/* File: twinprimes1.c
   Slow single-threaded version to find a twin prime
   between begin and end.

   compile with: g++ twinprimes1.c -o twinprimes1 
   to get timing, run wiht: time ./twinprimes1

*/

#include <stdio.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 ;


   for(int i=begin ; i < end ; i++) {
      if ( isPrime(i) && isPrime(i+2) ) {
         found = 1 ;
         first = i ;
         second = i+2 ;
      }
   }


   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 ;
}