src/statistics.cc

// Copyright 2016 Ismael Jimenez Martinez. All rights reserved.
// Copyright 2017 Roman Lebedev. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include "benchmark/benchmark.h"

#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <numeric>
#include "check.h"
#include "statistics.h"

namespace benchmark {

auto StatisticsSum = [](const std::vector<double>& v) {
  return std::accumulate(v.begin(), v.end(), 0.0);
};

double StatisticsMean(const std::vector<double>& v) {
  if (v.size() == 0) return 0.0;
  return StatisticsSum(v) * (1.0 / v.size());
}

double StatisticsMedian(const std::vector<double>& v) {
  if (v.size() < 3) return StatisticsMean(v);
  std::vector<double> partial;
  // we need roundDown(count/2)+1 slots
  partial.resize(1 + (v.size() / 2));
  std::partial_sort_copy(v.begin(), v.end(), partial.begin(), partial.end());
  // did we have odd number of samples?
  // if yes, then the last element of partially-sorted vector is the median
  // it no, then the average of the last two elements is the median
  if(v.size() % 2 == 1)
    return partial.back();
  return (partial[partial.size() - 2] + partial[partial.size() - 1]) / 2.0;
}

// Return the sum of the squares of this sample set
auto SumSquares = [](const std::vector<double>& v) {
  return std::inner_product(v.begin(), v.end(), v.begin(), 0.0);
};

auto Sqr = [](const double dat) { return dat * dat; };
auto Sqrt = [](const double dat) {
  // Avoid NaN due to imprecision in the calculations
  if (dat < 0.0) return 0.0;
  return std::sqrt(dat);
};

double StatisticsStdDev(const std::vector<double>& v) {
  const auto mean = StatisticsMean(v);
  if (v.size() == 0) return mean;

  // Sample standard deviation is undefined for n = 1
  if (v.size() == 1)
    return 0.0;

  const double avg_squares = SumSquares(v) * (1.0 / v.size());
  return Sqrt(v.size() / (v.size() - 1.0) * (avg_squares - Sqr(mean)));
}

std::vector<BenchmarkReporter::Run> ComputeStats(
    const std::vector<BenchmarkReporter::Run>& reports) {
  typedef BenchmarkReporter::Run Run;
  std::vector<Run> results;

  auto error_count =
      std::count_if(reports.begin(), reports.end(),
                    [](Run const& run) { return run.error_occurred; });

  if (reports.size() - error_count < 2) {
    // We don't report aggregated data if there was a single run.
    return results;
  }

  // Accumulators.
  std::vector<double> real_accumulated_time_stat;
  std::vector<double> cpu_accumulated_time_stat;
  std::vector<double> bytes_per_second_stat;
  std::vector<double> items_per_second_stat;

  real_accumulated_time_stat.reserve(reports.size());
  cpu_accumulated_time_stat.reserve(reports.size());
  bytes_per_second_stat.reserve(reports.size());
  items_per_second_stat.reserve(reports.size());

  // All repetitions should be run with the same number of iterations so we
  // can take this information from the first benchmark.
  int64_t const run_iterations = reports.front().iterations;
  // create stats for user counters
  struct CounterStat {
    Counter c;
    std::vector<double> s;
  };
  std::map< std::string, CounterStat > counter_stats;
  for(Run const& r : reports) {
    for(auto const& cnt : r.counters) {
      auto it = counter_stats.find(cnt.first);
      if(it == counter_stats.end()) {
        counter_stats.insert({cnt.first, {cnt.second, std::vector<double>{}}});
        it = counter_stats.find(cnt.first);
        it->second.s.reserve(reports.size());
      } else {
        CHECK_EQ(counter_stats[cnt.first].c.flags, cnt.second.flags);
      }
    }
  }

  // Populate the accumulators.
  for (Run const& run : reports) {
    CHECK_EQ(reports[0].benchmark_name, run.benchmark_name);
    CHECK_EQ(run_iterations, run.iterations);
    if (run.error_occurred) continue;
    real_accumulated_time_stat.emplace_back(run.real_accumulated_time);
    cpu_accumulated_time_stat.emplace_back(run.cpu_accumulated_time);
    items_per_second_stat.emplace_back(run.items_per_second);
    bytes_per_second_stat.emplace_back(run.bytes_per_second);
    // user counters
    for(auto const& cnt : run.counters) {
      auto it = counter_stats.find(cnt.first);
      CHECK_NE(it, counter_stats.end());
      it->second.s.emplace_back(cnt.second);
    }
  }

  // Only add label if it is same for all runs
  std::string report_label = reports[0].report_label;
  for (std::size_t i = 1; i < reports.size(); i++) {
    if (reports[i].report_label != report_label) {
      report_label = "";
      break;
    }
  }

  for(const auto& Stat : *reports[0].statistics) {
    // Get the data from the accumulator to BenchmarkReporter::Run's.
    Run data;
    data.benchmark_name = reports[0].benchmark_name + "_" + Stat.name_;
    data.report_label = report_label;
    data.iterations = run_iterations;

    data.real_accumulated_time = Stat.compute_(real_accumulated_time_stat);
    data.cpu_accumulated_time = Stat.compute_(cpu_accumulated_time_stat);
    data.bytes_per_second = Stat.compute_(bytes_per_second_stat);
    data.items_per_second = Stat.compute_(items_per_second_stat);

    data.time_unit = reports[0].time_unit;

    // user counters
    for(auto const& kv : counter_stats) {
      const auto uc_stat = Stat.compute_(kv.second.s);
      auto c = Counter(uc_stat, counter_stats[kv.first].c.flags);
      data.counters[kv.first] = c;
    }

    results.push_back(data);
  }

  return results;
}

}  // end namespace benchmark