Thread: subtraction of arbitrary big number

Code:

/* * used to add two numbers with the same sign
 * GIVEN FOR GUIDANCE
 */
int* add(int* input1, int* input2, int base) { 
	int len1 = bignum_length(input1);   //get length of first input
	int len2 = bignum_length(input2);   //get length of second input
	int resultlength = ((len1 > len2)? len1 : len2) + 2;  //if len1 > len2, return len1, if len2 > len1, return len 2. Then add 2 to the result
	int* result = (int*) malloc (sizeof(int) * resultlength);  //allocate a memeory address of the size of int x resultlength
	int r = 0;   //index
	int carry = 0;   //carry over
	int sign = input1[len1];  //last element of input1, which is -1
    	int num1, num2;


	len1--;
	len2--;


	while (len1 >= 0 || len2 >= 0) {
        if (len1 >= 0) {
            num1 = input1[len1];
        } else {
            num1 = 0;
        }


        if (len2 >= 0) {
            num2 = input2[len2];
        } else {
            num2 = 0;
        }
		result[r] = (num1 + num2 + carry) % base;
		carry = (num1 + num2 + carry) / base;
		len1--;
		len2--;
		r++;
	}
	if (carry > 0) {
        result[r] = carry; 
        r++; 
    }
	result[r] = sign;
	reverse(result);
	return result;
}

how can i write a subtraction method base on this add method?
or can someone explain what each line of this method code do?