globals
[ last-drop ; use by do-mouse
  mode      ; used by do-mouse
  top-row   ; used by do-fill
  drain-row ; used by do-drain
  field     ; the patches that are not border patches
  border    ; the border patches
]

breeds
[ balls ; balls are the particles
]

patches-own
[ void? ; true means patch is empty, and not an obstacle
]

to setup
   ca
   ask patches [ set void? true ]
   reset-screen-regions
   set top-row patches with [ pycor = screen-edge-y ]
   set drain-row patches with [ pycor = (- screen-edge-y) + 1 ]
   make-funnel
   make-splotches
   make-pins
end

to go
   ; if using mouse, all else pauses
   ifelse active-mouse? and mouse-down?
   [ do-mouse ]
   [ set mode 0 ; reset draw / erase mode
     ; evaluate the run-time effect switches
     if drain?   [ do-drain ]
     if fill?    [ do-fill  ]
     ; balls fall
     without-interruption
     [ ask balls [ do-fall ] ]
   ]
end

to define-field
   set field patches with 
   [     abs pxcor < screen-edge-x
     and pycor > (- screen-edge-y)
   ]
end

to define-border
   set border patches with
   [ abs pxcor = screen-edge-x or pycor = (- screen-edge-y)
   ]
end

to-report field-not-defined?
   report not is-patch-agentset? field
end

to-report border-not-defined?
   report not is-patch-agentset? border
end

to define-screen-regions
   if field-not-defined?   [ define-field  ]
   if border-not-defined?  [ define-border ]
end

to reset-screen-regions
   define-screen-regions
   clear-field
   set-border
end

to clear-field
   ask balls [ pop ]
   ask field [ clear-grid ]
end

to set-border
   ask border 
   [ set void? false
     set pcolor sky
   ]
end
       
to do-drain
   if random 100 < drain-rate
   [ let drain-balls balls with [ pycor = (- screen-edge-y) + 1 ]
     if any? drain-balls
     [ ask random-one-of drain-balls [ pop ] ]
   ]
end

to do-fill
   if random 100 < fill-rate
   [ let open-patches top-row with [ void? = true ]
     if any? open-patches
     [ ask random-one-of open-patches 
       [ make-ball]
     ]
   ]
end

to make-ball ; patch procedure
     sprout 1
     [ set breed balls
       set shape "circle"
       set heading 180
       set void? false
       let new-color who
       if new-color mod 10 <= 2 [ set new-color new-color + 2 ]
       if new-color mod 10 >= 8 [ set new-color new-color + 4 ]
       set color new-color 
     ]
end

to do-fall
   let below patch-at 0 -1
   let beside patches at-points [ [ -1 0 ] [ 1 0 ] ]
   let above patch-at 0  1
   ; if open space directly below, fall straight down
   ifelse void?-of below = true
   [ set void? true
     set ycor ycor - 1
     set void? false
   ]
   ; otherwise, if patch at left or right is clear,
   ; and patch below that is clear as well, fall down to the side
   [ set beside beside with [ void? = true and void?-of patch-at 0 -1 = true ]
     ifelse any? beside 
     [ set beside random-one-of beside
       set void? true
       set xcor pxcor-of beside
       set ycor pycor - 1
       set void? false
     ]
      ; otherwise, if patch above IS a ball, and any side patch is open, pick a direction, and go that way
     [ set beside beside with [ void? = true ]
       ifelse (     void?-of above = false
                and ( value-from above [ any? balls-here ] = true )
                and ( any? beside )
              )
        [ set beside random-one-of beside 
          set void? true
          set xcor pxcor-of beside
          set void? false
        ]
        ; otherwise, if patch above is (non ball) obstacle, and exactly one side is open, roll that way
        [ if (     void?-of above = false
               and (value-from above [ not any? balls-here ] = true )
               and any? beside with [ void? = true ]
               and any? beside with [ not void? = true ]
          )
          [  
            set beside one-of beside with [ void? = true ]
            set void? true
            set xcor pxcor-of beside
            set void? false
          ]
        ]
      ]
    ] 
end

to randomize-order
   if any? balls
   [ ask random-one-of balls
     [ hatch 1 [set color color + 1]
       die
     ]
   ]
end
  
to do-mouse
   let here patch-at mouse-xcor mouse-ycor
   if last-drop != here
   [ set last-drop here
     ask here
     [ ifelse draw-mode?
         [ if (mode != 2 and pcolor = black) or (mode = 1 and pcolor != blue )
           [ let nib here
             if fat-pen?
             [ set nib (field in-radius-nowrap 1.5 )
                 with [ void? = true or any? balls-here ]
             ]
             ask nib
             [ ask balls-here [ pop ]
               set-grid
             ]
             set mode 1
           ]  
           if (mode != 1 and pcolor = gray ) or (mode = 2 and pcolor != blue )
           [ let nib here
             if fat-pen?
             [ set nib (field in-radius-nowrap 1.5 )
                 with [ void? = false and (pcolor = gray or any? balls-here) ] 
             ]
             ask nib
             [ ask balls-here [ pop ]
               clear-grid
             ]
             set mode 2
           ]
         ]
         [ if not any? balls-here and void? = true
           [ make-ball
           ]
         ]
       ]
     ]
end

to clear-obstacles
   without-interruption
   [ ask patches with [ void? = false and pcolor = gray ] 
     [ clear-grid ]
   ]
end

to clear-particles
   without-interruption
   [ ask balls [ pop ] ]
end

to make-funnel
   let funnel-tip patch 0 (int (screen-edge-y * .25))
   ask field with [ pycor < ( screen-edge-y * .75 ) and self != ( funnel-tip ) and ( towards funnel-tip = 135 or towards funnel-tip = 225 ) ]
   [ set-grid ]
   ask field with [ pycor = int( screen-edge-y * .75 ) and abs pxcor > (screen-edge-x * .25) ]
   [ set-grid ]
   ask funnel-tip
   [ clear-grid ]
end   

to make-splotches
   let number 5
   let min-size screen-edge-x / 8
   ask random-n-of number field with [ pycor < screen-edge-y * -.33 and pycor > screen-edge-y * -.75 ]
   [ ask field in-radius-nowrap (min-size + random min-size )
     [ set-grid ]
   ]
end

to make-pins
   ask field with [ pycor >= screen-edge-y * -.2 and pycor <= screen-edge-y * .2 and  (pycor + 1) mod 2 = 0 and floor (pxcor + pycor / 2) mod 2 = 0 ]
   [ set-grid ]
end

to clear-grid
   set pcolor black
   set void? true
end

to set-grid
   set pcolor gray
   set void? false
end     
     
to pop
   hide-turtle
   set void? true
   die
end   
   
@#$#@#$#@
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@#$#@#$#@
WHAT IS IT?
-----------
This is a simple simulation of falling particles.

HOW IT WORKS
------------
Particles fall, pile-up, and flow over each other and obstacles, according to a collection of simple rules 

HOW TO USE IT
-------------
Click Setup.
   This sets of the field and draws some default obstacles
Click Go.
   This starts particles falling.
If active-mouse? is on, you can draw particles or obstacles
If draw-mode? is on, then the mouse will add and erase obstacles,
otherwise, the mouse will add particles.
The fat-pen? switch causes the mouse to draw or erase a larger block of obstacles at once.

If fill? is on, new particles are created at the top.
If drain? in on, particles are removed from the bottom.

The sliders fill-rate and drain-rate control how quickly particles are added and removed.

THINGS TO TRY
-------------
Draw in various configurations of obstacles.
Draw an hourglass.
Draw a cascade fountain.
Draw a waterfall
Draw conduits and pipes.

EXTENDING THE MODEL
-------------------
Add pressure
~~~~~~~~~~~~
There is no sense of "pressure" on the particles, so particles will not flow horizontally (except in certain cases, like along one-unit-high channels) or up-hill, like water.

So, add "pressure" : a force that can cause particles to move because of the state of particles that may be quite far away.

Add acceleration
~~~~~~~~~~~~~~~~
Falling objects accelerate as they fall. That is, they gain speed. In this model, the particles fall at a steady speed. Change the model so that the particles speed up as they fall. Careful! You need to make sure the particles don't skip past the obstacles, other particles, or the "ground"!

Add "bounce"
~~~~~~~~~~~~
Particles like sand and ball-bearings don't usually just fall, land, and sit there. They often bounce when they land. In this model, particles do not bounce. Should particles landing on "sloped" surfaces bounce at a particular angle? Would it be just as well for to bounce at a random angle? How much realism is "enough"?

Implement the obstacles as turtles
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The obstacles are patches. What if they were turtles? How could the model's code be optimized? Would it be faster or slower? What would that let you do?

Implement anti-particles
~~~~~~~~~~~~~~~~~~~~~~~~
What if there was another breed of particles that followed the same rules, but in reverse? That is, they fall *up*. How would the two kinds of particles interact? What if two particles (the particle and an anti-particle) wanted to occupy each-other's spots? Can they swap places? How would that change things?

Make a game
~~~~~~~~~~~
You can use this model as the basis for a game, like a Pachinko machine, or a game where the player must insert (or remove) obstacles to direct the particles into a goal. There may be un-removable obstacles, or un-fillable space that the player must work around. Perhaps a suite of special obstacles that add, destroy, move, or re-direct particles could be involved. Another game may involve the interaction of particles and anti-particles.

COPYRIGHT
---------
This work is copyright (c) 2004 James P. Steiner. It is licensed for use under a Creative Commons 2.0 Attribution, Non-commercial, Share-alike license.
@#$#@#$#@
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