GCD Of Big Number (String)

#include<iostream>
#include<cstdio>
#include<cstring>


using namespace std;


#define sz 100


char gg[sz];


int compare_number(char *dec1, char *dec2);
void reverse_string(char *string, int length);
void multiply_by_integer(char *num, int multiplier, char* result);
void character_subtraction(char *num1, char *num2, char *result);
int find_division_time(char *dividend, char *divisor);
void character_division(char *num1, char *num2, char *result, char *mod);
void gcd(char *a , char *b);


int main()
{
char num1[sz] , num2[sz];
while(scanf(" %s %s" , num1 , num2) == 2)
{
gcd(num1 , num2);


printf("%s\n" , gg);
}
return 0;
}


void gcd(char *a , char *b)
{
    char tmp[sz] , res[sz] , tmp1[sz];
    if(strcmp(a , "0") == 0)
{
strcpy(gg , b);
return ;
}
    while(strcmp(a , "0") != 0)
    {
character_division(b , a , res , tmp);
        if(strcmp(tmp , "0") == 0)
{
strcpy(gg , a);
return ;
}
        else
{
character_division(a , tmp , res , tmp1);
strcpy(a , tmp1);
strcpy(b , tmp);
}
}
strcpy(gg , b);
return ;
}


void reverse_string(char *string, int length)
{
int i;


for(i = 0; i < length / 2; i++)
{
string[i] ^= string[length - i - 1] ^= string[i] ^= string[length - i - 1];
}


return;
}


int compare_number(char *dec1, char *dec2)
{
int length1, length2, result;


//If one of the number is negative, process them here.
if (dec1[0] != '-' && dec2[0] == '-') return(1);
if (dec1[0] == '-' && dec2[0] != '-') return(-1);


//If the numbers are not having equal sign, they will not come here.
length1 = strlen(dec1);
length2 = strlen(dec2);


if (length1 == length2) result = strcmp(dec1, dec2);
   else result = length1 - length2;


   //If both the number are negative.
   if (dec1[0] == '-') return(-result);
   else return(result);
}


void character_subtraction(char *num1, char *num2, char *result)
{
int  i, j, k;


   //If 'num1' < 'num2', then swap them and put '-' at the first of the 'result'.
if (compare_number(num1, num2) < 0)
{
swap(num1 , num2);
        sprintf(result, "-%s", num1);
}


else strcpy(result, num1);


for(i = strlen(result) - 1, j = strlen(num2) - 1; j >= 0; i--, j--)
{
      if (result[i] >= num2[j])
      {
         result[i] = result[i] - num2[j] + '0';
      continue;
      }


result[i] = (result[i] + 10) - num2[j] + '0';
for(k = i - 1; k > 0 && result[k] == '0'; k--) result[k] = '9';
result[k] -= 1;
}


   //Ignoring all the leading zeros.
   for(i = (result[0] == '-') ? 1 : 0; result[i] == '0'; i++);


   //If the result is 0, then previous loop will end in 'NULL'.
   if (result[i] == NULL) --i;


   //Launch all the digits other than leading zeros.
   for(j = (result[0] == '-') ? 1 : 0; result[i] != NULL; j++, i++) result[j] = result[i];


   result[j] = NULL;


return;
}


void multiply_by_integer(char *num, int multiplier, char* result)
{
int i, index, carry, mul;


if (multiplier == 0)
{
sprintf(result, "0");
    return;
}


for(i = strlen(num) - 1, carry  = index = 0; i >= 0; i--)
{
mul = (num[i] - '0') * multiplier + carry;
carry = mul / 10;
result[index++] = mul % 10 + '0';
}


while(carry != 0)
{
result[index++] = carry % 10 + '0';
carry /= 10;
}


if (index == 0) result[index++] = '0';
result[index] = NULL;


reverse_string(result, index);


return;
}


int find_division_time(char *dividend, char *divisor)
{
int  num, length1, length2;
char sub[100 + 1], mul[100 + 3];


length1 = strlen(dividend);
length2 = strlen(divisor);


if (length1 < length2) return(0);


//Launch the first 1 or 2 digits of the dividend.
num = dividend[0] - '0';
if (length1 > length2 && length1 > 1) num = num * 10 + (dividend[1] - '0');


num = num / (divisor[0] - '0');
multiply_by_integer(divisor, num, mul);


while(compare_number(mul, dividend) > 0)
{
--num;
multiply_by_integer(divisor, num, mul);
}


character_subtraction(dividend, mul, sub);
strcpy(dividend, sub);


return(num);
}


void character_division(char *num1, char *num2, char *result, char *mod)
{
int i, index = 0;


//Launch the very first 'length2' characters in 'mod'.
for(i = 0; num1[i] != NULL && num2[i] != NULL; i++) mod[i] = num1[i];
mod[i] = NULL;


//If 'mod' >= 'num2'.
if (compare_number(mod, num2) >= 0)
{
result[index++] = find_division_time(mod, num2) + '0';
}


//Launch the rest characters one by one.
while(num1[i] != NULL)
{
strncat(mod, &num1[i++], 1);
result[index++] = find_division_time(mod, num2) + '0';
}


if (index == 0) result[index++] = '0';
result[index] = NULL;


    if (mod[0] == '0') mod[1] = NULL;


return;
}