123

patches-own
[
  new-color       ;; currently, always either white or black
  inner-neighbors ;; other cells in a circle around the cell
  outer-neighbors ;; other cells in a ring around the cell (but usually not touching the cell)
]

to setup
  clear-all
  ;; computes inner and outer neighbors in an ellipse around each cell
  ask patches
  [
    set inner-neighbors ellipse-in inner-radius-x inner-radius-y
    ;; outer-neighbors needs more computation because we want only the cells in the circular ring
    set outer-neighbors ellipse-ring outer-radius-x outer-radius-y inner-radius-x inner-radius-y
  ]

  ifelse any? patches with [ count outer-neighbors = 0 ]
    [ user-message word "It doesn't make sense that 'outer' is equal to or smaller than 'inner.' "
                        " Please reset the sliders and press Setup again."
      stop]
    [restart]
  reset-ticks
end 

;; this procedure sets approximately initial-density percent of the
;; cells white and the rest black; if initial-density is set at 50%
;; then about half the cells will be white and the rest black

to restart
  ask patches
    [ ifelse random-float 100.0 < initial-density
        [ set pcolor white ]
        [ set pcolor black ] ]
  reset-ticks
end 

to go
  ask patches [ pick-new-color ]
  ask patches [ set pcolor new-color ]
  tick
end 

to pick-new-color  ;; patch procedure
  let activator count inner-neighbors with [pcolor = white]
  let inhibitor count outer-neighbors with [pcolor = white]
  ;; we don't need to multiply 'activator' by a coefficient because
  ;; the ratio variable keeps the proportion intact
  let difference activator - ratio * inhibitor
  ifelse difference > 0
    [ set new-color white ]
    [ if difference < 0
        [ set new-color black ] ]
  ;; note that we did not deal with the case that difference = 0.
  ;; this is because we would then want cells not to change color.
end 

;;; procedures for defining elliptical neighborhoods

to-report ellipse-in [x-radius y-radius]  ;; patch procedure
  report patches in-radius (max list x-radius y-radius)
           with [1.0 >= ((xdistance myself ^ 2) / (x-radius ^ 2)) +
                        ((ydistance myself ^ 2) / (y-radius ^ 2))]
end 

to-report ellipse-ring [outx-radius outy-radius inx-radius iny-radius]  ;; patch procedure
  report patches in-radius (max list outx-radius outy-radius)
           with [1.0 >= ((xdistance myself ^ 2) / (outx-radius ^ 2)) +
                        ((ydistance myself ^ 2) / (outy-radius ^ 2))
             and 1.0 <  ((xdistance myself ^ 2) / (inx-radius ^ 2)) +
                        ((ydistance myself ^ 2) / (iny-radius ^ 2))
                ]
end 

;; The following two reporter give us the x and y distance magnitude.
;; you can think of a point at the tip of a triangle determining how much
;; "to the left" it is from another point and how far "over" it is from
;; that same point. These two numbers are important for computing total distances
;; in elliptical "neighborhoods."

;; Note that it is important to use the DISTANCEXY primitive and not
;; just take the absolute value of the difference in coordinates,
;; because DISTANCEXY handles wrapping around world edges correctly,
;; if wrapping is enabled (which it is by default in this model)

to-report xdistance [other-patch]  ;; patch procedure
  report distancexy [pxcor] of other-patch
                    pycor
end 

to-report ydistance [other-patch]  ;; patch procedure
  report distancexy pxcor
                    [pycor] of other-patch
end 


; Copyright 2003 Uri Wilensky.
; See Info tab for full copyright and license.