// This program demonstrates how to perform parallel exclusive scan.

#include <taskflow/taskflow.hpp>

#include <taskflow/algorithm/scan.hpp>

int main(int argc, char* argv[]) {

if(argc != 3) {

std::cerr << "usage: ./exclusive_scan num_workers num_elements\n";

std::exit(EXIT_FAILURE);

}

size_t W = std::atoi(argv[1]);

size_t N = std::atoi(argv[2]);

tf::Executor executor(W);

tf::Taskflow taskflow;

std::vector<int> elements(N), scan_seq(N), scan_par(N);

for(size_t i=0; i<N; i++) {

elements[i] = static_cast<int>(i);

}

// sequential exclusive scan

{

std::cout << "sequential exclusive scan ... ";

auto beg = std::chrono::steady_clock::now();

std::exclusive_scan(

elements.begin(), elements.end(), scan_seq.begin(), -1, std::plus<int>{}

);

auto end = std::chrono::steady_clock::now();

std::cout << std::chrono::duration_cast<std::chrono::nanoseconds>(end-beg).count()

<< "ns\n";

}

// create a parallel exclusive scan task

{

std::cout << "parallel exclusive scan ... ";

auto beg = std::chrono::steady_clock::now();

taskflow.exclusive_scan(

elements.begin(), elements.end(), scan_par.begin(), -1, std::plus<int>{}

);

executor.run(taskflow).wait();

auto end = std::chrono::steady_clock::now();

std::cout << std::chrono::duration_cast<std::chrono::nanoseconds>(end-beg).count()

<< "ns\n";

}

// verify the result

for(size_t i=0; i<N; i++) {

if(scan_seq[i] != scan_par[i]) {

printf(

"scan_seq[%zu]=%d..., scan_par[%zu]=%d...\n",

i, scan_seq[i], i, scan_par[i]

);

throw std::runtime_error("incorrect result");

}

}

printf("correct result\n");

return 0;

}