Coding Friends

#include <iostream>
#include <bitset>
#include <limits>
 
using namespace std;
 
/* mutiplication using the binary shift operator */
int multiplication(int mul, int by)
{
    int result = 0;
    if (by > 0)
    {
        // mulitiply by (mulby) comparsion for when to add a shifted value to the result.
        // moveby, how many shifts left to shift by if there is a match.
        int mulby = 2, moveby = 1;
        while (by >= mulby)
        {
            cout << "by      = " << bitset<numeric_limits<short>::digits>(by) << endl;
            cout << "mulby = " << bitset<numeric_limits<short>::digits>(mulby) << endl;
            // bitwise add and if it is the same value as the mulby then add to the result.
            if ((by & mulby) == mulby) {
                cout << "+ result= " << bitset<numeric_limits<short>::digits>((mul << moveby)) << endl;
                result += mul << moveby;
                cout << "result   = " << bitset<numeric_limits<short>::digits>(result) << endl;
            }
            mulby = mulby << 1; // the next test if the bits values are the same.
            moveby++;           // move the result by
        }
        // and if there is a modulus of 1 from 2 then the value was odd.
        if (by % 2 == 1)
        {
            result += mul;
        }
        cout << "end result = " << bitset<numeric_limits<short>::digits>(result) << endl;
    }
    return result;
}
 
int main ()
{
  cout << multiplication(6,7) << endl;
  return 0;
}

by           = 000000000000111
mulby        = 000000000000010
+ result     = 000000000001100
result       = 000000000001100
by           = 000000000000111
mulby        = 000000000000100
+ result     = 000000000011000
result       = 000000000100100
end result   = 000000000101010
42

6 * 7 = 42