factoring... it works for some input

I am at a loss... I just can't figure why my program will factor most numbers but some it'll just give out the wrong answer.

2000000009: 7 * 285714287

But I get something else.. please someone help. The problem is in factor().

Code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>

int issquare(int i) 
{
	int j, s;

	s = 0; 

	if(i < 0)
		return 0; 

	j=i&3;

	/*weeding out the obvious non squares*/
	if((j != 0) && (j != 1))
		return 0; 

	for(j = 1; s < i; j += 2)
		s += j;

	return (s == i); 
}

int isprime(int x)
{
	int i;
	static long int SmallPrimes[] =
		{0,0,1,1,0,1,0,1,0,0};

	if (x<10) return SmallPrimes[x];
	if (x%2==0) return 0;
	if (x%3==0) return 0;
	if (x%5==0) return 0;

	for (i = 7; i*i<=x; i++)
		if (x%i==0) return 0;

	return 1;
}

int factor(int c)
{
	int r, h, k, s;

	if (c % 2 == 0)
		return 2;

	r = sqrt(c);

	if(issquare(c)) {
		if(isprime(r))
			return r;
		else
			return factor(r);
	}

	h = (2 * r) - 1;

	if(c % 4 == 1)
		k = 5;
	else 
		k = 3;

	if(h % 4 != c % 4)
		h = h - 2;

	s = ((h + k) / 2) * ((2 + h - k) / 2);

	while(s != c) {
		if(s > c) {
			s = s - (2 * k + 2);
			k = k + 4;
		}
		if(s < c) {
			h = h + 4;
			s = s + (2 * h - 2);
		}
	}
	return((2+h-k)/2);
}

int main(int argc, char **argv)
{
	int n, t;
	uclock_t start, stop;

	n = atoi(argv[1]);

	if (argc != 2) {
		fprintf(stderr, "\nusage: %s [number]\n", argv[0]);
		return -1;
	}

	if(isprime(n)) {
		printf("\n%d is prime\n", n);
		return 0;
	}

	printf("\n%d: ", n);
	start=uclock();

	while(!isprime(n)) {
		t = factor(n);
		if (t==0) break;
		printf("%d ", t);
		n /= t;
	}

	stop=uclock();
	printf("%d", n);
	printf("\n\n%.4f seconds\n", (double)(stop-start)/(double) \
		UCLOCKS_PER_SEC);

	return 0;
}

Your code seems to work for a large number , could you tell me the name of algorithm you have used ?

i am using fermat's factorization method, but it's a little different now. hmmm... i wonder what's wrong.

Originally posted by skeptik
i am using fermat's factorization method, but it's a little different now. hmmm... i wonder what's wrong.

Thanks for the info.