compute the fact fo a very large number.

fact.c

#include <stdio.h>


int multi(char *a, int len, int b)
{
    int i;
    int carry = 0;
    for (i = 0; i < len; i++) {
        int tmp = (a[i] - '0') * b + carry;
        a[i] = tmp % 10 + '0';
        carry = tmp / 10;
    }

    while (carry != 0) {
        a[len] = carry % 10 + '0';
        carry /= 10;
        len += 1;
    }

    return len;
}

void swap(char *array, int a, int b)
{
    char tmp = array[a];
    array[a] = array[b];
    array[b] = tmp;
}

void reverse(char *array, int len)
{
    int i;
    int mid = len / 2;

    for (i = 0; i < mid; i++) {
        swap(array, i, len-1 - i);
    }
}

void CalcNN(int n, char *pOut)
{
    int i;
    int len = 1;
    pOut[0] = '1';
    for (i = 1; i <= n; i++) {
        len = multi(pOut, len, i);
    }

    reverse(pOut, len);
    pOut[len] = '\0';
}

int main(int argc, const char *argv[])
{
    char buf[BUFSIZ];
    int num;
    scanf("%d", &num);
    CalcNN(num, buf);
    printf("result: %s\n", buf);
    return 0;
}