611 def compact(grid, walls):
612     """
613     Converts a grid in the form of FTTTTFTTF to FTFFTFTTF, thus reducing the #
614     of pellets in the grid.
615     """
616     compact = grid.copy()
617
618     def cross(x, y):
619         xValues = [x - 1, x, x + 1]
620         yValues = [y - 1, y, y + 1]
621         numSurroundingPellets = 0
622         for xValue in xValues:
623             if xValue >= 0 and xValue < compact.width:
624                 for yValue in yValues:
625                     if yValue >= 0 and yValue < compact.height:
626                         if not compact[xValue, yValue] and not walls[xValue, yValue]:
627                             return True
628                         elif compact[xValue, yValue]:
629                             numSurroundingPellets += 1
630         return numSurroundingPellets == 8
631
632     for i in range(compact.width):
633         for j in range(compact.height):
634             if compact[i][j]:
635                 compact[i][j] = cross(i, j)
636
637     return compact