Refactoring Eight Queens: Step 5

Doing the next refactoring, adding an IsSpotUnderAtatck method to get rid of some nesting, results in something like:

namespace Eight
{
  template <int SIZE> class Queens
  {
    bool board[SIZE][SIZE]={false}, colOccupied[SIZE]={false}, diag1Occupied[SIZE*2 - 1]={false}, diag2Occupied[SIZE*2 - 1]={false};

    int Solve(int row = 0) // assumes all rows before "row" have correctly-placed queens so far
    {
      if (row >= SIZE)
        return 1;

      int count = 0;
      for (int col=0; col<SIZE; col++)
      {
        if (IsSpotUnderAttack(row, col))
          continue;

        int diag1 = row + col;
        int diag2 = SIZE - row + col - 1;
        board[row][col] = colOccupied[col] = diag1Occupied[diag1] = diag2Occupied[diag2] = true;
        count += Solve(row + 1);
        board[row][col] = colOccupied[col] = diag1Occupied[diag1] = diag2Occupied[diag2] = false;
      }
      return count;
    }
    bool IsSpotUnderAttack(int row, int col) const
    {
      return colOccupied[col] || diag1Occupied[row+col] || diag2Occupied[SIZE-row+col-1];
    }

  public:
    int CountSolutions() { return Solve(); }
  };
}

All the tests still pass.

Next, let’s add PlaceQueen and RemoveQueen methods, which will clarify the code because we won’t be looking at the diag1 and diag2 calculations.

When that’s done, click Next.