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When GenerateNewPieces() is called with true passed as dropSquares, the looping logic processes one column at a time from the bottom up. By using the step 1 in the for loop for the Y coordinate, we can make the loop run backwards instead of the default forward direction. When it finds an empty square, it calls FillFromAbove() to pull a filled square from above into that location.

The reason the processing order is important here is that by filling a lower square from a higher position, that higher position will become empty. It, in turn, will need to be filled from above.

After the holes are filled (or if dropSquares is set to false), GenerateNewPieces() examines each square in boardSquares and asks it to generate random pieces for each square that contains an empty piece.

Water-filled pipes

Whether or not a pipe is filled with water, it is managed separately from its orientation. Rotating a single pipe could change the water-filled status of any number of other pipes, without changing their rotation.

Instead of filling and emptying individual pipes, it is easier to empty all of the pipes and then re-fill the pipes that need to be marked as having water in them.