/***********************************************
 *                                             *
 *          "DON'T MESS WITH MARILYN"          *
 *                                             *
 *            MONTE CARLO SIMULATION           *
 *                                             *
 * 02/21/91, M.E. Brandt, Ph.D.                *
 *                                             *
 ***********************************************/
 
#include <stdio.h>


#define BIT_MASK      7
#define NO_DOORS      3.0
#define RANDFACTOR    (32768.0 * (1.0/NO_DOORS))


main()
{

    int i, n, car, choice, open, count;
    
    unsigned char u[2], doors, noprize, mask, not_open,
                  not_choice; 
    
    double prob;
    
    
    srand(1);

    do {
    
        fprintf(stderr, 
         "\nHow many games of not-switching, and switching\
 would you like?: ");
        scanf("%d", &n);

        printf("\n\nNumber of trials in this series is %d\n",
               n);
       

        /* LOOP IN WHICH CONTESTANT DOES NOT SWITCH */
   
        for(count=i=0; i<n; i++) {
   
            doors = 1;
    
            /* get a random number between 0 and 2 to hide 
               car behind door # */
    
            car =  (int)((double)rand()/RANDFACTOR);
    
            doors <<= car;               /* status of doors */
    
            noprize = ~doors & BIT_MASK; /* doors with goats */
        
            /* contestant picks a door */
        
            choice =  (int)((double)rand()/RANDFACTOR);
    
            /* host reveals a door which is not 'choice' 
               and not 'car' (a goat!) */
    
            mask = noprize & ((~(1 << choice)) & BIT_MASK);
   
            /* host always picks first one not satisfying 
               condition */
         
            open = 0;
            while(!(mask & 1)) {
               mask >>= 1;
               open++;
            }
    
            if(choice == car) count++;
    
            /* since the contestant did not switch, 'choice' 
               does not depend on 'open'! */
    
     
        }
    
        prob = (double)count/(double)n;

        printf(
         "\nPercentage of wins for non-switching is %.1f",
               prob * 100.0);
        


/* LOOP IN WHICH CONTESTANT SWITCHES CHOICE */

       for(count=i=0; i<n; i++) {
   
           doors = 1;
    
        /* get a random number between 0 and 2 to hide car 
           behind door # */
    
           car =  (int)((double)rand()/RANDFACTOR);
    
           doors <<= car;               /* status of doors */
    
           noprize = ~doors & BIT_MASK; /* doors with goats */
        
          /* contestant picks a door */
        
           choice =  (int)((double)rand()/RANDFACTOR);
    
          /* host reveals a door which is not 'choice' and not
             'car' */
    
           mask = noprize & ((~(1 << choice)) & BIT_MASK);
    
           open = 0;
           while(!(mask & 1)) {
              mask >>= 1;
              open++;
           }


         /* now the contestant switches to door which is not 
            'choice' and not 'open' */
       
           not_choice = ~(1 << choice) & BIT_MASK;
           not_open   = ~(1 << open)   & BIT_MASK;
       
           choice = not_open & not_choice;        


         /* if switched choice = original status of 'doors' 
            it's a "win"! */
      
           if(choice == doors) count++;
    
     
        }

    
        prob = (double)count/(double)n;

        printf("\nPercentage of wins for switching is %.1f\n",
               prob * 100.0);
        printf("\n-----------------------------------------\
-----\n");

        fprintf(stderr, "\nPlay Again?: ");
        scanf("%s", u);
    
  }
  while(toupper(*u) == 'Y');
    

}