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8.15: Checking if a Slide or Rotation is Valid

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                elif event.type == KEYDOWN:
                    # moving the piece sideways
                    if (event.key == K_LEFT or event.key == K_a) and isValidPosition(board, fallingPiece, adjX=-1):
                        fallingPiece['x'] -= 1
                        movingLeft = True
                        movingRight = False
                        lastMoveSidewaysTime = time.time()
                    elif (event.key == K_RIGHT or event.key == K_d) and isValidPosition(board, fallingPiece, adjX=1):
                        fallingPiece['x'] += 1
                        movingRight = True
                        movingLeft = False
                        lastMoveSidewaysTime = time.time()
                    # rotating the piece (if there is room to rotate)
                    elif (event.key == K_UP or event.key == K_w):
                        fallingPiece['rotation'] = (fallingPiece['rotation'] + 1) % len(PIECES[fallingPiece['shape']])
                        if not isValidPosition(board, fallingPiece):
                            fallingPiece['rotation'] = (fallingPiece['rotation'] - 1) % len(PIECES[fallingPiece['shape']])
                    elif (event.key == K_q): # rotate the other direction
                        fallingPiece['rotation'] = (fallingPiece['rotation'] - 1) % len(PIECES[fallingPiece['shape']])
                        if not isValidPosition(board, fallingPiece):
                            fallingPiece['rotation'] = (fallingPiece['rotation'] + 1) % len(PIECES[fallingPiece['shape']])
                    # making the piece fall faster with the down key
                    elif (event.key == K_DOWN or event.key == K_s):
                        movingDown = True
                        if isValidPosition(board, fallingPiece, adjY=1):
                            fallingPiece['y'] += 1
                        lastMoveDownTime = time.time()

    When the left arrow key is pressed down (and moving to the left is a valid move for the falling piece, as determined by the call to isValidPosition()), then we should change the position to one space to the left by subtracting the value of fallingPiece['x'] by 1. The isValidPosition() function has optional parameters called adjX and adjY. Normally the isValidPosition() function checks the position of the data provided by the piece object that is passed for the second parameter. However, sometimes we don’t want to check where the piece is currently located, but rather a few spaces over from that position.

    If we pass -1 for the adjX (a short name for "adjusted X"), then it doesn’t check the validity of the position in the piece’s data structure, but rather if the position of where the piece would be if it was one space to the left. Passing 1 for adjX would check one space to the right. There is also an adjY optional parameter. Passing -1 for adjY checks one space above where the piece is currently positioned, and passing a value like 3 for adjY would check three spaces down from where the piece is.

    The movingLeft variable is set to True, and just to make sure the falling piece won’t move both left and right, the movingRight variable is set to False on line 6 [228]. The lastMoveSidewaysTime variable will be updated to the current time on line 7 [229].

    These variables are set so that the player can just hold down the arrow key to keep moving the piece over. If the movingLeft variable is set to True, the program can know that the left arrow key (or A key) has been pressed and not yet let go. And if 0.15 seconds (the number stored in MOVESIDEWAYSFREQ) has passed since the time stored in lastMoveSidewaysTime, then it is time for the program to move the falling piece to the left again.

    The lastMoveSidewaysTime works just like how the lastClickTime variable did in the Simulate chapter.

    The code on lines 9 [231] to 13 [235] is almost identical to lines 3 [225] to 7 [229], except that it handles moving the falling piece to the right when the right arrow key (or D key) has been pressed.

    The up arrow key (or W key) will rotate the falling piece to its next rotation. All the code has to do is increment the 'rotation' key’s value in the fallingPiece dictionary by 1. However, if incrementing the 'rotation' key’s value makes it larger than the total number of rotations, then "modding" by the total number of possible rotations for that shape (which is what len(SHAPES[fallingPiece['shape']]) is) then it will "roll over" to 0.

    Here’s an example of this modding with the J shape, which has 4 possible rotations:

    >>> 0 % 4
    >>> 1 % 4
    >>> 2 % 4
    >>> 3 % 4
    >>> 5 % 4
    >>> 6 % 4
    >>> 7 % 4
    >>> 8 % 4

    If the new rotated position is not valid because it overlaps some boxes already on the board, then we want to switch it back to the original rotation by subtracting 1 from fallingPiece['rotation']. We can also mod it by len(SHAPES[fallingPiece['shape']]) so that if the new value is -1, the modding will change it back to the last rotation in the list. Here’s an example of modding a negative number:

    >>> -1 % 4

    Lines 20 [242] to 23 [245] do the same thing 16 [238] to 19 [241], except they handle the case where the player has pressed the Q key which rotates the piece in the opposite direction. In this case, we subtract 1 from fallingPiece['rotation'] (which is done on line 21 [243]) instead of adding 1.

    If the down arrow or S key is pressed down, then the player wants the piece to fall faster than normal. Line 29 [251] moves the piece down one space on the board (but only if it is a valid space). The movingDown variable is set to True and lastMoveDownTime is reset to the current time. These variables will be checked later so that the piece keeps falling at the faster rate as long as the down arrow or S key is held down.

    This page titled 8.15: Checking if a Slide or Rotation is Valid is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Al Sweigart via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.