/**
 * @title Computing an Inner Product with RcppParallel
 * @author JJ Allaire
 * @license GPL (>= 2)
 */

#include <Rcpp.h>
using namespace Rcpp;

#include <algorithm>

// [[Rcpp::export]]
double innerProduct(NumericVector x, NumericVector y) {
   return std::inner_product(x.begin(), x.end(), y.begin(), 0.0);
}

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

struct InnerProduct : public Worker
{
   // source vectors
   const RVector<double> x;
   const RVector<double> y;

   // product that I have accumulated
   double product;

   // constructors
   InnerProduct(const NumericVector x, const NumericVector y)
      : x(x), y(y), product(0) {}
   InnerProduct(const InnerProduct& innerProduct, Split)
      : x(innerProduct.x), y(innerProduct.y), product(0) {}

   // process just the elements of the range I have been asked to
   void operator()(std::size_t begin, std::size_t end) {
      product += std::inner_product(x.begin() + begin,
                                    x.begin() + end,
                                    y.begin() + begin,
                                    0.0);
   }

   // join my value with that of another InnerProduct
   void join(const InnerProduct& rhs) {
     product += rhs.product;
   }
};

// [[Rcpp::export]]
double parallelInnerProduct(NumericVector x, NumericVector y) {

   // declare the InnerProduct instance that takes a pointer to the vector data
   InnerProduct innerProduct(x, y);

   // call paralleReduce to start the work
   parallelReduce(0, x.length(), innerProduct);

   // return the computed product
   return innerProduct.product;
}