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// Copyright 2016 Google Inc. 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.
#ifndef SRC_WEIGHTED_RESERVOIR_SAMPLER_H_
#define SRC_WEIGHTED_RESERVOIR_SAMPLER_H_
#include <cassert>
#include <random>
namespace protobuf_mutator {
// Algorithm pick one item from the sequence of weighted items.
// https://en.wikipedia.org/wiki/Reservoir_sampling#Algorithm_A-Chao
//
// Example:
// WeightedReservoirSampler<int> sampler;
// for(int i = 0; i < size; ++i)
// sampler.Pick(weight[i], i);
// return sampler.GetSelected();
template <class T, class RandomEngine = std::default_random_engine>
class WeightedReservoirSampler {
public:
explicit WeightedReservoirSampler(RandomEngine* random) : random_(random) {}
void Try(uint64_t weight, const T& item) {
if (Pick(weight)) selected_ = item;
}
const T& selected() const { return selected_; }
bool IsEmpty() const { return total_weight_ == 0; }
private:
bool Pick(uint64_t weight) {
if (weight == 0) return false;
total_weight_ += weight;
return weight == total_weight_ || std::uniform_int_distribution<uint64_t>(
1, total_weight_)(*random_) <= weight;
}
T selected_ = {};
uint64_t total_weight_ = 0;
RandomEngine* random_;
};
} // namespace protobuf_mutator
#endif // SRC_WEIGHTED_RESERVOIR_SAMPLER_H_
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