#ifndef lint
static char *rcsid = "$Header: /tmp_mnt/vida/disks/disk5/Users/terry/r/gassy/RCS/next_prime.c,v 1.3 1992/08/09 22:07:40 terry Exp terry $";
#endif

#include "types.h"

#include <math.h>

#define ERROR      -1
#define NEITHER    0
#define COMPOSITE  1
#define PRIME      2

INT
prime(n)
LONG n;
{
    extern DOUBLE sqrt();
    register LONG lim;
    register LONG i;

    if (n < 0){
	return ERROR;
    }

    if (n == 0L || n == 1L){
	return NEITHER;
    }

    if (n % 2 == 0 || n % 3 == 0 || n % 5 == 0){
	return COMPOSITE;
    }
    
    lim = 1 + (LONG)sqrt((DOUBLE)n);
    
    for (i = 7; i <= lim; i += 2){
	if (n % i == 0){
	    return COMPOSITE;
	}
    }
    
    return PRIME;
}

INT
next_prime(n)
INT n;
{
    if (n < (INT) 0){
	error("Next prime called with negative (%d) argument.", n);
    }
    
    if (n % 2 == 0){
	n++;
    }

    while (prime(n) != PRIME){
	n += 2;
    }

    return n;
}