Add MovingMedianFilter to rtc_base/numerics

This class will be used for filtering remote clock offset in rtp streams.
It is a separate wrapper around PercentileFilter because it will be used
in that form in several places.

Bug: webrtc:8468
Change-Id: If1f6c38ac1ffa02232c1aed5512b92878b1c346a
Reviewed-on: https://webrtc-review.googlesource.com/17841
Commit-Queue: Ilya Nikolaevskiy <ilnik@webrtc.org>
Reviewed-by: Tommi <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20531}

rtc_base/BUILD.gn

@@ -402,6 +402,7 @@ rtc_static_library("rtc_numerics") {
   sources = [
     "numerics/exp_filter.cc",
     "numerics/exp_filter.h",
+    "numerics/moving_median_filter.h",
     "numerics/percentile_filter.h",
     "numerics/sequence_number_util.h",
   ]
@@ -997,6 +998,7 @@ if (rtc_include_tests) {
     }
     sources = [
       "numerics/exp_filter_unittest.cc",
+      "numerics/moving_median_filter_unittest.cc",
       "numerics/percentile_filter_unittest.cc",
       "numerics/sequence_number_util_unittest.cc",
     ]

rtc_base/numerics/moving_median_filter.h

@@ -0,0 +1,69 @@
+/*
+ *  Copyright (c) 2017 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef RTC_BASE_NUMERICS_MOVING_MEDIAN_FILTER_H_
+#define RTC_BASE_NUMERICS_MOVING_MEDIAN_FILTER_H_
+
+#include <list>
+
+#include "rtc_base/checks.h"
+#include "rtc_base/constructormagic.h"
+#include "rtc_base/numerics/percentile_filter.h"
+
+namespace webrtc {
+
+// Class to efficiently get moving median filter from a stream of samples.
+template <typename T>
+class MovingMedianFilter {
+ public:
+  // Construct filter. |window_size| is how many latest samples are stored and
+  // used to take median. |window_size| must be positive.
+  explicit MovingMedianFilter(size_t window_size);
+
+  // Insert a new sample.
+  void Insert(const T& value);
+
+  // Get median over the latest window.
+  T GetFilteredValue() const;
+
+ private:
+  PercentileFilter<T> percentile_filter_;
+  std::list<T> samples_;
+  size_t samples_stored_;
+  const size_t window_size_;
+
+  RTC_DISALLOW_COPY_AND_ASSIGN(MovingMedianFilter);
+};
+
+template <typename T>
+MovingMedianFilter<T>::MovingMedianFilter(size_t window_size)
+    : percentile_filter_(0.5f), samples_stored_(0), window_size_(window_size) {
+  RTC_CHECK_GT(window_size, 0);
+}
+
+template <typename T>
+void MovingMedianFilter<T>::Insert(const T& value) {
+  percentile_filter_.Insert(value);
+  samples_.emplace_back(value);
+  ++samples_stored_;
+  if (samples_stored_ > window_size_) {
+    percentile_filter_.Erase(samples_.front());
+    samples_.pop_front();
+    --samples_stored_;
+  }
+}
+
+template <typename T>
+T MovingMedianFilter<T>::GetFilteredValue() const {
+  return percentile_filter_.GetPercentileValue();
+}
+
+}  // namespace webrtc
+#endif  // RTC_BASE_NUMERICS_MOVING_MEDIAN_FILTER_H_

rtc_base/numerics/moving_median_filter_unittest.cc

@@ -0,0 +1,51 @@
+/*
+ *  Copyright 2017 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "rtc_base/numerics/moving_median_filter.h"
+#include "test/gtest.h"
+
+namespace webrtc {
+
+TEST(MovingMedianFilterTest, ProcessesNoSamples) {
+  MovingMedianFilter<int> filter(2);
+  EXPECT_EQ(0, filter.GetFilteredValue());
+}
+
+TEST(MovingMedianFilterTest, ReturnsMovingMedianWindow5) {
+  MovingMedianFilter<int> filter(5);
+  const int64_t kSamples[5] = {1, 5, 2, 3, 4};
+  const int64_t kExpectedFilteredValues[5] = {1, 1, 2, 2, 3};
+  for (int i = 0; i < 5; ++i) {
+    filter.Insert(kSamples[i]);
+    EXPECT_EQ(kExpectedFilteredValues[i], filter.GetFilteredValue());
+  }
+}
+
+TEST(MovingMedianFilterTest, ReturnsMovingMedianWindow3) {
+  MovingMedianFilter<int> filter(3);
+  const int64_t kSamples[5] = {1, 5, 2, 3, 4};
+  const int64_t kExpectedFilteredValues[5] = {1, 1, 2, 3, 3};
+  for (int i = 0; i < 5; ++i) {
+    filter.Insert(kSamples[i]);
+    EXPECT_EQ(kExpectedFilteredValues[i], filter.GetFilteredValue());
+  }
+}
+
+TEST(MovingMedianFilterTest, ReturnsMovingMedianWindow1) {
+  MovingMedianFilter<int> filter(1);
+  const int64_t kSamples[5] = {1, 5, 2, 3, 4};
+  const int64_t kExpectedFilteredValues[5] = {1, 5, 2, 3, 4};
+  for (int i = 0; i < 5; ++i) {
+    filter.Insert(kSamples[i]);
+    EXPECT_EQ(kExpectedFilteredValues[i], filter.GetFilteredValue());
+  }
+}
+
+}  // namespace webrtc