Defines the rules of Sudoku

sudoku_rules.cpp

// Returns a boolean which indicates whether any assigned entry
// in the specified row matches the given number. 
bool used_in_row(int grid[DIM][DIM], int row, int num)
{
	for (int col = 0; col < DIM; col++)
		if (grid[row][col] == num)
		{
			return true;
		}
	return false;
}

// Returns a boolean which indicates whether any assigned entry
// in the specified column matches the given number. 
bool used_in_col(int grid[DIM][DIM], int col, int num)
{
	for (int row = 0; row < DIM; row++)
		if (grid[row][col] == num)
		{
			return true;
		}
	return false;
}

// Returns a boolean which indicates whether any assigned entry
// within the specified 3x3 box matches the given number. 
bool used_in_box(int grid[DIM][DIM], int box_start_rpw, int box_start_col, int num)
{
	for (int row = 0; row < 3; row++)
		for (int col = 0; col < 3; col++)
			if (grid[row + box_start_rpw][col + box_start_col] == num) 
			{
				return true;
			}
	return false;
}

// Returns a boolean which indicates whether it will be legal to assign
// num to the given row,col location.
bool is_safe(int grid[DIM][DIM], int row, int col, int num)
{
	// Check if 'num' is not already placed in current row,
	// current column and current 3x3 box 
	return !used_in_row(grid, row, num) &&
		!used_in_col(grid, col, num) &&
		!used_in_box(grid, row - row % 3, col - col % 3, num);
}


// Searches the grid to find an entry that is still unassigned. If
// found, the reference parameters row, col will be set the location
// that is unassigned, and true is returned. If no unassigned entries
// remain, false is returned. 
std::pair<int, int> get_unassigned_location(int grid[DIM][DIM])
{
	for (int row = 0; row < DIM; row++)
		for (int col = 0; col < DIM; col++)
			if (grid[row][col] == BLANK)
			{
				return std::make_pair(row, col);
			}
	return GRID_FULL;
}