- Solution

// dotProductAsync.cpp
#include <chrono>
#include <iostream>
#include <future>
#include <numeric>
#include <random>
#include <thread>
#include <vector>
static const int NUM = 100000000;
long long getDotProduct(std::vector<int>& v, std::vector<int>& w){
  auto future1 = std::async([&]{return std::inner_product(&v[0], &v[v.size()/4], &w[0], 0LL);});
  auto future2 = std::async([&]{return std::inner_product(&v[v.size()/4], &v[v.size()/2], &w[v.size()/4], 0LL);});
  auto future3 = std::async([&]{return std::inner_product(&v[v.size()/2], &v[v.size()*3/4], &w[v.size()/2], 0LL);});
  auto future4 = std::async([&]{return std::inner_product(&v[v.size()*3/4], &v[v.size()], &w[v.size()*3/4], 0LL);});
  return future1.get() + future2.get() + future3.get() + future4.get();
}
int main(){
  std::cout << std::endl;
  // get NUM random numbers from 0 .. 100
  std::random_device seed;
  // generator
  std::mt19937 engine(seed());
  // distribution
  std::uniform_int_distribution<int> dist(0, 100);
  // fill the vectors
  std::vector<int> v, w;
  v.reserve(NUM);
  w.reserve(NUM);
  for (int i=0; i< NUM; ++i){
    v.push_back(dist(engine));
    w.push_back(dist(engine));
  }
  // measure the execution time
  std::chrono::system_clock::time_point start = std::chrono::system_clock::now();
  std::cout << "getDotProduct(v, w): " << getDotProduct(v, w) << std::endl;
  std::chrono::duration<double> dur  = std::chrono::system_clock::now() - start;
  std::cout << "Parallel Execution: "<< dur.count() << std::endl;
  std::cout << std::endl;
}