Bitwise Calculator

/*
 * File:   main.cpp
 * Author: James
 *
 * Created on November 23, 2014, 2:41 AM
 */

#include <iostream>
#include <string>
#include <sstream>
#include <cmath>
using namespace std;

#define nl "\n"

/*
 * This is a bitwise calculator
 */

bool is_number(const string& astring)
{
    return !astring.empty() &&
            astring.find_first_not_of("-0123456789") == string::npos;
}

int badd (int a, int b)
{
    int c;

    do
    {
        c = a & b;
        c = c << 1;
        b = a ^ b;

        a = b & c;
        a = a << 1;
        c = b ^ c;

        b = c & a;
        b = b << 1;
        a = c ^ a;
    } while (b != 0);
    return a;
}

int bsub (int a, int b)
{
    int c = 0;
    b = badd(~b, 1); // generates the twos compliment of b
    c = badd(a, b); // adds the twos compliment to a
    return c;
}

int bmul (int a, int b)
{
    int A = a;

    if ((a == 0) || (b == 0))
        return 0;

    for (int b2 = (0 - b); (abs(b) > 1) || (abs(b2) > 1); b--, b2++)
        A = badd(A, a);

    return A;
}

int bmod (int a, int b)
{
    int B = b;

    while (a <= B)
    {
        B = bsub(B, a);
    }
    return B;
}

string bdiv (int a, int b)
{
    int i = 0;
    stringstream conv;

    while (a >= b)
    {
        a = bsub(a, b);
        i++;
    }

    conv << i << ".";

    for (int c = 1; c <= 5; c++)
    {
        int r = 0;
        a = bmul(10, a);

        while (a >= b)
        {
            a = bsub(a, b);
            r++;
        }
        conv << r;
    }
    return conv.str();
}

string op_select(string op,double a,double b)
{
    string out;
    stringstream convert;

    if (op == "+")
        convert << badd(a, b);
    if (op == "-")
        convert << bsub(a, b);
    if (op == "*")
        convert << bmul(a, b);
    if (op == "%")
        convert << bmod(a, b);
    if (op == "/")
        convert << bdiv(a, b);
    if (!(op.find_first_not_of("+-*/%") == string::npos))
        convert << "f";
    out = convert.str();
    return out;
}

int main()
{
    double a, b;
    string input, result;

    cout << "enter an integer\n";
    getline (cin, input);

    if ( is_number(input) )
        stringstream(input) >> a;
    else
    {
        cout << "that's not a number...";
        return 1;
    }

    cout << "enter another integer\n";
    getline (cin, input);

    if ( is_number(input) )
        stringstream(input) >> b;
    else
    {
        cout << "that's not a number...";
        return 1;
    }

    cout << "enter a simple math operator (+,-,*,/,%)\n";
    getline (cin, input);

    result = op_select(input,a,b);
    if (result == "f")
    {
        cout << "invalid operator " << input;
        return 1;
    }

    cout << "the answer to " << a << " " << input << " " << b
            << " is: " << result << ".\n";

    return 0;
}