Runnng lengths in Rcpp

rle2.cpp

#include <Rcpp.h>
using namespace Rcpp;


// [[Rcpp::export]]
List cpp_rle2(
    const NumericVector& x,
    const double h = 0,
    const double eps = 1e-16
) {
  
  int n = x.length();
  
  if (n == 1) {
    return Rcpp::List::create(
      Rcpp::Named("lengths") = wrap(1),
      Rcpp::Named("group") = wrap(1),
      Rcpp::Named("changepoints") = wrap(1)
    );
  }
  
  std::vector<int> labels(n), lengths(n);
  std::vector<int> changes;
  
  int tmpCount = 1;
  int prevStart = 0;
  labels[0] = 1;
  changes.push_back(1);
  
  for (int i = 0; i < (n-1); i++) {
    
    if ( std::abs((x[i] + h) - x[i+1]) <= eps ) {
      tmpCount += 1;
      labels[i+1] = labels[i];
    } else {
      for (int j = prevStart; j <= i; j++)
        lengths[j] = tmpCount;
      tmpCount = 1;
      prevStart = i+1;
      labels[i+1] = labels[i]+1;
      changes.push_back(i+2);
    }
    
  }
  
  for (int j = prevStart; j < n; j++)
    lengths[j] = tmpCount;
  
  return Rcpp::List::create(
    Rcpp::Named("lengths") = lengths,
    Rcpp::Named("group") = labels,
    Rcpp::Named("changepoints") = changes
  );
}