pairwise_sum.h

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template <typename T, typename L>
inline double pairwise_sum(T* vector, L length) {

  double sum = 0;

  /* Base case: if length is less than 16, perform summation */
  if (length < 16) {

#pragma omp simd reduction(+:sum)
    for (L i=0; i < length; i++)
      sum += vector[i];
  }

  else {
    L offset = length % 2;
    length = floor(length / 2);
    sum = pairwise_sum<T>(&vector[0], length) +
          pairwise_sum<T>(&vector[length], length+offset);
  }

  return sum;
}