Question

Im trying to convert my arbitrary precision integer class to be able to use digits that are not just 8 bits per digit. Ive stumbled upon a weird problem: I am able to use uint16_t
for my base digit type, but not uint32_t. My code will return bad results. The example I used to find what was going wrong is 0x1111111111111111 * 0x1111111111111111, which should be0x123456789abcdf00fedcba987654321. However, I am getting 0x123456789abcdf0fedcba987654321.

I thought that I had changed all of the hardcoded types so that changing the base digit type would not matter, but apparently not.

Here is the relevant code:

typedef uint32_t                          digit;            // original code uses uint8_t; uint16_t works too
typedef uint64_t                          double_digit;     // int type to hold overflow values

typedef std::deque <digit>                base;

const digit NEG1 = -1;                    // uint8_t -> 255, uint32_t -> 4294967295
const digit BITS = sizeof(digit) << 3;    // sizeof gives the number of bytes, so multiply that by 8 to get the number of bits
const digit HIGH_BIT = 1 << (BITS - 1);   // uint8_t -> 128

        // left bit shift. sign is maintained
        integer operator<<(uint64_t shift){
            if (!*this || !shift)
                return *this;
            base out = digits;
            for(uint64_t i = 0; i < (shift / BITS); i++)
                out.push_back(0);
            shift %= BITS;
            if (shift){
                out.push_back(0);
                return integer(out, _sign) >> (BITS - shift);
            }
            return integer(out, _sign);
        }

        // right bit shift. sign is maintained
        integer operator>>(uint64_t shift){
            if (shift >= bits())
                return integer(0);
            base out = digits;
            for(uint64_t i = 0; i < (shift / BITS); i++)
                out.pop_back();
            shift %= BITS;
            if (shift){
                base v;
                for(d_size i = out.size() - 1; i != 0; i--)
                    v.push_front(((out[i] >> shift) | (out[i - 1] << (BITS - shift))) & NEG1);
                v.push_front(out[0] >> shift);
                out = v;
            }
            return integer(out, _sign);
        }

        // operator+ calls this
        integer add(integer & lhs, integer & rhs){
            base out;
            base::reverse_iterator i = lhs.digits.rbegin(), j = rhs.digits.rbegin();
            bool carry = false;
            double_digit sum;
            for(; ((i != lhs.digits.rend()) && (j != rhs.digits.rend())); i++, j++){
                sum = *i + *j + carry;
                out.push_front(sum);
                carry = (sum > NEG1);
            }
            for(; i != lhs.digits.rend(); i++){
                sum = *i + carry;
                out.push_front(sum);
                carry = (sum > NEG1);
            }
            for(; j != rhs.digits.rend(); j++){
                sum = *j + carry;
                out.push_front(sum);
                carry = (sum > NEG1);
            }
            if (carry)
                out.push_front(1);
            return integer(out);
        }

        // operator* calls this
        // Long multiplication
        integer long_mult(integer & lhs, integer & rhs){
            unsigned int zeros = 0;
            integer row, out = 0;
            for(base::reverse_iterator i = lhs.digits.rbegin(); i != lhs.digits.rend(); i++){
                row.digits = base(zeros++, 0); // zeros on the right hand side
                digit carry = 0;
                for(base::reverse_iterator j = rhs.digits.rbegin(); j != rhs.digits.rend(); j++){
                    double_digit prod = (double_digit(*i) * double_digit(*j)) + carry;// multiply through
                    row.digits.push_front(prod & NEG1);
                    carry = prod >> BITS;
                }
                if (carry)
                    row.digits.push_front(carry);
                out = add(out, row);
            }
            return out;
        }

Is there something obvious that I missed that might cause incorrect calculations? I have stared at this code for a little too long in one burst.

The full modified code is here.

EDIT: I have tested the code on ideone, and it is returning the correct value for this calculation, but my computer still does not. Is there any good explanation for this?

Answer 1

The problem appears to be due to unintended excessive right-shifting. You mention that your ideone post doesn’t reproduce the problem. While testing, I noticed that in your ideone post, the type of digit was actually still set to 16-bit. The moment I changed this to 32 bit (to reflect the SO code snippet) and tried to compile under gcc, I got a bunch of warnings about excessive right shift. Running the program resulted in an infinite loop, which, combined with the right-shift warnings and the fact that you get different behavior on your platform strongly suggests undefined behavior is occurring.

More specifically, there are places where you’re getting bit-size mismatches between int types (ie, you still have a few chunks of code to update).

The trouble maker appears to be setFromZ. Here you have the integer type specified by Z, and the integer type specified by base::value_type. The two are not guaranteed to have the same number of bits, so the bit-shift and bit-mask code in that function are not working correctly when a bit mismatch occurs. And a mismatch is occurring for several instantiations of that template that occur in your ideone code, when uint32_t is the digit type (at least in my environment).

Edit: Confirmation that excessive right-shift is undefined according to standard: Reference: SO Post, see accepted answer, which quotes the standard.

(C++98 §5.8/1) states that:
The behavior is
undefined if the right operand is negative, or greater than or equal
to the length in bits of the promoted left operand.