#ifndef REGH
#define REGH
#include <iostream>
#include <cassert>
#include <iterator>
using namespace std;

class sudoku;
class box;

class reg {
	static const size_t root = 3;
	static const size_t n = root * root;

	sudoku *p;
	size_t x;
	size_t y;
public:
	friend class sudoku;

	class iterator: public std::iterator<forward_iterator_tag, box> {
		sudoku *const p;
		size_t x;
		size_t y;
public:
		iterator(sudoku *initial_p, size_t initial_x, size_t initial_y):
			p(initial_p), x(initial_x), y(initial_y) {}

		box& operator*() const;
		box *operator->() const {return &**this;}

		iterator& operator++() {
			const size_t remainder = x % root;
			if (remainder < root - 1) {
				++x;
			} else {
				x -= remainder;
				++y;
			}
			return *this;
		}

		friend bool operator==(const iterator& it1,
			const iterator& it2) {
			return it1.p == it2.p
				&& it1.x == it2.x
				&& it1.y == it2.y;
		}
	};

	iterator begin() const {return iterator(p, x * root, y * root);}
	iterator   end() const {return iterator(p, x * root, y * root + root);}
};

inline bool operator!=(const reg::iterator& it1, const reg::iterator& it2) {
	return !(it1 == it2);
}
#endif