Matrix digital rain

Implement the Matrix Digital Rain visual effect from the movie "The Matrix" as described in Wikipedia.

Provided is a reference implementation in Common Lisp to be run in a terminal.

Common Lisp

Runs in the terminal (using the Ncurses C library and the croatoan Lisp wrapper).

<lang lisp> (defun matrix-digital-rain ()

 (with-screen (scr :input-echoing nil :input-blocking nil :cursor-visibility nil)
   (let* ((width (.width scr))
          (height (.height scr))
          ;; start at a random height in each column.
          (pos (loop repeat width collect (random height)))
          ;; run each column at a random speed.
          (speeds (loop repeat width collect (random 4))))
     ;; hit the q key to exit the main loop.
     (add-event-handler (scr #\q) 'exit-event-loop)
     (add-event-handler (scr nil)
       (lambda (win event)
         ;; generate a random ascii char
         (flet ((randch () (+ 64 (random 58))))
           (loop for col from 0 to (1- width) do
             (loop repeat (nth col speeds) do
               (setf (.attributes win) '(:bold))
               (setf (.color-pair win) '(:white :black))
               (add win (randch) :y (mod (nth col pos) height) :x col)
               (setf (.color-pair win) '(:green :black))
               (add win (randch) :y (mod (- (nth col pos) 1) height) :x col)
               (add win (randch) :y (mod (- (nth col pos) 2) height) :x col)
               (setf (.attributes win) '())
               (add win (randch) :y (mod (- (nth col pos) 3) height) :x col)
               ;; overwrite the last char half the height from the first char.
               (add win #\space  :y (mod (- (nth col pos) (floor height 2)) height) :x col)
               (refresh win)
               ;; advance the column
               (setf (nth col pos) (mod (1+ (nth col pos)) height)))))))
     (setf (.frame-rate scr) 20)
     (run-event-loop scr))))

</lang>