RcppFunction

#include <Rcpp.h>
// [[Rcpp::depends(RcppParallel)]]
#include <RcppParallel.h>

struct EuclidianDistanceCalculator: RcppParallel::Worker{
  const RcppParallel::RMatrix<double> inputMatrix;
  RcppParallel::RVector<double> result;
  const RcppParallel::RVector<int> oldColumns;

  EuclidianDistanceCalculator(const Rcpp::NumericMatrix& inputMatrix,
                              Rcpp::NumericVector& result, const Rcpp::LogicalVector& oldColumns)
    : inputMatrix(inputMatrix), result(result), oldColumns(oldColumns){
  }

  void operator()(std::size_t begin, std::size_t end){
    for (std::size_t i = begin; i < end; i++){
      RcppParallel::RMatrix<double>::Row row = inputMatrix.row(i);

      double partialEuclidianDistance = 0;
      for (unsigned int j = 0; j < row.length(); j++){
        if (oldColumns[j]){
          partialEuclidianDistance += row[j] * row[j];
        }
      }
      result[i] = partialEuclidianDistance;
    }
  }
};

// [[Rcpp::export]]
void calculateEuclidianDistances(const Rcpp::NumericMatrix& deltaMatrix, const Rcpp::LogicalVector& oldColumns, Rcpp::NumericVector& resultEuclidianDistances){
  int length = deltaMatrix.nrow();
  EuclidianDistanceCalculator edc(deltaMatrix, resultEuclidianDistances, oldColumns);
  RcppParallel::parallelFor(0, length, edc);
}