====== Step 3.3.1: The update loop ======

This loop is responsive for a updating the falling piece as long as it doesn't hit the bottom. Its algorithm is very simple:

  - Draw the piece
  - Check if there was input from joystick
  - Check if move is possible
  - If possible, move piece
  - Do the above 3 more times
  - Fall the piece by one position if possible or exit if not

Please read the following code. We'll discuss it bellow.

  REM -- The update loop
  REPEAT
    REM -- Draw the shape
    CALL draw_shape(shape, piece_x!, piece_y!, shape_color!, 1)
    REM -- The player has four chances to move the piece
    REM -- before it falls one position
    FOR i! = 0 TO 3
      REM -- Check if there's input from joystick
      IF PEEK!(JOY_PORT1) <> 255 THEN
        REM -- Copy piece position to temp variables to be able
        REM -- to check for overlapping without actually
        REM -- updating the piece
        tmp_x! = piece_x! : tmp_y! = piece_y! : tmp_r! = piece_r!
        REM -- Check what input comes from joystick
        IF joy_1_left!()  = 1 THEN DEC tmp_x!
        IF joy_1_right!() = 1 THEN INC tmp_x!
        IF joy_1_down!()  = 1 THEN INC tmp_y!
        IF joy_1_fire!()  = 1 THEN \tmp_r! = (\piece_r! + 1) & %00000011
        REM -- Check if move is possible
        IF overlaps!(shape_no!, tmp_r!, tmp_x!, tmp_y!) = 0 THEN
          REM -- It is possible, erase the piece off of screen
          CALL draw_shape(shape, piece_x!, piece_y!, shape_color!, 0)      
          REM -- Update piece position and shape and draw again
          piece_x! = tmp_x! : piece_y! = tmp_y! : piece_r! = tmp_r!
          shape = get_shape(shape_no!, piece_r!)
          CALL draw_shape(shape, piece_x!, piece_y!, shape_color!, 1)      
        ENDIF
      ENDIF
      FOR j! = 0 TO delay! : WATCH VICII_RASTER, 255 : NEXT
    NEXT
    REM -- Erase the shape
    CALL draw_shape(shape, piece_x!, piece_y!, shape_color!, 0)
    REM -- Fall piece by one
    INC piece_y!
  UNTIL overlaps!(shape_no!, piece_r!, piece_x!, piece_y!) = 1

The code is wrapped in a ''[[:repeat|REPEAT ... UNTIL]]'' block that means it is a //post-test loop//. The loop is executed at least once, checking the //exit condition// at the end. If the condition evaluates to true, the loop exits immediately.

The ''[[:call|CALL]]'' statement calls a procedure.

We haven't defined the constant ''JOY_PORT1'' and there are no such functions in XC=BASIC as ''joy_1_left!()''. This is because these are not part of the XC=BASIC core language but provided by an external library, or extension. Here's what you need to do before being able to use joystic functions:

  * Download the file //xcb-ext-joystick.bas// from [[https://github.com/neilsf/xcb-ext-joystick|GitHub]] and put it in a directory that's accessible from within the source file's directory. For example if the path to your  source file is ///home/mark/xcbasic-tutorial/tetris.bas// then you might want to add the extension to ///home/mark/xcbasic-tutorial/inc/xcb-ext-joystick.bas//
  * Use the ''[[:include|INCLUDE]]'' statement to read the extension and make it part of your program:

  REM -- Put this line at the top of your program
  INCLUDE "inc/xcb-ext-joystick.bas"

The path specified in the ''INCLUDE'' statement must be a path relative to the current source file.

All the rest should be straigth forward. We have only implemented what we previously designed using calls to the procedures previously defined.

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