Percentile(0.0));
+  EXPECT_EQ(base::TimeDelta(), empty_window.Percentile(50.0));
+  EXPECT_EQ(base::TimeDelta(), empty_window.Percentile(100.0));
+}
+
+TEST(RollingSampleWindowTest, SizeOneWindow) {
+  RollingSampleWindow size_one_window(1);
+  base::TimeDelta sample1 = base::TimeDelta::FromMilliseconds(10);
+  base::TimeDelta sample2 = base::TimeDelta::FromMilliseconds(20);
+
+  EXPECT_EQ(base::TimeDelta(), size_one_window.Percentile(0.0));
+  EXPECT_EQ(base::TimeDelta(), size_one_window.Percentile(50.0));
+  EXPECT_EQ(base::TimeDelta(), size_one_window.Percentile(100.0));
+
+  size_one_window.InsertSample(sample1);
+  EXPECT_EQ(base::TimeDelta(), size_one_window.Percentile(0.0));

[brianderson, 2013/05/31 18:42:05]

Would it make sense for the 0th percentile to return the smallest value?

[ajuma, 2013/05/31 20:41:11]

My initial thought was for Percentile(X) to return the smallest value greater
than or equal to X% of the samples. In this case, the 0th percentile would be
base::TimeDelta() (since, after all, it is indeed greater than or equal to 0% of
the samples). But I agree that the smallest sample is a much more useful thing
for Percentile(0.0) to return, so I've changed Percentile to return the smallest
*sample* greater than or equal to X% of the samples (of course, with an
exception for the case of an empty history).

+  EXPECT_EQ(sample1, size_one_window.Percentile(50.0));
+  EXPECT_EQ(sample1, size_one_window.Percentile(100.0));
+
+  size_one_window.InsertSample(sample2);
+  EXPECT_EQ(base::TimeDelta(), size_one_window.Percentile(0.0));
+  EXPECT_EQ(sample2, size_one_window.Percentile(50.0));
+  EXPECT_EQ(sample2, size_one_window.Percentile(100.0));
+
+  size_one_window.Clear();
+  EXPECT_EQ(base::TimeDelta(), size_one_window.Percentile(0.0));
+  EXPECT_EQ(base::TimeDelta(), size_one_window.Percentile(50.0));
+  EXPECT_EQ(base::TimeDelta(), size_one_window.Percentile(100.0));
+}
+
+TEST(RollingSampleWindowTest, LargeWindow) {
+  RollingSampleWindow large_window(100);
+  base::TimeDelta sample1 = base::TimeDelta::FromMilliseconds(150);
+  base::TimeDelta sample2 = base::TimeDelta::FromMilliseconds(250);
+  base::TimeDelta sample3 = base::TimeDelta::FromMilliseconds(200);
+
+  large_window.InsertSample(sample1);
+  large_window.InsertSample(sample2);
+
+  EXPECT_EQ(base::TimeDelta(), large_window.Percentile(0.0));
+  EXPECT_EQ(sample1, large_window.Percentile(25.0));
+  EXPECT_EQ(sample2, large_window.Percentile(75.0));
+  EXPECT_EQ(sample2, large_window.Percentile(100.0));
+
+  large_window.InsertSample(sample3);
+  EXPECT_EQ(base::TimeDelta(), large_window.Percentile(0.0));
+  EXPECT_EQ(sample1, large_window.Percentile(25.0));
+  EXPECT_EQ(sample3, large_window.Percentile(50.0));
+  EXPECT_EQ(sample2, large_window.Percentile(100.0));
+
+  // Fill the window.
+  for (int i = 1; i <= 97; i++)
+    large_window.InsertSample(base::TimeDelta::FromMilliseconds(i));
+
+  EXPECT_EQ(base::TimeDelta(), large_window.Percentile(0.0));
+  for (int i = 1; i <= 97; i++) {
+    EXPECT_EQ(base::TimeDelta::FromMilliseconds(i),
+              large_window.Percentile(i - 0.5));
+  }
+  EXPECT_EQ(sample1, large_window.Percentile(97.5));
+  EXPECT_EQ(sample3, large_window.Percentile(98.5));
+  EXPECT_EQ(sample2, large_window.Percentile(99.5));
+
+  // Continue inserting samples, causing the oldest samples to be discarded.
+  base::TimeDelta sample4 = base::TimeDelta::FromMilliseconds(100);
+  base::TimeDelta sample5 = base::TimeDelta::FromMilliseconds(102);
+  base::TimeDelta sample6 = base::TimeDelta::FromMilliseconds(104);
+  large_window.InsertSample(sample4);
+  large_window.InsertSample(sample5);
+  large_window.InsertSample(sample6);
+  EXPECT_EQ(sample4, large_window.Percentile(97.5));
+  EXPECT_EQ(sample5, large_window.Percentile(98.5));
+  EXPECT_EQ(sample6, large_window.Percentile(99.5));
+
+  large_window.Clear();
+  EXPECT_EQ(base::TimeDelta(), large_window.Percentile(0.0));
+  EXPECT_EQ(base::TimeDelta(), large_window.Percentile(50.0));
+  EXPECT_EQ(base::TimeDelta(), large_window.Percentile(100.0));
+}
+
+}  // namespace
+}  // namespace cc