add

add.c

// Got a minute to revisit this question this morning and cleaned it up a lot
// - Jack

#define MAX(a,b) (((a)>(b))?(a):(b))

int ctoi(char c){
	return (int)(c - '0');
}

char itoc(int i){
	return (char)i+'0';
}

char* add(char * a,char*b){
	int aLen = strlen(a);
	int bLen = strlen(b);
	//make room for null terminator and carry in result
	int rLen = MAX(aLen,bLen)+1;
	char* result = new char[rLen+1]; 
	result[rLen] = 0;

	int ai = aLen-1;
	int bi = bLen-1;
	int ri = rLen-1;

	int carry = 0;
	while(ri >= 0){
		
		int aDigit = 0;
		if(ai >= 0){
			aDigit = ctoi(a[ai]);
		}
		int bDigit = 0;
		if(bi >= 0){
			bDigit = ctoi(b[bi]);
		}

		
		int sum = aDigit + bDigit + carry;
		if(sum > 9){
			sum -= 10;
			carry = 1;
		}
		else{
			carry = 0;
		}

		result[ri] = itoc(sum);
		ri--;
		bi--;
		ai--;
	}
		
	return result;
	
}