Russian peasant multiplication

/*
    rbm.c multiplies two numbers given at the command line using the "Russian
    peasant multiplication“ and shows the steps of calculation.
    To compile run "gcc -ansi -o rbm rbm.c".
    Copyright (C) <September 22, 2013> Henning Polzer,
    send comments and error reports to: h underscore polzer at gmx dot de.

    This program is free software; you can redistribute it and/or
    modify it under the terms of the GNU General Public License
    as published by the Free Software Foundation; either version 2
    of the License, or (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
*/


#include <math.h>                           /* fabs (), floor () */
#include <stdio.h>                          /* printf () */
#include <stdlib.h>                         /* atol () */

typedef unsigned long ganzzahl;

void rbm (ganzzahl a, ganzzahl b)
{
    ganzzahl doppel, flag=0, halb, links=a, rechts=b, summe=0;

    if (b < a) {                        /* kleineren Faktor nach links */
      links=b;
      rechts=a;
      flag=1;
    }

    printf ("\n%9ld %9ld", links, rechts);          /* Startwerte ausgeben */
    if (links%2==1) summe += rechts;                /* Startwert ggf. addieren */
      else printf ("<gestrichen");      /* Links gerade? Rechts streichen (s. u.)*/

    printf ("\t\tStartwerte");
    if (flag==1) printf (" (Positionen getauscht)");    /* weil kleiner Wert links */

    printf ("\n");

    do {
      halb=floor(links/2);
      doppel=rechts*2;
      printf ("%9ld ", halb);
      if (halb%2==0) printf ("%9ld<gestrichen", doppel);    /* <-- links gerade (s. o.)*/
        else {
              printf ("%9ld", doppel);      /* Links ungerade: Rechts addieren*/
              summe+=doppel;
        } /* else */
       printf ("\n");
       links=halb;              /* Wert anpassen fuer naechsten Schritt */
       rechts=doppel;
    } while (links>1);                      /* 1 Untergrenze: Ende */

    printf ("\n");
    if (flag==0) printf ("\t%ld * %ld", a, b);
      else printf ("\t%ld * %ld", b, a);
    /* pruefen, ob Rechenergebnis stimmt: */
    printf (" = %ld stimmt mit konventioneller Berechnung", summe);
    if (a*b!=summe) printf (" nicht");
    printf (" ueberein.\n");
} /* rbm */


int main (int argc, char *argv[])
{
    ganzzahl zahl1, zahl2;
    if ((argc==3) &&                /* genau zwei ganze Zahlen > 0 eingeben */
       ((zahl1=fabs(atol(argv[1])))!=0) &&
       ((zahl2=fabs(atol(argv[2])))!=0)) rbm (zahl1, zahl2);
      else printf ("Richtiger Aufruf: rbm zahl>0 zahl>0\n");
} /* main */