#ifndef FESA_MATH_COMPENSATED_SUM_H_
#define FESA_MATH_COMPENSATED_SUM_H_

#include <cmath>

namespace fesa::math_internal {

/// @brief Preserves the compensated scalar accumulation graph and caller order.
inline void AddCompensated(const double term, double* sum, double* correction) {
  const double next = *sum + term;
  *correction += std::abs(*sum) >= std::abs(term)
                     ? (*sum - next) + term
                     : (term - next) + *sum;
  *sum = next;
}

/// @brief Owns one sum and its correction without changing reduction order.
class CompensatedSum {
 public:
  void Add(const double term) { AddCompensated(term, &sum_, &correction_); }
  double Value() const { return sum_ + correction_; }

 private:
  double sum_ = 0.0;
  double correction_ = 0.0;
};

}  // namespace fesa::math_internal

#endif  // FESA_MATH_COMPENSATED_SUM_H_