/*
 *  Copyright 2011 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_ROLLING_ACCUMULATOR_H_
#define RTC_BASE_ROLLING_ACCUMULATOR_H_

#include <stddef.h>

#include <algorithm>
#include <vector>

#include "rtc_base/checks.h"
#include "rtc_base/numerics/running_statistics.h"

namespace rtc {

// RollingAccumulator stores and reports statistics
// over N most recent samples.
//
// T is assumed to be an int, long, double or float.
template <typename T>
class RollingAccumulator {
 public:
  explicit RollingAccumulator(size_t max_count) : samples_(max_count) {
    RTC_DCHECK(max_count > 0);
    Reset();
  }
  ~RollingAccumulator() {}

  RollingAccumulator(const RollingAccumulator&) = delete;
  RollingAccumulator& operator=(const RollingAccumulator&) = delete;

  size_t max_count() const { return samples_.size(); }

  size_t count() const { return static_cast<size_t>(stats_.Size()); }

  void Reset() {
    stats_ = webrtc::webrtc_impl::RunningStatistics<T>();
    next_index_ = 0U;
    max_ = T();
    max_stale_ = false;
    min_ = T();
    min_stale_ = false;
  }

  void AddSample(T sample) {
    if (count() == max_count()) {
      // Remove oldest sample.
      T sample_to_remove = samples_[next_index_];
      stats_.RemoveSample(sample_to_remove);
      if (sample_to_remove >= max_) {
        max_stale_ = true;
      }
      if (sample_to_remove <= min_) {
        min_stale_ = true;
      }
    }
    // Add new sample.
    samples_[next_index_] = sample;
    if (count() == 0 || sample >= max_) {
      max_ = sample;
      max_stale_ = false;
    }
    if (count() == 0 || sample <= min_) {
      min_ = sample;
      min_stale_ = false;
    }
    stats_.AddSample(sample);
    // Update next_index_.
    next_index_ = (next_index_ + 1) % max_count();
  }

  double ComputeMean() const { return stats_.GetMean().value_or(0); }

  T ComputeMax() const {
    if (max_stale_) {
      RTC_DCHECK(count() > 0)
          << "It shouldn't be possible for max_stale_ && count() == 0";
      max_ = samples_[next_index_];
      for (size_t i = 1u; i < count(); i++) {
        max_ = std::max(max_, samples_[(next_index_ + i) % max_count()]);
      }
      max_stale_ = false;
    }
    return max_;
  }

  T ComputeMin() const {
    if (min_stale_) {
      RTC_DCHECK(count() > 0)
          << "It shouldn't be possible for min_stale_ && count() == 0";
      min_ = samples_[next_index_];
      for (size_t i = 1u; i < count(); i++) {
        min_ = std::min(min_, samples_[(next_index_ + i) % max_count()]);
      }
      min_stale_ = false;
    }
    return min_;
  }

  // O(n) time complexity.
  // Weights nth sample with weight (learning_rate)^n. Learning_rate should be
  // between (0.0, 1.0], otherwise the non-weighted mean is returned.
  double ComputeWeightedMean(double learning_rate) const {
    if (count() < 1 || learning_rate <= 0.0 || learning_rate >= 1.0) {
      return ComputeMean();
    }
    double weighted_mean = 0.0;
    double current_weight = 1.0;
    double weight_sum = 0.0;
    const size_t max_size = max_count();
    for (size_t i = 0; i < count(); ++i) {
      current_weight *= learning_rate;
      weight_sum += current_weight;
      // Add max_size to prevent underflow.
      size_t index = (next_index_ + max_size - i - 1) % max_size;
      weighted_mean += current_weight * samples_[index];
    }
    return weighted_mean / weight_sum;
  }

  // Compute estimated variance.  Estimation is more accurate
  // as the number of samples grows.
  double ComputeVariance() const { return stats_.GetVariance().value_or(0); }

 private:
  webrtc::webrtc_impl::RunningStatistics<T> stats_;
  size_t next_index_;
  mutable T max_;
  mutable bool max_stale_;
  mutable T min_;
  mutable bool min_stale_;
  std::vector<T> samples_;
};

}  // namespace rtc

#endif  // RTC_BASE_ROLLING_ACCUMULATOR_H_