Background

Consider the following:

template <unsigned N>
struct Fibonacci
{
    enum
    {
    	value = Fibonacci<N-1>::value + Fibonacci<N-2>::value
    };
};

template <>
struct Fibonacci<1>
{
    enum
    {
    	value = 1
    };
};

template <>
struct Fibonacci<0>
{
    enum
    {
    	value = 0
    };
};

This is a common example and we can get the value of a Fibonacci number as a compile-time constant:

int main(void)
{
    std::cout << "Fibonacci(15) = ";
    std::cout << Fibonacci<15>::value;
    std::cout << std::endl;
}

But you obviously cannot get the value at runtime:

int main(void)
{
    std::srand(static_cast<unsigned>(std::time(0)));

    // ensure the table exists up to a certain size
    // (even though the rest of the code won't work)
    static const unsigned fibbMax = 20;
    Fibonacci<fibbMax>::value;

    // get index into sequence
    unsigned fibb = std::rand() % fibbMax;

    std::cout << "Fibonacci(" << fibb << ") = ";
    std::cout << Fibonacci<fibb>::value;
    std::cout << std::endl;
}

Because fibb is not a compile-time constant.

Question

So my question is:

What is the best way to peek into this table at run-time? The most obvious solution (and "solution" should be taken lightly), is to have a large switch statement:

unsigned fibonacci(unsigned index)
{
    switch (index)
    {
    case 0:
    	return Fibonacci<0>::value;
    case 1:
    	return Fibonacci<1>::value;
    case 2:
    	return Fibonacci<2>::value;
    .
    .
    .
    case 20:
    	return Fibonacci<20>::value;
    default:
    	return fibonacci(index - 1) + fibonacci(index - 2);
    }
}

int main(void)
{
    std::srand(static_cast<unsigned>(std::time(0)));

    static const unsigned fibbMax = 20;    

    // get index into sequence
    unsigned fibb = std::rand() % fibbMax;

    std::cout << "Fibonacci(" << fibb << ") = ";
    std::cout << fibonacci(fibb);
    std::cout << std::endl;
}

But now the size of the table is very hard coded and it wouldn't be easy to expand it to say, 40.

The only one I came up with that has a similiar method of query is this:

template <int TableSize = 40>
class FibonacciTable
{
public:
    enum
    {
    	max = TableSize
    };

    static unsigned get(unsigned index)
    {
    	if (index == TableSize)
    	{
    		return Fibonacci<TableSize>::value;
    	}
    	else
    	{
    		// too far, pass downwards
    		return FibonacciTable<TableSize - 1>::get(index);
    	}
    }
};

template <>
class FibonacciTable<0>
{
public:
    enum
    {
    	max = 0
    };

    static unsigned get(unsigned)
    {
    	// doesn't matter, no where else to go.
    	// must be 0, or the original value was
    	// not in table
    	return 0;
    }
};

int main(void)
{
    std::srand(static_cast<unsigned>(std::time(0)));

    // get index into sequence
    unsigned fibb = std::rand() % FibonacciTable<>::max;

    std::cout << "Fibonacci(" << fibb << ") = ";
    std::cout << FibonacciTable<>::get(fibb);
    std::cout << std::endl;
}

Which seems to work great. The only two problems I see are:

• Potentially large call stack, since calculating Fibonacci<2> requires we go through TableMax all the way to 2, and:

• If the value is outside of the table, it returns zero as opposed to calculating it.

So is there something I am missing? It seems there should be a better way to pick out these values at runtime.

A template metaprogramming version of a switch statement perhaps, that generates a switch statement up to a certain number?

Thanks in advance.