text: // Copyright 2010-2014 Google
  // 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 OR_TOOLS_BASE_ACCURATE_SUM_H_
  #define OR_TOOLS_BASE_ACCURATE_SUM_H_
  
  namespace operations_research {
  
  // Kahan summation compensation algorithm.
  //
  //   http://en.wikipedia.org/wiki/Kahan_summation_algorithm
  template <typename FpNumber>
  class AccurateSum {
   public:
    // You may copy-construct an AccurateSum.
    AccurateSum() : sum_(), error_sum_() {}
  
    // Adds an FpNumber to the sum.
    void Add(const FpNumber& value) {
      error_sum_ += value;
      const FpNumber new_sum = sum_ + error_sum_;
      error_sum_ += sum_ - new_sum;
      sum_ = new_sum;
    }
  
    // Gets the value of the sum.
    FpNumber Value() const { return sum_; }
  
   private:
    FpNumber sum_;
    FpNumber error_sum_;
  };
  
  }  // namespace operations_research
  
  #endif  // OR_TOOLS_BASE_ACCURATE_SUM_H_
  