C++ Ranges

gistfile1.cpp

#include <cstdint>
#include <iostream>

template <typename T>
class DInputRange
{
public:
	struct iterator
	{
		const T* r;
		iterator(T* rng) : r(rng) {}

		auto operator*() -> decltype(r->front()) const { return static_cast<const T*>(r)->front(); }
		void operator++()  { ((T*)r)->popFront(); }
		bool operator!=(iterator) const { return !static_cast<const T*>(r)->empty(); }
	};

	iterator begin() { return static_cast<T*>(this); }
	iterator end() { return nullptr; }
};

int bitcount(uint64_t v)
{
#ifdef _MSC_VER
	unsigned long r = 0;
	_BitScanForward64(&r, v);
	return (int)r;
#else
	return __builtin_ctzll(v);
#endif

}
// D-style input range, consumes input
class DBitRange : public DInputRange<DBitRange>
{
public:
	explicit DBitRange(uint64_t& b) : b_{ b } {}

	bool empty() const { return b_ == 0; }
	void popFront() { b_ &= b_ - 1; }
	int front() const { return bitcount(b_); }
private:
	uint64_t& b_;
};

void C_style(uint64_t primes) {
	for (; primes != 0; primes &= primes - 1)
		std::cout << bitcount(primes) << ", ";
	std::cout << "\n";
}

void D_style(uint64_t primes) {
	for (auto bitrange = DBitRange{ primes }; !bitrange.empty(); bitrange.popFront())
		std::cout << bitrange.front() << ", ";
	std::cout << "\n";
}

void CPP_style(uint64_t primes) {
	for (auto b : DBitRange{ primes })
		std::cout << b << ", ";
	std::cout << "\n";
}

#if 1
int main()
{
	int vals[] = { 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61 };
	uint64_t primes = 0, primes2;
	for (auto v : vals)
		primes ^= 1ull << v;
	primes2 = primes;

	// C-style iteration over bitmap
	for (auto bitmap = primes; bitmap != 0; bitmap &= bitmap - 1)
		std::cout << bitcount(bitmap) << ", ";
	std::cout << "\n";

	// D-style iteration over bitmap
	// translates 1-on-1 to C code, no proxy iterators and references
	for (auto bitrange = DBitRange{ primes }; !bitrange.empty(); bitrange.popFront())
		std::cout << bitrange.front() << ", ";
	std::cout << "\n";

	// CHALLENGE: define C++ range-based for iteration over bitmap
	// that translates 1-on-1 into the equivalent C/D code
	for (auto b : DBitRange{ primes2 })
		std::cout << b << ", ";
	std::cout << "\n";
}
#endif