Amortize data usage using TrafficStats on Android.

components/data_usage/android/traffic_stats_amortizer.cc

+// 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 "components/data_usage/android/traffic_stats_amortizer.h"
+
+#include <cmath>
+
+#include "net/android/traffic_stats.h"
+
+namespace data_usage {
+namespace android {
+
+namespace {
+
+int64_t AmortizeByRatio(int64_t bytes, double ratio, double* remainder) {

[tbansal1, 2015/10/12 21:23:20]

please add comments.

+  DCHECK(remainder);
+
+  double intpart;

[tbansal1, 2015/10/12 21:23:20]

may be initialize to -1, than DCHECK_GE() below would ensure that it is >=0

+  *remainder =
+      std::modf(static_cast<double>(bytes) * ratio + (*remainder), &intpart);
+
+  DCHECK_GE(intpart, 0.0);
+  DCHECK_LE(intpart, static_cast<double>(INT64_MAX));
+  return static_cast<int64_t>(intpart);
+}
+
+}  // namespace
+
+TrafficStatsAmortizer::TrafficStatsAmortizer()
+    : are_traffic_stats_available_(false),
+      traffic_stats_tx_bytes_(0),
+      traffic_stats_rx_bytes_(0) {}
+
+TrafficStatsAmortizer::~TrafficStatsAmortizer() {}
+
+void TrafficStatsAmortizer::Amortize(std::vector<DataUse>* data_use_sequence,
+                                     int64_t extra_tx_bytes,
+                                     int64_t extra_rx_bytes) {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  DCHECK(!data_use_sequence->empty());
+
+  bool previous_are_traffic_stats_available = are_traffic_stats_available_;
+  int64_t previous_traffic_stats_tx_bytes = traffic_stats_tx_bytes_;
+  int64_t previous_traffic_stats_rx_bytes = traffic_stats_rx_bytes_;
+
+  // TODO(sclittle): Currently, the TrafficStats numbers don't seem to update
+  // fast enough for the amortization to be accurate at this frequency. Find
+  // some way to smooth out these numbers nicely, e.g. amortize over a longer
+  // period of time, not just the most recent delta.
+  RefreshTrafficStats();

[bengr, 2015/10/12 21:33:08]

Say more. How far behind is TrafficStats?

+
+  if (!previous_are_traffic_stats_available || !are_traffic_stats_available_)
+    return;
+
+  int64_t traffic_stats_tx_delta_bytes =
+      traffic_stats_tx_bytes_ - previous_traffic_stats_tx_bytes;
+  int64_t traffic_stats_rx_delta_bytes =
+      traffic_stats_rx_bytes_ - previous_traffic_stats_rx_bytes;
+
+  int64_t total_unamortized_tx_delta_bytes = extra_tx_bytes;
+  int64_t total_unamortized_rx_delta_bytes = extra_rx_bytes;
+  for (const DataUse& data_use : *data_use_sequence) {
+    total_unamortized_tx_delta_bytes += data_use.tx_bytes;
+    total_unamortized_rx_delta_bytes += data_use.rx_bytes;
+  }
+
+  const double tx_ratio = static_cast<double>(traffic_stats_tx_delta_bytes) /
+                          static_cast<double>(total_unamortized_tx_delta_bytes);

[tbansal1, 2015/10/12 21:23:20]

can this give divide by 0 error? It probably can depending on how fast traffic
stats updates.

[bengr, 2015/10/12 21:33:08]

Agreed.

+  const double rx_ratio = static_cast<double>(traffic_stats_rx_delta_bytes) /
+                          static_cast<double>(total_unamortized_rx_delta_bytes);
+
+  double tx_remainder = 0.0;
+  double rx_remainder = 0.0;
+  for (DataUse& data_use : *data_use_sequence) {
+    data_use.tx_bytes =
+        AmortizeByRatio(data_use.tx_bytes, tx_ratio, &tx_remainder);
+    data_use.rx_bytes =
+        AmortizeByRatio(data_use.rx_bytes, rx_ratio, &rx_remainder);
+  }

[tbansal1, 2015/10/12 21:23:20]

DCHECK that tx_remainder and rx_remainder < 1 and >=0?

+
+  // TODO(sclittle): Maybe record some UMA comparing amortized values to
+  // unamortized values?
+}
+
+bool TrafficStatsAmortizer::GetTrafficStatsTxBytes(int64_t* tx_bytes) const {
+  return net::android::traffic_stats::GetCurrentUidTxBytes(tx_bytes);

[tbansal1, 2015/10/12 21:23:20]

DCHECK(thread_checker_...)

+}
+
+bool TrafficStatsAmortizer::GetTrafficStatsRxBytes(int64_t* rx_bytes) const {
+  return net::android::traffic_stats::GetCurrentUidRxBytes(rx_bytes);
+}
+
+void TrafficStatsAmortizer::RefreshTrafficStats() {
+  DCHECK(thread_checker_.CalledOnValidThread());
+
+  are_traffic_stats_available_ =
+      GetTrafficStatsTxBytes(&traffic_stats_tx_bytes_) &&

[bengr, 2015/10/12 21:33:08]

Can tx_bytes_ or rx_bytes_ be 0?

+      GetTrafficStatsRxBytes(&traffic_stats_rx_bytes_);
+}
+
+}  // namespace android
+}  // namespace data_usage