// Copyright 2016 Google Inc. All rights reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef SRC_WEIGHTED_RESERVOIR_SAMPLER_H_
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#define SRC_WEIGHTED_RESERVOIR_SAMPLER_H_
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#include <cassert>
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#include <random>
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namespace protobuf_mutator {
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// Algorithm pick one item from the sequence of weighted items.
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// https://en.wikipedia.org/wiki/Reservoir_sampling#Algorithm_A-Chao
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//
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// Example:
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// WeightedReservoirSampler<int> sampler;
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// for(int i = 0; i < size; ++i)
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// sampler.Pick(weight[i], i);
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// return sampler.GetSelected();
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template <class T, class RandomEngine = std::default_random_engine>
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class WeightedReservoirSampler {
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public:
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explicit WeightedReservoirSampler(RandomEngine* random) : random_(random) {}
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void Try(uint64_t weight, const T& item) {
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if (Pick(weight)) selected_ = item;
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}
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const T& selected() const { return selected_; }
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bool IsEmpty() const { return total_weight_ == 0; }
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private:
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bool Pick(uint64_t weight) {
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if (weight == 0) return false;
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total_weight_ += weight;
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return weight == total_weight_ || std::uniform_int_distribution<uint64_t>(
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1, total_weight_)(*random_) <= weight;
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}
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T selected_ = {};
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uint64_t total_weight_ = 0;
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RandomEngine* random_;
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};
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} // namespace protobuf_mutator
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#endif // SRC_WEIGHTED_RESERVOIR_SAMPLER_H_
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