/* File: twinprimes2.cpp
   Multi-threaded version to find a twin prime
   between begin and end.

   This version can be buggy because writes to
   found, first and second can be made simultanenously
   by different threads.

   compile with: g++ twinprimes2.cpp -o twinprimes2
   to get timing, run wiht: time ./twinprimes2

*/

#include <stdio.h>
#include <omp.h>
#include <math.h>


int isPrime (unsigned int n) {
   unsigned int m = sqrt(n) ;

   for (unsigned int k=2 ; k <= m ; k++) {
      if (n % k == 0) return 0 ;
   }
   return 1 ;
}


int main() {

   int found=0 ;
   int begin=10000000 ; // 10 million
   int end=20000000 ;   // 20 million
   int first, second ;

   #pragma omp parallel shared(found,begin,end,first,second) 
   {
      #pragma omp for schedule(dynamic) reduction(||:found)
      for(int i=begin ; i < end ; i++) {
         if ( isPrime(i) && isPrime(i+2) ) {
	    found = 1 ;  // Danger!!
	    first = i ;
	    second = i+2 ;
	 }
      }
   }  // end parallel region


   if (found) {
      printf("Yes, there are twin primes between %d and %d.\n", begin, end) ;
      printf("One example is %d and %d.\n", first, second) ;
   } else {
      printf("No, there are no twin primes between %d and %d.\n", begin, end) ;
   }
   
   return 0 ;
}