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/*
** bigfac.c -- put into the public domain by Carl Declerck
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUFFLEN 8192
#define BUFFER ((char *) malloc(BUFFLEN))
void main (void);
void multiply (char *, char *, char *);
void zero_buffer (char *);
void minus_one (char *);
int isnull (char *);
void factorial (char *);
void main (void)
{
char *g = BUFFER;
printf ("Enter a number: ");
scanf ("%s", g);
printf ("Factorial of %s is: ", g);
factorial (g);
printf ("%s\n", g);
free (g);
}
void multiply (char *g1, char *g2, char *g3)
{
int gp1, gp2, cumpos, respos, mod, div;
int cmod, cdiv, resoff, wdig1, wdig2, base;
zero_buffer (g3);
for (gp2 = strlen(g2) - 1; gp2 >= 0; gp2--)
{
wdig2 = *(g2 + gp2) - 48;
resoff = strlen(g2) - gp2 - 1;
respos = BUFFLEN - resoff - 2;
for (gp1 = strlen(g1) - 1; gp1 >= 0; gp1--)
{
wdig1 = *(g1 + gp1) - 48;
mod = (wdig1 * wdig2) % 10;
div = (wdig1 * wdig2) / 10;
base = *(g3 + respos) - 48;
cmod = (base + mod) % 10;
cdiv = (base + mod) / 10 + div;
*(g3 + respos) = (char)(cmod + 48);
cumpos = --respos;
while (cdiv > 0)
{
base = *(g3 + cumpos) - 48;
*(g3 + cumpos--) = (char)((base + cdiv) % 10 + 48);
cdiv = (base + cdiv) / 10;
}
}
}
for (respos = 0; *(g3 + respos) == '0'; respos++)
;
strcpy (g3, (char *) (g3 + respos));
if (*g3 == 0)
strcpy (g3, "0");
}
void zero_buffer (char *buff)
{
int cnt;
for (cnt= 0; cnt < BUFFLEN; cnt++)
*(buff + cnt) = '0';
*(buff + BUFFLEN - 1) = 0;
}
void minus_one (char *g)
{
int p;
char digit;
p = strlen(g) - 1;
digit = *(g + p);
while (digit == '0')
{
*(g + p--) = '9';
digit = *(g + p);
}
*(g + p) -= 1;
}
int isnull (char *g)
{
int p, ok = 1;
for (p = 0; p < (int)(strlen(g)); p++)
if (*(g + p) != '0')
ok = 0;
return (ok);
}
void factorial (char *g)
{
char *h1 = BUFFER, *h2 = BUFFER;
strcpy (h1, "1");
while (!isnull(g))
{
multiply (h1, g, h2);
strcpy (h1, h2);
minus_one (g);
}
strcpy (g, h1);
free (h1);
free (h2);
}
/*
** The principal function is multiply(), it 'multiplies' two
** character-strings of arbritrary length and puts the result
** into a third. 8192 bytes is enough for 1000!, beyond that
** the buffer-size may need to be incremented.
*/
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