enum state { CORRECT = 0, INCORRECT, INCOMPLETE };

	/** Finds the current state of a sudoku game */
	state sudoku_state(short grid[9][9])
	{
		/* This program initializes a buffer for each digit from 1 to 9.
		 * It then proceeds to test, for each row, each line and each sub-grid,
		 * that numbers are not repeated, using `buffer` as a bitset.
		 * In order to avoid resetting `buffer` before each check, we make use of
		 * the fact that between successful checks, all the numbers in `buffer`
		 * are equal to a given n. We keep this n in variable `curr`.
		 * It is then sufficient to test differences with n to spot a problem in
		 * the currently analysed subspace.
		 * The algorithm short-circuits the call stack on invalid state by
		 * throwing the state and catching it at call-site.
		 */
		int buffer[9] = {0}; // nbs from 1 to 9
		int curr = 0;
		auto test = [&](int d) {
			if (d <= 0 || d > 9) throw INCOMPLETE;
			if (buffer[d-1]++ != curr) throw INCORRECT;
		};
		auto rows_or_lines = [&](bool lines /* or rows */) {
			for (int i = 0; i < 9; ++i) {
				for (int j = 0; j < 9; ++j)
					if (lines) test(grid[i][j]);
					else test(grid[j][i]);
				++curr;
			}
		};
		auto subgrids = [&](int x, int y) {
			for (int i = x; i < x+3; ++i)
				for (int j = y; j < y+3; ++j)
					test(grid[j][i]);
			++curr;
		};
		try {
			rows_or_lines(true); rows_or_lines(false);
			for (int i = 0; i < 3; i++) for (int j = 0; j < 3; j++)
				subgrids(i*3,j*3);
		}
		catch (state err_state) { return err_state; }
		return CORRECT;
	}