/* We need standard IO */
**#include** <stdio.h>
/* factor a number by Han's method. */
/* Generating relation is N = pq = ss+r = (q-2x)q = qq-2xq */
/* DN wrt x = -2q */
/* DN wrt q = 2(q-x)+1 = 2q-2x+1 */
/* DN2 wrt q = 4q+4-4x = 4(q-x+1) */
/* (And observing that q must be odd) */
/* (And factoring out the factor of two in both branches) */
main () {
long n, p, q, x, t;
printf ("Factor a positive odd number by Han's method\n");
printf ("N = \n"); scanf ("%ld", &n);
printf ("Factoring %ld\n", n);
if (n<1) { printf ("N must be greater than zero\n"); return 1;}
if (n%2==0) { printf ("N must be odd\n"); return 1;}
q=1; x=0; t = (q*q-2*x*q -n)/2;
while (t!=0) if (t<0) { t += 2*(q-x+1); q+=2; }
else { t -= q; x+=1; }
p = q - 2*x;
printf ("%ld*%ld=%ld\n",p,q,n);
}

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© Copyright 2003 by John Halleck, All Rights Reserved.
This snapshot was last modified on December 24th, 2013