metrics: Add RandomSelector, a class that randomly selects items with odds

chrome/browser/metrics/perf/random_selector.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 "chrome/browser/metrics/perf/random_selector.h"
+
+#include "base/logging.h"
+#include "base/rand_util.h"
+#include "base/strings/string_number_conversions.h"
+#include "base/strings/string_split.h"
+#include "base/strings/string_util.h"
+
+RandomSelector::RandomSelector() : sum_of_weights_(0) {}
+
+RandomSelector::~RandomSelector() {}
+
+double RandomSelector::SumWeights(const std::vector<WeightAndValue>& odds) {
+  double sum = 0.0;
+  for (const auto& odd : odds) {
+    sum += odd.weight;
+  }
+  return sum;
+}
+
+void RandomSelector::SetOdds(const std::vector<WeightAndValue>& odds) {
+  odds_ = odds;
+  sum_of_weights_ = SumWeights(odds_);
+}
+
+const std::string& RandomSelector::Select() {
+  // Get a random double between 0 and the sum.
+  double random = RandDoubleUpTo(sum_of_weights_);
+  // Figure out what it belongs to.
+  return GetValueFor(random);
+}
+
+double RandomSelector::RandDoubleUpTo(double max) {
+  CHECK_GT(max, 0.0);
+  return max * base::RandDouble();
+}
+
+const std::string& RandomSelector::GetValueFor(double random) {
+  double current = 0.0;
+  for (const auto& odd : odds_) {
+    current += odd.weight;
+    if (random < current)
+      return odd.value;
+  }
+  NOTREACHED() << "Invalid value for key: " << random;
+  return base::EmptyString();
+}