Merge sort: Rcpp integration code

/*sorting function using the merge sort algorithm
*/

#include <Rcpp.h>
using namespace Rcpp;

void intercal(int p, int q, int r, NumericVector v,  NumericVector w)
{
  int i, j, k;
  i = p;
  j = q;
  k = 0;
  while (i < q && j < r) {
    if (v[i] < v[j]) {
      w[k] = v[i];
      i++;
    }
    else {
      w[k] = v[j];
      j++;
    }
    k++;
  }
  while (i < q) {
    w[k] = v[i];
    i++;
    k++;
  }
  while (j < r) {
    w[k] = v[j];
    j++;
    k++;
  }
  for (i = p; i < r; i++)
    v[i] = w[i-p];
}

void mergesort(int p, int r, NumericVector v, NumericVector aux)
{
  int q;
  if (p < r - 1) {
    q = (p + r) / 2;
    mergesort(p, q, v,aux);
    mergesort(q, r, v,aux);
    intercal(p, q, r, v,aux);
  }
}

// [[Rcpp::export]]
NumericVector sort_merge(NumericVector vetor) {
  Rcpp::NumericVector res = Rcpp::clone(vetor);
  Rcpp::NumericVector aux = Rcpp::clone(vetor);
  int n = res.size();
  mergesort(0,n,res,aux);
  return res;}