pi_better_critical.cc
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pi_better_critical.cc
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			/*
			This exercise is taken from the class Parallel Programming Workshop (MPI,
			OpenMP and Advanced Topics) at HLRS given by Rolf Rabenseifner
			*/
			#include <chrono>
			#include <cstdio>
			#include <cmath>
			#include <omp.h>

			using clk = std::chrono::high_resolution_clock;
			using second = std::chrono::duration<double>;
			using time_point = std::chrono::time_point<clk>;

			inline int digit(double x, int n) {
			return std::trunc(x * std::pow(10., n)) - std::trunc(x * std::pow(10., n - 1)) *10.;
			}

			inline double f(double a) { return (4. / (1. + a * a)); }

			const int n = 10000000;

			int main(int /* argc / , char * /* argv */) {
			int i;
			double dx, x, sum, pi;

			auto nthreads = omp_get_max_threads();

			auto t1 = clk::now();

			/* calculate pi = integral [0..1] 4 / (1 + x*2) dx /
			dx = 1. / n;
			sum = 0.0;
			#pragma omp parallel shared(sum) private(x)
			{
			double lsum = 0.; // private by definition
			#pragma omp for
			for (i = 0; i < n; i++) {
			x = 1. * i * dx;
			lsum = lsum + f(x);
			}
			#pragma omp critical
			sum += lsum;
			}
			pi = dx * sum;

			second elapsed = clk::now() - t1;

			std::printf("computed pi = %.16g\n", pi);
			std::printf("wall clock time (chrono) = %.4gs\n", elapsed.count());
			std::printf("using n threads = %.d\n", nthreads);

			for(int d = 1; d <= 15; ++d) {
			std::printf("%d", digit(pi, d));
			}

			return 0;
			}