What is the best way to add two numbers without using the + operator?

Question

A friend and I are going back and forth with brain-teasers and I have no idea how to solve this one. My assumption is that it\'s possible with some bitwise operators, but n

Answer 1

Note, this would be for an adder known as a ripple-carry adder, which works, but does not perform optimally. Most binary adders built into hardware are a form of fast adder such as a carry-look-ahead adder.

My ripple-carry adder works for both unsigned and 2's complement integers if you set carry_in to 0, and 1's complement integers if carry_in is set to 1. I also added flags to show underflow or overflow on the addition.

#define BIT_LEN 32
#define ADD_OK 0
#define ADD_UNDERFLOW 1
#define ADD_OVERFLOW 2

int ripple_add(int a, int b, char carry_in, char* flags) {
    int result = 0;
    int current_bit_position = 0;
    char a_bit = 0, b_bit = 0, result_bit = 0;

    while ((a || b) && current_bit_position < BIT_LEN) {
        a_bit = a & 1;
        b_bit = b & 1;
        result_bit = (a_bit ^ b_bit ^ carry_in);
        result |= result_bit << current_bit_position++;
        carry_in = (a_bit & b_bit) | (a_bit & carry_in) | (b_bit & carry_in);
        a >>= 1;
        b >>= 1;
    }

    if (current_bit_position < BIT_LEN) {
        *flags = ADD_OK;
    }
    else if (a_bit & b_bit & ~result_bit) {
        *flags = ADD_UNDERFLOW;
    }
    else if (~a_bit & ~b_bit & result_bit) {
        *flags = ADD_OVERFLOW;
    }
    else {
        *flags = ADD_OK;
    }

    return result;
}