2.11BSD/src/games/factor.c

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```#ifndef lint
static char sccsid[] = "@(#)factor.c	4.1 (Wollongong) 6/13/83";
#endif

/*
*              factor [ number ]
*
* Written to replace factor.s in Bell V7 distribution
*/

main(argc, argv)
char	*argv[];
{
int	n;

if (argc >= 2) {
sscanf(argv[1], "%d", &n);
if (n > 0)
printfactors(n);
} else {
while (scanf("%d", &n) == 1)
if (n > 0)
printfactors(n);
}
}

/*
* Print all prime factors of integer n > 0, smallest first, one to a line
*/
printfactors(n)
register int	n;
{
register int	prime;

if (n == 1)
printf("\t1\n");
else while (n != 1) {
prime = factor(n);
printf("\t%d\n", prime);
n /= prime;
}
}

/*
* Return smallest prime factor of integer N > 0
*
* Algorithm from E.W. Dijkstra (A Discipline of Programming, Chapter 20)
*/

int
factor(N)
int	N;
{
int		p;
register int	f;
static struct {
int	hib;
int	val[24];
} ar;

{	register int	x, y;

ar.hib = -1;
x = N; y = 2;
while (x != 0) {
ar.val[++ar.hib] = x % y;
x /= y;
y += 1;
}
}

f = 2;

while (ar.val[0] != 0 && ar.hib > 1) {
register int	i;

f += 1;
i = 0;
while (i != ar.hib) {
register int	j;

j = i + 1;
ar.val[i] -= j * ar.val[j];
while (ar.val[i] < 0) {
ar.val[i] += f + i;
ar.val[j] -= 1;
}
i = j;
}
while (ar.val[ar.hib] == 0)
ar.hib--;
}

if (ar.val[0] == 0)
p = f;
else
p = N;

return(p);
}
```