// Copyright 2015 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 "net/base/network_quality_estimator.h"

#include <float.h>
#include <algorithm>
#include <cmath>
#include <limits>
#include <vector>

#include "base/logging.h"
#include "base/metrics/histogram.h"
#include "base/metrics/histogram_base.h"
#include "base/strings/string_number_conversions.h"
#include "build/build_config.h"
#include "net/base/load_flags.h"
#include "net/base/load_timing_info.h"
#include "net/base/net_util.h"
#include "net/base/network_interfaces.h"
#include "net/url_request/url_request.h"
#include "url/gurl.h"

#if defined(OS_ANDROID)
#include "net/android/network_library.h"
#endif  // OS_ANDROID

namespace {

// Default value of the half life (in seconds) for computing time weighted
// percentiles. Every half life, the weight of all observations reduces by
// half. Lowering the half life would reduce the weight of older values faster.
const int kDefaultHalfLifeSeconds = 60;

// Name of the variation parameter that holds the value of the half life (in
// seconds) of the observations.
const char kHalfLifeSecondsParamName[] = "HalfLifeSeconds";

// Returns a descriptive name corresponding to |connection_type|.
const char* GetNameForConnectionType(
    net::NetworkChangeNotifier::ConnectionType connection_type) {
  switch (connection_type) {
    case net::NetworkChangeNotifier::CONNECTION_UNKNOWN:
      return "Unknown";
    case net::NetworkChangeNotifier::CONNECTION_ETHERNET:
      return "Ethernet";
    case net::NetworkChangeNotifier::CONNECTION_WIFI:
      return "WiFi";
    case net::NetworkChangeNotifier::CONNECTION_2G:
      return "2G";
    case net::NetworkChangeNotifier::CONNECTION_3G:
      return "3G";
    case net::NetworkChangeNotifier::CONNECTION_4G:
      return "4G";
    case net::NetworkChangeNotifier::CONNECTION_NONE:
      return "None";
    case net::NetworkChangeNotifier::CONNECTION_BLUETOOTH:
      return "Bluetooth";
    default:
      NOTREACHED();
      break;
  }
  return "";
}

// Suffix of the name of the variation parameter that contains the default RTT
// observation (in milliseconds). Complete name of the variation parameter
// would be |ConnectionType|.|kDefaultRTTMsecObservationSuffix| where
// |ConnectionType| is from |kConnectionTypeNames|. For example, variation
// parameter for Wi-Fi would be "WiFi.DefaultMedianRTTMsec".
const char kDefaultRTTMsecObservationSuffix[] = ".DefaultMedianRTTMsec";

// Suffix of the name of the variation parameter that contains the default
// downstream throughput observation (in Kbps).  Complete name of the variation
// parameter would be |ConnectionType|.|kDefaultKbpsObservationSuffix| where
// |ConnectionType| is from |kConnectionTypeNames|. For example, variation
// parameter for Wi-Fi would be "WiFi.DefaultMedianKbps".
const char kDefaultKbpsObservationSuffix[] = ".DefaultMedianKbps";

// Computes and returns the weight multiplier per second.
// |variation_params| is the map containing all field trial parameters
// related to NetworkQualityEstimator field trial.
double GetWeightMultiplierPerSecond(
    const std::map<std::string, std::string>& variation_params) {
  int half_life_seconds = kDefaultHalfLifeSeconds;
  int32_t variations_value = 0;
  auto it = variation_params.find(kHalfLifeSecondsParamName);
  if (it != variation_params.end() &&
      base::StringToInt(it->second, &variations_value) &&
      variations_value >= 1) {
    half_life_seconds = variations_value;
  }
  DCHECK_GT(half_life_seconds, 0);
  return exp(log(0.5) / half_life_seconds);
}

// Returns the histogram that should be used to record the given statistic.
// |max_limit| is the maximum value that can be stored in the histogram.
base::HistogramBase* GetHistogram(
    const std::string& statistic_name,
    net::NetworkChangeNotifier::ConnectionType type,
    int32_t max_limit) {
  const base::LinearHistogram::Sample kLowerLimit = 1;
  DCHECK_GT(max_limit, kLowerLimit);
  const size_t kBucketCount = 50;

  // Prefix of network quality estimator histograms.
  const char prefix[] = "NQE.";
  return base::Histogram::FactoryGet(
      prefix + statistic_name + GetNameForConnectionType(type), kLowerLimit,
      max_limit, kBucketCount, base::HistogramBase::kUmaTargetedHistogramFlag);
}

}  // namespace

namespace net {

const int32_t NetworkQualityEstimator::kInvalidThroughput = 0;

NetworkQualityEstimator::NetworkQualityEstimator(
    scoped_ptr<ExternalEstimateProvider> external_estimates_provider,
    const std::map<std::string, std::string>& variation_params)
    : NetworkQualityEstimator(external_estimates_provider.Pass(),
                              variation_params,
                              false,
                              false) {}

NetworkQualityEstimator::NetworkQualityEstimator(
    scoped_ptr<ExternalEstimateProvider> external_estimates_provider,
    const std::map<std::string, std::string>& variation_params,
    bool allow_local_host_requests_for_tests,
    bool allow_smaller_responses_for_tests)
    : allow_localhost_requests_(allow_local_host_requests_for_tests),
      allow_small_responses_(allow_smaller_responses_for_tests),
      last_connection_change_(base::TimeTicks::Now()),
      current_network_id_(
          NetworkID(NetworkChangeNotifier::ConnectionType::CONNECTION_UNKNOWN,
                    std::string())),
      downstream_throughput_kbps_observations_(
          GetWeightMultiplierPerSecond(variation_params)),
      rtt_msec_observations_(GetWeightMultiplierPerSecond(variation_params)),
      external_estimates_provider_(external_estimates_provider.Pass()) {
  static_assert(kMinRequestDurationMicroseconds > 0,
                "Minimum request duration must be > 0");
  static_assert(kDefaultHalfLifeSeconds > 0,
                "Default half life duration must be > 0");
  static_assert(kMaximumNetworkQualityCacheSize > 0,
                "Size of the network quality cache must be > 0");
  // This limit should not be increased unless the logic for removing the
  // oldest cache entry is rewritten to use a doubly-linked-list LRU queue.
  static_assert(kMaximumNetworkQualityCacheSize <= 10,
                "Size of the network quality cache must <= 10");

  ObtainOperatingParams(variation_params);
  NetworkChangeNotifier::AddConnectionTypeObserver(this);
  if (external_estimates_provider_)
    external_estimates_provider_->SetUpdatedEstimateDelegate(this);
  current_network_id_ = GetCurrentNetworkID();
  AddDefaultEstimates();
}

// static
const base::TimeDelta NetworkQualityEstimator::InvalidRTT() {
  return base::TimeDelta::Max();
}

void NetworkQualityEstimator::ObtainOperatingParams(
    const std::map<std::string, std::string>& variation_params) {
  DCHECK(thread_checker_.CalledOnValidThread());

  for (size_t i = 0; i <= NetworkChangeNotifier::CONNECTION_LAST; ++i) {
    NetworkChangeNotifier::ConnectionType type =
        static_cast<NetworkChangeNotifier::ConnectionType>(i);
    int32_t variations_value = kMinimumRTTVariationParameterMsec - 1;
    // Name of the parameter that holds the RTT value for this connection type.
    std::string rtt_parameter_name =
        std::string(GetNameForConnectionType(type))
            .append(kDefaultRTTMsecObservationSuffix);
    auto it = variation_params.find(rtt_parameter_name);
    if (it != variation_params.end() &&
        base::StringToInt(it->second, &variations_value) &&
        variations_value >= kMinimumRTTVariationParameterMsec) {
      default_observations_[i] =
          NetworkQuality(base::TimeDelta::FromMilliseconds(variations_value),
                         default_observations_[i].downstream_throughput_kbps());
    }

    variations_value = kMinimumThroughputVariationParameterKbps - 1;
    // Name of the parameter that holds the Kbps value for this connection
    // type.
    std::string kbps_parameter_name =
        std::string(GetNameForConnectionType(type))
            .append(kDefaultKbpsObservationSuffix);
    it = variation_params.find(kbps_parameter_name);
    if (it != variation_params.end() &&
        base::StringToInt(it->second, &variations_value) &&
        variations_value >= kMinimumThroughputVariationParameterKbps) {
      default_observations_[i] =
          NetworkQuality(default_observations_[i].rtt(), variations_value);
    }
  }
}

void NetworkQualityEstimator::AddDefaultEstimates() {
  DCHECK(thread_checker_.CalledOnValidThread());
  if (default_observations_[current_network_id_.type].rtt() != InvalidRTT()) {
    rtt_msec_observations_.AddObservation(Observation(
        default_observations_[current_network_id_.type].rtt().InMilliseconds(),
        base::TimeTicks::Now()));
  }
  if (default_observations_[current_network_id_.type]
          .downstream_throughput_kbps() != kInvalidThroughput) {
    downstream_throughput_kbps_observations_.AddObservation(
        Observation(default_observations_[current_network_id_.type]
                        .downstream_throughput_kbps(),
                    base::TimeTicks::Now()));
  }
}

NetworkQualityEstimator::~NetworkQualityEstimator() {
  DCHECK(thread_checker_.CalledOnValidThread());
  NetworkChangeNotifier::RemoveConnectionTypeObserver(this);
}

void NetworkQualityEstimator::NotifyHeadersReceived(const URLRequest& request) {
  DCHECK(thread_checker_.CalledOnValidThread());

  if (!RequestProvidesUsefulObservations(request))
    return;

  // Update |estimated_median_network_quality_| if this is a main frame request.
  if (request.load_flags() & LOAD_MAIN_FRAME) {
    estimated_median_network_quality_ = NetworkQuality(
        GetRTTEstimateInternal(base::TimeTicks(), 50),
        GetDownlinkThroughputKbpsEstimateInternal(base::TimeTicks(), 50));
  }

  base::TimeTicks now = base::TimeTicks::Now();
  LoadTimingInfo load_timing_info;
  request.GetLoadTimingInfo(&load_timing_info);

  // If the load timing info is unavailable, it probably means that the request
  // did not go over the network.
  if (load_timing_info.send_start.is_null() ||
      load_timing_info.receive_headers_end.is_null()) {
    return;
  }

  // Time when the resource was requested.
  base::TimeTicks request_start_time = load_timing_info.send_start;

  // Time when the headers were received.
  base::TimeTicks headers_received_time = load_timing_info.receive_headers_end;

  // Duration between when the resource was requested and when response
  // headers were received.
    base::TimeDelta observed_rtt = headers_received_time - request_start_time;
    DCHECK_GE(observed_rtt, base::TimeDelta());
    if (observed_rtt < peak_network_quality_.rtt()) {
      peak_network_quality_ = NetworkQuality(
          observed_rtt, peak_network_quality_.downstream_throughput_kbps());
    }

    rtt_msec_observations_.AddObservation(
        Observation(observed_rtt.InMilliseconds(), now));

    // Compare the RTT observation with the estimated value and record it.
    if (estimated_median_network_quality_.rtt() != InvalidRTT()) {
      RecordRTTUMA(estimated_median_network_quality_.rtt().InMilliseconds(),
                   observed_rtt.InMilliseconds());
    }
}

void NetworkQualityEstimator::NotifyRequestCompleted(
    const URLRequest& request) {
  DCHECK(thread_checker_.CalledOnValidThread());

  if (!RequestProvidesUsefulObservations(request))
    return;

  base::TimeTicks now = base::TimeTicks::Now();
  LoadTimingInfo load_timing_info;
  request.GetLoadTimingInfo(&load_timing_info);

  // If the load timing info is unavailable, it probably means that the request
  // did not go over the network.
  if (load_timing_info.send_start.is_null() ||
      load_timing_info.receive_headers_end.is_null()) {
    return;
  }

  // Time since the resource was requested.
  // TODO(tbansal): Change the start time to receive_headers_end, once we use
  // NetworkActivityMonitor.
  base::TimeDelta request_start_to_completed =
      now - load_timing_info.send_start;
  DCHECK_GE(request_start_to_completed, base::TimeDelta());

  // Ignore tiny transfers which will not produce accurate rates.
  // Ignore short duration transfers.
  // Skip the checks if |allow_small_responses_| is true.
  if (!allow_small_responses_ &&
      (request.GetTotalReceivedBytes() < kMinTransferSizeInBytes ||
       request_start_to_completed < base::TimeDelta::FromMicroseconds(
                                        kMinRequestDurationMicroseconds))) {
    return;
  }

  double downstream_kbps = request.GetTotalReceivedBytes() * 8.0 / 1000.0 /
                           request_start_to_completed.InSecondsF();
  DCHECK_GE(downstream_kbps, 0.0);

  // Check overflow errors. This may happen if the downstream_kbps is more than
  // 2 * 10^9 (= 2000 Gbps).
  if (downstream_kbps >= std::numeric_limits<int32_t>::max())
    downstream_kbps = std::numeric_limits<int32_t>::max();

  int32_t downstream_kbps_as_integer = static_cast<int32_t>(downstream_kbps);

  // Round up |downstream_kbps_as_integer|. If the |downstream_kbps_as_integer|
  // is less than 1, it is set to 1 to differentiate from case when there is no
  // connection.
  if (downstream_kbps - downstream_kbps_as_integer > 0)
    downstream_kbps_as_integer++;

  DCHECK_GT(downstream_kbps_as_integer, 0.0);
  if (downstream_kbps_as_integer >
      peak_network_quality_.downstream_throughput_kbps())
    peak_network_quality_ =
        NetworkQuality(peak_network_quality_.rtt(), downstream_kbps_as_integer);

  downstream_throughput_kbps_observations_.AddObservation(
      Observation(downstream_kbps_as_integer, now));
}

void NetworkQualityEstimator::RecordRTTUMA(int32_t estimated_value_msec,
                                           int32_t actual_value_msec) const {
  DCHECK(thread_checker_.CalledOnValidThread());

  // Record the difference between the actual and the estimated value.
  if (estimated_value_msec >= actual_value_msec) {
    base::HistogramBase* difference_rtt =
        GetHistogram("DifferenceRTTEstimatedAndActual.",
                     current_network_id_.type, 10 * 1000);  // 10 seconds
    difference_rtt->Add(estimated_value_msec - actual_value_msec);
  } else {
    base::HistogramBase* difference_rtt =
        GetHistogram("DifferenceRTTActualAndEstimated.",
                     current_network_id_.type, 10 * 1000);  // 10 seconds
    difference_rtt->Add(actual_value_msec - estimated_value_msec);
  }

  // Record all the RTT observations.
  base::HistogramBase* rtt_observations =
      GetHistogram("RTTObservations.", current_network_id_.type,
                   10 * 1000);  // 10 seconds upper bound
  rtt_observations->Add(actual_value_msec);

  if (actual_value_msec == 0)
    return;

  int32 ratio = (estimated_value_msec * 100) / actual_value_msec;

  // Record the accuracy of estimation by recording the ratio of estimated
  // value to the actual value.
  base::HistogramBase* ratio_median_rtt = GetHistogram(
      "RatioEstimatedToActualRTT.", current_network_id_.type, 1000);
  ratio_median_rtt->Add(ratio);
}

bool NetworkQualityEstimator::RequestProvidesUsefulObservations(
    const URLRequest& request) const {
  return request.url().is_valid() &&
         (allow_localhost_requests_ || !IsLocalhost(request.url().host())) &&
         request.url().SchemeIsHTTPOrHTTPS() &&
         // Verify that response headers are received, so it can be ensured that
         // response is not cached.
         !request.response_info().response_time.is_null() &&
         !request.was_cached() &&
         request.creation_time() >= last_connection_change_;
}

void NetworkQualityEstimator::OnConnectionTypeChanged(
    NetworkChangeNotifier::ConnectionType type) {
  DCHECK(thread_checker_.CalledOnValidThread());
  if (peak_network_quality_.rtt() != InvalidRTT()) {
    switch (current_network_id_.type) {
      case NetworkChangeNotifier::CONNECTION_UNKNOWN:
        UMA_HISTOGRAM_TIMES("NQE.FastestRTT.Unknown",
                            peak_network_quality_.rtt());
        break;
      case NetworkChangeNotifier::CONNECTION_ETHERNET:
        UMA_HISTOGRAM_TIMES("NQE.FastestRTT.Ethernet",
                            peak_network_quality_.rtt());
        break;
      case NetworkChangeNotifier::CONNECTION_WIFI:
        UMA_HISTOGRAM_TIMES("NQE.FastestRTT.Wifi", peak_network_quality_.rtt());
        break;
      case NetworkChangeNotifier::CONNECTION_2G:
        UMA_HISTOGRAM_TIMES("NQE.FastestRTT.2G", peak_network_quality_.rtt());
        break;
      case NetworkChangeNotifier::CONNECTION_3G:
        UMA_HISTOGRAM_TIMES("NQE.FastestRTT.3G", peak_network_quality_.rtt());
        break;
      case NetworkChangeNotifier::CONNECTION_4G:
        UMA_HISTOGRAM_TIMES("NQE.FastestRTT.4G", peak_network_quality_.rtt());
        break;
      case NetworkChangeNotifier::CONNECTION_NONE:
        UMA_HISTOGRAM_TIMES("NQE.FastestRTT.None", peak_network_quality_.rtt());
        break;
      case NetworkChangeNotifier::CONNECTION_BLUETOOTH:
        UMA_HISTOGRAM_TIMES("NQE.FastestRTT.Bluetooth",
                            peak_network_quality_.rtt());
        break;
      default:
        NOTREACHED() << "Unexpected connection type = "
                     << current_network_id_.type;
        break;
    }
  }

  if (peak_network_quality_.downstream_throughput_kbps() !=
      kInvalidThroughput) {
    switch (current_network_id_.type) {
      case NetworkChangeNotifier::CONNECTION_UNKNOWN:
        UMA_HISTOGRAM_COUNTS(
            "NQE.PeakKbps.Unknown",
            peak_network_quality_.downstream_throughput_kbps());
        break;
      case NetworkChangeNotifier::CONNECTION_ETHERNET:
        UMA_HISTOGRAM_COUNTS(
            "NQE.PeakKbps.Ethernet",
            peak_network_quality_.downstream_throughput_kbps());
        break;
      case NetworkChangeNotifier::CONNECTION_WIFI:
        UMA_HISTOGRAM_COUNTS(
            "NQE.PeakKbps.Wifi",
            peak_network_quality_.downstream_throughput_kbps());
        break;
      case NetworkChangeNotifier::CONNECTION_2G:
        UMA_HISTOGRAM_COUNTS(
            "NQE.PeakKbps.2G",
            peak_network_quality_.downstream_throughput_kbps());
        break;
      case NetworkChangeNotifier::CONNECTION_3G:
        UMA_HISTOGRAM_COUNTS(
            "NQE.PeakKbps.3G",
            peak_network_quality_.downstream_throughput_kbps());
        break;
      case NetworkChangeNotifier::CONNECTION_4G:
        UMA_HISTOGRAM_COUNTS(
            "NQE.PeakKbps.4G",
            peak_network_quality_.downstream_throughput_kbps());
        break;
      case NetworkChangeNotifier::CONNECTION_NONE:
        UMA_HISTOGRAM_COUNTS(
            "NQE.PeakKbps.None",
            peak_network_quality_.downstream_throughput_kbps());
        break;
      case NetworkChangeNotifier::CONNECTION_BLUETOOTH:
        UMA_HISTOGRAM_COUNTS(
            "NQE.PeakKbps.Bluetooth",
            peak_network_quality_.downstream_throughput_kbps());
        break;
      default:
        NOTREACHED() << "Unexpected connection type = "
                     << current_network_id_.type;
        break;
    }
  }

  base::TimeDelta rtt = GetRTTEstimateInternal(base::TimeTicks(), 50);
  if (rtt != InvalidRTT()) {
    // Add the 50th percentile value.
    base::HistogramBase* rtt_percentile =
        GetHistogram("RTT.Percentile50.", current_network_id_.type,
                     10 * 1000);  // 10 seconds
    rtt_percentile->Add(rtt.InMilliseconds());

    // Add the remaining percentile values.
    static const int kPercentiles[] = {0, 10, 90, 100};
    for (size_t i = 0; i < arraysize(kPercentiles); ++i) {
      rtt = GetRTTEstimateInternal(base::TimeTicks(), kPercentiles[i]);

      rtt_percentile = GetHistogram(
          "RTT.Percentile" + base::IntToString(kPercentiles[i]) + ".",
          current_network_id_.type, 10 * 1000);  // 10 seconds
      rtt_percentile->Add(rtt.InMilliseconds());
    }
  }

  // Write the estimates of the previous network to the cache.
  CacheNetworkQualityEstimate();

  // Clear the local state.
  last_connection_change_ = base::TimeTicks::Now();
  peak_network_quality_ = NetworkQuality();
  downstream_throughput_kbps_observations_.Clear();
  rtt_msec_observations_.Clear();
  current_network_id_ = GetCurrentNetworkID();

  // Read any cached estimates for the new network. If cached estimates are
  // unavailable, add the default estimates.
  if (!ReadCachedNetworkQualityEstimate())
    AddDefaultEstimates();
  estimated_median_network_quality_ = NetworkQuality();
}

bool NetworkQualityEstimator::GetRTTEstimate(base::TimeDelta* rtt) const {
  DCHECK(thread_checker_.CalledOnValidThread());
  DCHECK(rtt);
  if (rtt_msec_observations_.Size() == 0) {
    *rtt = InvalidRTT();
    return false;
  }
  *rtt = GetRTTEstimateInternal(base::TimeTicks(), 50);
  return (*rtt != InvalidRTT());
}

bool NetworkQualityEstimator::GetDownlinkThroughputKbpsEstimate(
    int32_t* kbps) const {
  DCHECK(thread_checker_.CalledOnValidThread());
  DCHECK(kbps);
  if (downstream_throughput_kbps_observations_.Size() == 0) {
    *kbps = kInvalidThroughput;
    return false;
  }
  *kbps = GetDownlinkThroughputKbpsEstimateInternal(base::TimeTicks(), 50);
  return (*kbps != kInvalidThroughput);
}

bool NetworkQualityEstimator::GetRecentMedianRTT(
    const base::TimeTicks& begin_timestamp,
    base::TimeDelta* rtt) const {
  DCHECK(thread_checker_.CalledOnValidThread());
  DCHECK(rtt);
  *rtt = GetRTTEstimateInternal(begin_timestamp, 50);
  return (*rtt != InvalidRTT());
}

bool NetworkQualityEstimator::GetRecentMedianDownlinkThroughputKbps(
    const base::TimeTicks& begin_timestamp,
    int32_t* kbps) const {
  DCHECK(thread_checker_.CalledOnValidThread());
  DCHECK(kbps);
  *kbps = GetDownlinkThroughputKbpsEstimateInternal(begin_timestamp, 50);
  return (*kbps != kInvalidThroughput);
}

NetworkQualityEstimator::Observation::Observation(int32_t value,
                                                  base::TimeTicks timestamp)
    : value(value), timestamp(timestamp) {
  DCHECK_GE(value, 0);
  DCHECK(!timestamp.is_null());
}

NetworkQualityEstimator::Observation::~Observation() {
}

NetworkQualityEstimator::ObservationBuffer::ObservationBuffer(
    double weight_multiplier_per_second)
    : weight_multiplier_per_second_(weight_multiplier_per_second) {
  static_assert(kMaximumObservationsBufferSize > 0U,
                "Minimum size of observation buffer must be > 0");
  DCHECK_GE(weight_multiplier_per_second_, 0.0);
  DCHECK_LE(weight_multiplier_per_second_, 1.0);
}

NetworkQualityEstimator::ObservationBuffer::~ObservationBuffer() {
}

void NetworkQualityEstimator::ObservationBuffer::AddObservation(
    const Observation& observation) {
  DCHECK_LE(observations_.size(),
            static_cast<size_t>(kMaximumObservationsBufferSize));
  // Evict the oldest element if the buffer is already full.
  if (observations_.size() == kMaximumObservationsBufferSize)
    observations_.pop_front();

  observations_.push_back(observation);
  DCHECK_LE(observations_.size(),
            static_cast<size_t>(kMaximumObservationsBufferSize));
}

size_t NetworkQualityEstimator::ObservationBuffer::Size() const {
  return observations_.size();
}

void NetworkQualityEstimator::ObservationBuffer::Clear() {
  observations_.clear();
  DCHECK(observations_.empty());
}

base::TimeDelta NetworkQualityEstimator::GetRTTEstimateInternal(
    const base::TimeTicks& begin_timestamp,
    int percentile) const {
  DCHECK(thread_checker_.CalledOnValidThread());
  DCHECK_GE(percentile, 0);
  DCHECK_LE(percentile, 100);
  if (rtt_msec_observations_.Size() == 0)
    return InvalidRTT();

  // RTT observations are sorted by duration from shortest to longest, thus
  // a higher percentile RTT will have a longer RTT than a lower percentile.
  base::TimeDelta rtt = InvalidRTT();
  int32_t rtt_result = -1;
  if (rtt_msec_observations_.GetPercentile(begin_timestamp, &rtt_result,
                                           percentile)) {
    rtt = base::TimeDelta::FromMilliseconds(rtt_result);
  }
  return rtt;
}

int32_t NetworkQualityEstimator::GetDownlinkThroughputKbpsEstimateInternal(
    const base::TimeTicks& begin_timestamp,
    int percentile) const {
  DCHECK(thread_checker_.CalledOnValidThread());
  DCHECK_GE(percentile, 0);
  DCHECK_LE(percentile, 100);
  if (downstream_throughput_kbps_observations_.Size() == 0)
    return kInvalidThroughput;

  // Throughput observations are sorted by kbps from slowest to fastest,
  // thus a higher percentile throughput will be faster than a lower one.
  int32_t kbps = kInvalidThroughput;
  downstream_throughput_kbps_observations_.GetPercentile(begin_timestamp, &kbps,
                                                         100 - percentile);
  return kbps;
}

void NetworkQualityEstimator::ObservationBuffer::ComputeWeightedObservations(
    const base::TimeTicks& begin_timestamp,
    std::vector<WeightedObservation>& weighted_observations,
    double* total_weight) const {
  weighted_observations.clear();
  double total_weight_observations = 0.0;
  base::TimeTicks now = base::TimeTicks::Now();

  for (const auto& observation : observations_) {
    if (observation.timestamp < begin_timestamp)
      continue;
    base::TimeDelta time_since_sample_taken = now - observation.timestamp;
    double weight =
        pow(weight_multiplier_per_second_, time_since_sample_taken.InSeconds());
    weight = std::max(DBL_MIN, std::min(1.0, weight));

    weighted_observations.push_back(
        WeightedObservation(observation.value, weight));
    total_weight_observations += weight;
  }

  // Sort the samples by value in ascending order.
  std::sort(weighted_observations.begin(), weighted_observations.end());
  *total_weight = total_weight_observations;
}

bool NetworkQualityEstimator::ObservationBuffer::GetPercentile(
    const base::TimeTicks& begin_timestamp,
    int32_t* result,
    int percentile) const {
  DCHECK(result);
  // Stores WeightedObservation in increasing order of value.
  std::vector<WeightedObservation> weighted_observations;

  // Total weight of all observations in |weighted_observations|.
  double total_weight = 0.0;

  ComputeWeightedObservations(begin_timestamp, weighted_observations,
                              &total_weight);
  if (weighted_observations.empty())
    return false;

  DCHECK(!weighted_observations.empty());
  DCHECK_GT(total_weight, 0.0);

  // weighted_observations may have a smaller size than observations_ since the
  // former contains only the observations later than begin_timestamp.
  DCHECK_GE(observations_.size(), weighted_observations.size());

  double desired_weight = percentile / 100.0 * total_weight;

  double cumulative_weight_seen_so_far = 0.0;
  for (const auto& weighted_observation : weighted_observations) {
    cumulative_weight_seen_so_far += weighted_observation.weight;

    // TODO(tbansal): Consider interpolating between observations.
    if (cumulative_weight_seen_so_far >= desired_weight) {
      *result = weighted_observation.value;
      return true;
    }
  }

  // Computation may reach here due to floating point errors. This may happen
  // if |percentile| was 100 (or close to 100), and |desired_weight| was
  // slightly larger than |total_weight| (due to floating point errors).
  // In this case, we return the highest |value| among all observations.
  // This is same as value of the last observation in the sorted vector.
  *result = weighted_observations.at(weighted_observations.size() - 1).value;
  return true;
}

NetworkQualityEstimator::NetworkID
NetworkQualityEstimator::GetCurrentNetworkID() const {
  DCHECK(thread_checker_.CalledOnValidThread());

  // TODO(tbansal): crbug.com/498068 Add NetworkQualityEstimatorAndroid class
  // that overrides this method on the Android platform.

  // It is possible that the connection type changed between when
  // GetConnectionType() was called and when the API to determine the
  // network name was called. Check if that happened and retry until the
  // connection type stabilizes. This is an imperfect solution but should
  // capture majority of cases, and should not significantly affect estimates
  // (that are approximate to begin with).
  while (true) {
    NetworkQualityEstimator::NetworkID network_id(
        NetworkChangeNotifier::GetConnectionType(), std::string());

    switch (network_id.type) {
      case NetworkChangeNotifier::ConnectionType::CONNECTION_UNKNOWN:
      case NetworkChangeNotifier::ConnectionType::CONNECTION_NONE:
      case NetworkChangeNotifier::ConnectionType::CONNECTION_BLUETOOTH:
      case NetworkChangeNotifier::ConnectionType::CONNECTION_ETHERNET:
        break;
      case NetworkChangeNotifier::ConnectionType::CONNECTION_WIFI:
#if defined(OS_ANDROID) || defined(OS_LINUX) || defined(OS_CHROMEOS) || \
    defined(OS_WIN)
        network_id.id = GetWifiSSID();
#endif
        break;
      case NetworkChangeNotifier::ConnectionType::CONNECTION_2G:
      case NetworkChangeNotifier::ConnectionType::CONNECTION_3G:
      case NetworkChangeNotifier::ConnectionType::CONNECTION_4G:
#if defined(OS_ANDROID)
        network_id.id = android::GetTelephonyNetworkOperator();
#endif
        break;
      default:
        NOTREACHED() << "Unexpected connection type = " << network_id.type;
        break;
    }

    if (network_id.type == NetworkChangeNotifier::GetConnectionType())
      return network_id;
  }
  NOTREACHED();
}

bool NetworkQualityEstimator::ReadCachedNetworkQualityEstimate() {
  DCHECK(thread_checker_.CalledOnValidThread());

  // If the network name is unavailable, caching should not be performed.
  if (current_network_id_.id.empty())
    return false;

  CachedNetworkQualities::const_iterator it =
      cached_network_qualities_.find(current_network_id_);

  if (it == cached_network_qualities_.end())
    return false;

  NetworkQuality network_quality(it->second.network_quality());

  DCHECK_NE(InvalidRTT(), network_quality.rtt());
  DCHECK_NE(kInvalidThroughput, network_quality.downstream_throughput_kbps());

  downstream_throughput_kbps_observations_.AddObservation(Observation(
      network_quality.downstream_throughput_kbps(), base::TimeTicks::Now()));
  rtt_msec_observations_.AddObservation(Observation(
      network_quality.rtt().InMilliseconds(), base::TimeTicks::Now()));
  return true;
}

void NetworkQualityEstimator::OnUpdatedEstimateAvailable() {
  DCHECK(thread_checker_.CalledOnValidThread());
  DCHECK(external_estimates_provider_);
  // TODO(tbansal): Query provider for the recent value.
}

void NetworkQualityEstimator::CacheNetworkQualityEstimate() {
  DCHECK(thread_checker_.CalledOnValidThread());
  DCHECK_LE(cached_network_qualities_.size(),
            static_cast<size_t>(kMaximumNetworkQualityCacheSize));

  // If the network name is unavailable, caching should not be performed.
  if (current_network_id_.id.empty())
    return;

  NetworkQuality network_quality = NetworkQuality(
      GetRTTEstimateInternal(base::TimeTicks(), 50),
      GetDownlinkThroughputKbpsEstimateInternal(base::TimeTicks(), 50));
  if (network_quality.rtt() == InvalidRTT() ||
      network_quality.downstream_throughput_kbps() == kInvalidThroughput) {
    return;
  }

  if (cached_network_qualities_.size() == kMaximumNetworkQualityCacheSize) {
    // Remove the oldest entry.
    CachedNetworkQualities::iterator oldest_entry_iterator =
        cached_network_qualities_.begin();

    for (CachedNetworkQualities::iterator it =
             cached_network_qualities_.begin();
         it != cached_network_qualities_.end(); ++it) {
      if ((it->second).OlderThan(oldest_entry_iterator->second))
        oldest_entry_iterator = it;
    }
    cached_network_qualities_.erase(oldest_entry_iterator);
  }
  DCHECK_LT(cached_network_qualities_.size(),
            static_cast<size_t>(kMaximumNetworkQualityCacheSize));

  cached_network_qualities_.insert(std::make_pair(
      current_network_id_, CachedNetworkQuality(network_quality)));
  DCHECK_LE(cached_network_qualities_.size(),
            static_cast<size_t>(kMaximumNetworkQualityCacheSize));
}

NetworkQualityEstimator::CachedNetworkQuality::CachedNetworkQuality(
    const NetworkQuality& network_quality)
    : last_update_time_(base::TimeTicks::Now()),
      network_quality_(network_quality) {
}

NetworkQualityEstimator::CachedNetworkQuality::CachedNetworkQuality(
    const CachedNetworkQuality& other)
    : last_update_time_(other.last_update_time_),
      network_quality_(other.network_quality_) {
}

NetworkQualityEstimator::CachedNetworkQuality::~CachedNetworkQuality() {
}

bool NetworkQualityEstimator::CachedNetworkQuality::OlderThan(
    const CachedNetworkQuality& cached_network_quality) const {
  return last_update_time_ < cached_network_quality.last_update_time_;
}

NetworkQualityEstimator::NetworkQuality::NetworkQuality()
    : NetworkQuality(NetworkQualityEstimator::InvalidRTT(),
                     NetworkQualityEstimator::kInvalidThroughput) {}

NetworkQualityEstimator::NetworkQuality::NetworkQuality(
    const base::TimeDelta& rtt,
    int32_t downstream_throughput_kbps)
    : rtt_(rtt), downstream_throughput_kbps_(downstream_throughput_kbps) {
  DCHECK_GE(rtt_, base::TimeDelta());
  DCHECK_GE(downstream_throughput_kbps_, 0);
}

NetworkQualityEstimator::NetworkQuality::NetworkQuality(
    const NetworkQuality& other)
    : NetworkQuality(other.rtt_, other.downstream_throughput_kbps_) {}

NetworkQualityEstimator::NetworkQuality::~NetworkQuality() {}

NetworkQualityEstimator::NetworkQuality&
    NetworkQualityEstimator::NetworkQuality::
    operator=(const NetworkQuality& other) {
  rtt_ = other.rtt_;
  downstream_throughput_kbps_ = other.downstream_throughput_kbps_;
  return *this;
}

}  // namespace net