to setup
  ca
  set-default-shape turtles "circle"
  draw-walls
  cct 5                                 ;; create some turtles
    [ randomize ]                       ;; set random coordinates and random direction
end

; draws the boundaries (walls) of the "billiard table"
to draw-walls
  ; draw left and right walls
  ask patches with [abs pxcor = screen-edge-x]
    [ set pcolor blue ]
  ; draw top and bottom walls
  ask patches with [abs pycor = screen-edge-y]
    [ set pcolor blue ]
end

; set random coordinates and random direction
to randomize
  setxy (random screen-size-x) (random screen-size-y)
  rt random 360
  if pcolor = blue       ; if it's on the wall...
    [ randomize ]        ; ...try again
end

to go  ;; turtle procedure
  every 0.5 - speed / 10
    [ ifelse leave-trace?             ;; the turtle puts its pen up or down depending on the 
        [ pd ]                        ;; value of the LEAVE-TRACE? switch
        [ pu ]
       bounce
       fd 1 ]
end

;; this procedure checks the coordinates and makes the turtles
;; reflect according to the law that the angle of reflection is 
;; equal to the angle of incidence
to bounce  ;; turtle procedure
  locals [ new-px new-py ]
  ; get the coordinates of the patch we'll be on if we go forward 1
  set new-px round (xcor + dx)
  set new-py round (ycor + dy)
  ; check: hitting left or right wall?
  if (abs new-px = screen-edge-x)
    ; if so, reflect heading around x axis
    [ set heading (0 - heading) ]
  ; check: hitting top or bottom wall?
  if (abs new-py = screen-edge-y)
    ; if so, reflect heading around y axis
    [ set heading (180 - heading) ]
end


; This model was created as part of the project: CONNECTED MATHEMATICS:
; MAKING SENSE OF COMPLEX PHENOMENA THROUGH BUILDING OBJECT-BASED PARALLEL
; MODELS (OBPML)
; The project gratefully acknowledges the support of the National Science
; Foundation (Applications of Advanced Technologies Program) -- grant numbers
; RED #9552950 and REC #9632612.
; 
; Converted from StarLogoT to NetLogo, 2001
; Copyright 1999  by Uri Wilensky. All rights reserved.
; 
; Permission to use, copy, or modify this software and its documentation for
; educational and research purposes only and without fee is hereby granted,
; provided that this copyright notice and the original authors' names appear
; on all copies and supporting documentation. For any other uses of this
; software, in original or modified form, including but not limited to
; distribution in whole or in part, specific prior permission must be
; obtained from Uri Wilensky. These programs shall not be used, rewritten, or
; adapted as the basis of a commercial software or hardware product without
; first obtaining appropriate licenses from Uri Wilensky. We make no
; representations about the suitability of this software for any purpose. It
; is provided "as is" without express or implied warranty.
@#$#@#$#@
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CC-WINDOW
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Command Center

BUTTON
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NIL
setup
NIL
1
T
OBSERVER

BUTTON
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go
go
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TURTLE

SWITCH
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SLIDER
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Speed
Speed
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@#$#@#$#@
WHAT IS IT?
------------
This demo shows how to make turtles bounce off the walls.
@#$#@#$#@
default
true
0
Polygon -7566196 true true 150 5 40 250 150 205 260 250

circle
true
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@#$#@#$#@
NetLogo 1.1
@#$#@#$#@
@#$#@#$#@
@#$#@#$#@