/* I've witten the following code example after while reading the great book
 * "C++ Templates - The Complete Guide"
 * by David Vandevoorde and Nicolai M. Josuttis, Addison-Wesley, 2002
 *
 * In this book an example for calculating primes is given, which outputs
 * the results as compiler-error-messages. This is an example I've written
 * to get the primes as program output instead of error messages.
 * My example is not bullet proof, anyway it was just for fun. :)
 *
 * (C) Copyright Andre Alexander Bell <andre.bell@gmx.de> 2003.
 * Permission to copy, use, modify, sell and distribute this software
 * is granted provided this copyright notice appears in all copies.
 * This software is provided "as is" without express or implied
 * warranty, and with no claim as to its suitability for any purpose.
 */
#include <iostream>
#include <iomanip>

// the following three IfThenElse Templates are taken from
// "C++ Templates - The Complete Guide"
template<bool CONDITION, typename THEN, typename ELSE>
struct IfThenElse;

template<typename THEN, typename ELSE>
struct IfThenElse<true,THEN,ELSE>
{
	typedef THEN result_type;
};

template<typename THEN, typename ELSE>
struct IfThenElse<false,THEN,ELSE>
{
	typedef ELSE result_type;
};

// This is an adaption of the Value template shown in Chapter 17 of
// "C++ Templates - The Complete Guide"
template<bool b>
struct Value
{
	enum { result = b };
};

// This is where the work ist done :)
template<int N, int I=2>
struct IsPrime
{
	// if (below sqrt(N) and N is not divisible by I)
	//   typedef IsPrime<N,I+1> SubT;
	// else
	//   typedef Value<true> SubT; // This one will give SubT::result==true
	typedef typename IfThenElse<
			(I*I<N) && (N%I!=0),
			IsPrime<N,I+1>,
			Value<true> >::result_type
		SubT;

	enum { result = (N==2) || ((N%I)!=0) && (SubT::result) };
};

// Ok, here we go, let's see if these are primes
int main()
{
	// Set boolalpha to get bools printed as 'true' and 'false'
	std::cout.setf(std::ios_base::boolalpha);
	// static_cast<bool> makes result be a bool for output
	std::cout << "IsPrime<2>::result = " << static_cast<bool>(IsPrime<2>::result) << std::endl;
	std::cout << "IsPrime<5>::result = " << static_cast<bool>(IsPrime<5>::result) << std::endl;
	std::cout << "IsPrime<23>::result = " << static_cast<bool>(IsPrime<23>::result) << std::endl;
	// The following number is prime. To get this compiled you will need a template recursion
	// depth > 1000. With g++ try this:
	//
	// g++ -ftemplate-depth-1001 -o isprime isprime.cpp
	//
	// std::cout << "IsPrime<1000003>::result = " << static_cast<bool>(IsPrime<1000003>::result) << std::endl;

	return 0;
}