cc65/samples/sieve.c

/*
 * Calculate all primes up to a specific number.
 */



#include <stdlib.h>
#include <stdio.h>
#include <ctype.h>
#include <time.h>
#include <conio.h>


/* Workaround missing clock stuff */
#ifdef __APPLE2__
#  define clock()               0
#  define CLOCKS_PER_SEC        1
#endif



/*****************************************************************************/
/*                                   Data                                    */
/*****************************************************************************/



#define COUNT           16384           /* Up to what number? */
#define SQRT_COUNT      128             /* Sqrt of COUNT */

static unsigned char Sieve[COUNT];



/*****************************************************************************/
/*                                   Code                                    */
/*****************************************************************************/



#pragma static-locals(1);



static char ReadUpperKey (void)
/* Read a key from console, convert to upper case and return */
{
    return toupper (cgetc ());
}



int main (void)
{
    /* Clock variable */
    clock_t Ticks;
    unsigned Sec;
    unsigned Milli;

    /* This is an example where register variables make sense */
    register unsigned char* S;
    register unsigned       I;
    register unsigned       J;

    /* Output a header */
    printf ("Sieve benchmark - calculating primes\n");
    printf ("between 2 and %u\n", COUNT);
    printf ("Please wait patiently ...\n");

    /* Read the clock */
    Ticks = clock();

    /* Execute the sieve */
    I = 2;
    while (I < SQRT_COUNT) {
        if (Sieve[I] == 0) {
            /* Prime number - mark multiples */
            J = I*2;
            S = &Sieve[J];
            while (J < COUNT) {
                *S = 1;
                S += I;
                J += I;
            }
        }
        ++I;
    }

    /* Calculate the time used */
    Ticks = clock() - Ticks;
    Sec = (unsigned) (Ticks / CLOCKS_PER_SEC);
    Milli = ((Ticks % CLOCKS_PER_SEC) * 1000) / CLOCKS_PER_SEC;

    /* Print the time used */
    printf ("Time used: %u.%03u seconds\n", Sec, Milli);
    printf ("Q to quit, any other key for list\n");

    /* Wait for a key and print the list if not 'Q' */
    if (ReadUpperKey () != 'Q') {
        /* Print the result */
        J = 0;
        for (I = 2; I < COUNT; ++I) {
            if (Sieve[I] == 0) {
                printf ("%4d\n", I);
                if (++J == 23) {
                    printf ("Q to quit, any other key continues\n");
                    if (ReadUpperKey () == 'Q') {
                        break;
                    }
                    J = 0;
                }
            }
            if (kbhit() && ReadUpperKey == 'Q') {
                break;
            }
        }
    }

    return EXIT_SUCCESS;
}