breed [ birds bird ]
breed [ dead-birds dead-bird ]


birds-own [
  flockmates         ;; agentset of nearby turtles
  nearest-neighbor   ;; closest one of our flockmates
  velocity           ;; vector with x y and z components it is determined by the previous velocity
                     ;; and the current acceleration at each step
  acceleration       ;; vector with x y and z components, determined by the six urges
]

;;
;; Setup Procedures
;;
to setup
  clear-all
  setxyz 0 -30 0
  create-birds population
    [
      place-bird
      set size 0.75
      set velocity [ 0 0 0 ]
      set acceleration [ 0 0 0 ]
      set color yellow + 2 - random-float 6
      set flockmates no-turtles
    ]
  ask turtle 0 [ set color red ]

  ;; checkered floor
  ask patches
    with [ pzcor = min-pzcor ]
      [ set pcolor 3 + green + ( pxcor + pycor ) mod 2 ]

  reset-ticks
end

to place-bird ;; turtle procedure
  setxyz random-xcor random-ycor random-zcor
  ;; make sure that the birds are not on the floor or inside the
  ;; any obstacles that have been created.
  while [ pcolor != black ]
  [ setxyz random-xcor random-ycor random-zcor ]
end

;;
;; Runtime Procedures
;;
to go
  ask birds
  [ flock
    ;; if we've hit something
    ;; we're a goner
    if pcolor != black
    [ hatch-dead-birds 1
      [ bk 1 ]
      die ]
  ]
  ;; ask the dead ones to continue falling
  ask dead-birds
    [ fall ]
  tick
end

to flock  ;; bird procedure
  ;; look for birds in my vicinity
  find-flockmates

  ;; acceleration at each step is determined
  ;; entirely by the urges
  set acceleration (list 0 0 0)

  add-urge world-center-urge world-center-constant
  add-urge wander-urge wander-constant
  add-urge avoid-obstacle-urge avoidance-constant

  ;; if I'm not in a flock ignore the flock related
  ;; urges
  if count flockmates > 0
  [ add-urge spacing-urge spacing-constant
    add-urge center-urge center-constant
    add-urge velocity-urge velocity-constant ]

  ;; keep the acceleration within the accepted range
  if magnitude acceleration > max-acceleration
  [ set acceleration
    (scale
        max-acceleration
        normalize acceleration) ]

  ;; the new velocity of the bird is sum of the acceleration
  ;; and the old velocity.
  set velocity (add velocity acceleration)

  ;; keep the velocity within the accepted range
  if magnitude velocity > max-velocity
  [ set velocity
    (scale
        max-velocity
        normalize velocity) ]

  let nxcor xcor + ( item 0 velocity )
  let nycor ycor + ( item 1 velocity )
  let nzcor zcor + ( item 2 velocity )
  facexyz nxcor nycor nzcor
  fd 0.1 * magnitude velocity
end

to add-urge [urge factor] ;; bird procedure
  set acceleration add acceleration scale factor normalize urge
end

to-report world-center-urge  ;; bird reporter
  ifelse distancexyz 0 0 0 > 10
  [ report (list (- xcor) (- ycor) (- zcor) ) ]
  [ report (list 0 0 0) ]
end

to-report center-urge ;; bird reporter
  ;; report the average distance from my flockmates
  ;; in each direction
  if count flockmates = 0 or center-constant = 0
  [ report (list 0 0 0) ]
  report
    (map
      [ [a b] -> b - a ]
      (list xcor ycor zcor)
      (list
        mean [ xcor ] of flockmates
        mean [ ycor ] of flockmates
        mean [ zcor ] of flockmates ) )
end

to-report velocity-urge ;; bird reporter
  ;; report the average difference in velocity
  ;; from my flock mates
  if count flockmates = 0 or velocity-constant = 0
  [ report (list 0 0 0) ]
  report
    ( map
      [ [a b] -> a - b ]
      (list
        mean [ item 0 velocity ] of flockmates
        mean [ item 1 velocity ] of flockmates
        mean [ item 2 velocity ] of flockmates )
      velocity )

end

to-report wander-urge ;; turtle reporter
  ;; report 3 random numbers between -1 and 1
  report n-values 3 [ (random-float 2) - 1 ]
end

to-report spacing-urge ;; turtle reporter
  let urge [ 0 0 0 ]
  ;; report the sum of the distances to birds
  ;; in my flock that are closer to me than
  ;; cruise-distance
  ask flockmates with [ distance myself < cruise-distance ] [
    set urge
      add
        urge
        (subtract
          (list [xcor] of myself [ycor] of myself [zcor] of myself)
          (list xcor ycor zcor))
  ]
  ;; when vertical spacing is not on ignore the
  ;; z component of the spacing urge.
  if not vertical-spacing? [ set urge lput 0 but-last urge ]
  report urge
end

to-report avoid-obstacle-urge ;; turtle reporter
  let urge (list 0 0 0)
  if avoidance-constant = 0 [ report urge ]
  ;; report the sum of the distances from
  ;; any patches that are obstacles
  ;; in each direction
  ask patches in-radius vision with [ pcolor != black ]
  [ set urge
      add
        urge
        subtract
          (list [xcor] of myself [ycor] of myself [zcor] of myself)
          (list pxcor pycor pzcor)
  ]
  report urge
end

to find-flockmates  ;; turtle reporter
  ;; latch on to the nearby birds
  set flockmates other birds in-radius vision
end

to fall ;; dead-bird procedure
  if zcor > min-pzcor + 1
    [ set zcor zcor - 0.5 ]
end

;;
;; world building
;;
to build-cubes
  ask n-of 20 patches
  [ set pcolor blue + random 3
    ask neighbors [ set pcolor blue + random 3 ] ]
end

to build-wall
  let x max-pxcor - random world-width
  let ydir 1 - random 2
  if ydir = 0 [ set ydir -1 ]
  let height random max-pxcor
  ask patches
    with [ pxcor = x and ydir * pycor < 0 and pzcor < height ]
    [ set pcolor gray + random 2 ]
end

;;
;; vector operations
;;
to-report add [ v1 v2 ]
  report (map [ [a b] -> a + b ] v1 v2)
end

to-report subtract [ v1 v2 ]
  report (map [ [a b] -> a - b ] v1 v2)
end

to-report scale [ scalar vector ]
  report map [ n -> scalar * n ] vector
end

to-report magnitude [ vector ]
  report sqrt sum map [ n -> n * n ] vector
end

to-report normalize [ vector ]
  let m magnitude vector
  if m = 0 [ report vector ]
  report map [ n -> n / m ] vector
end


; Copyright 2005 Uri Wilensky.
; See Info tab for full copyright and license.
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@#$#@#$#@
## WHAT IS IT?

This is a vector-based 3D flocking model, based on Jon Klein's implementation of Craig Reynolds' Boids algorithm. Each bird is influenced by a series of urges. By assigning different weights to each urge, the birds exhibit different flocking behaviors.

## HOW IT WORKS

Each bird has vectors for velocity and acceleration, that is, a velocity and acceleration in the x, y, and z directions.  Each of these vectors is influenced by six urges, to be near the center of the flock, to have the same velocity as the rest of the flock, to be keep spacing correct, to avoid colliding with obstacles, to be near to the center of the world and to wander throughout the world.  Each of these factors affects the resulting velocity and acceleration; however, the sliders in the interface weight the amount that each has an effect.

## HOW TO USE IT

Choose the number of birds with the POPULATION slider. Press SETUP to create the birds. Press GO to start the simulation.

BUILD-CUBES randomly places floating-ball obstacles throughout the world. BUILD-WALL creates a randomly placed wall.

VISION is the distance that a bird can see around it. MAX-VELOCITY is the fastest a bird can go and MAX-ACCELERATION is the fastest a bird can accelerate. CRUISE-DISTANCE is the minimum distance from a nearby bird for a bird to feel comfortable.

A bird's movement is determined by the set of urges acting on that bird:

CENTER-CONSTANT - urge to move towards the center of its flock
VELOCITY-CONSTANT - urge to align its velocity with the velocity of her flockmates
SPACING-CONSTANT - urge to be no closer than CRUISE-DISTANCE from other birds
AVOIDANCE-CONSTANT - urge to avoid colliding with obstacles
WORLD-CENTER-CONSTANT - urge to avoid the edges of the world
WANDER-CONSTANT - urge to move in a random way

The urge-constant sliders control the weight of each urge in determining the birds' behavior.

If the VERTICAL-SPACING? switch is off the spacing urge only operates horizontally.

If a bird hits an obstacle, the bird dies. The COUNT DEAD-BIRDS monitor shows the number of dead birds.

## THINGS TO NOTICE

Compare how long it takes the birds to form flocks in this model to how long it takes in the regular 3D Flocking model.

Given the default settings the flocks travel in a circular pattern.

## THINGS TO TRY

Turn VERTICAL-SPACING? off and notice how it changes the shape of the flocks.

Try changing the different constants so that the birds become better or worse at avoiding obstacles.

## EXTENDING THE MODEL

Try adding different urges to the model; also try making different types of obstacles for birds to avoid.

Create different breeds of birds that behave differently from each other and see how they interact.

## NETLOGO FEATURES

This model uses lists and `map` fairly extensively to represent and manipulate vectors.

## CREDITS AND REFERENCES

This model is based on Jon Klein's "Swarm" demo for breve, (see http://www.spiderland.org/s/) which was inspired by Craig Reynolds classic Boids flocking algorithm.

## HOW TO CITE

If you mention this model or the NetLogo software in a publication, we ask that you include the citations below.

For the model itself:

* Wilensky, U. (2005).  NetLogo Flocking 3D Alternate model.  http://ccl.northwestern.edu/netlogo/models/Flocking3DAlternate.  Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

Please cite the NetLogo software as:

* Wilensky, U. (1999). NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

## COPYRIGHT AND LICENSE

Copyright 2005 Uri Wilensky.

![CC BY-NC-SA 3.0](http://ccl.northwestern.edu/images/creativecommons/byncsa.png)

This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License.  To view a copy of this license, visit https://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.

Commercial licenses are also available. To inquire about commercial licenses, please contact Uri Wilensky at uri@northwestern.edu.

<!-- 2005 -->
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