Question

This is a rather theoretical question, so while the language is specifically Java, any general solution will suffice.

Suppose I wanted to write a trivial factorial function:

long factorial(int n)
{
    //handle special cases like negatives, etc.

    long p = 1;
    for(int i = 1; i <= n; i++)
    {
        p = p * n;
    }
    return p;
}

But now, I also want to check if the factorial overflows (without simply hard coding a MAX_FACTORIAL_PARAMETER or something of the like). In general checking for overflow during multiplication is as simple as checking the result against the original inputs, but in this case, since overflow can occur at any point, it would be rather expensive to perform more divisions and comparisons in every single loop.

The question then, is twofold–is there any way to solve the factorial problem of overflow without checking for multiplication overflow at every step or hard coding a maximum allowed parameter?

And in general, how should I approach problems that involve many stages of iteration/recursion that could silently fail at every stage without compromising performance by introducing expensive checks at each?

Answer 1

While Java does not help you with this, there certainly are languages that help with overflow. For example, C# provides the checked keyword. Underneath, this feature probably uses hardware support in the form of the overflow flag.