Grazing

patches-own [
  nutrient ;; the total of the manure and chicken-manure
  manure   ;; the amount of manure added to patches by cows and sheep
  chicken-manure ;; the amount of manure added to patches by chickens
  energy   ;; the health of the pasture (as measured by the amount of grass)
]
breed [sheep a-sheep]
breed [chickens chicken]
breed [cows cow]

to setup
  clear-all
  setup-patches
  setup-turtles
  reset-ticks
end 

to setup-patches
  ask patches [
    set pcolor 65
    set energy 9
    set manure 0
    set chicken-manure 0
  ]
end 

to setup-turtles
  set-default-shape cows "cow"
  set-default-shape sheep "sheep"
  set-default-shape chickens "chicken"
  create-cows cow-number [
    set color black
    set size 2
    setxy random-xcor random-ycor
  ]
  create-sheep sheep-number [
    set color white
    setxy random-xcor random-ycor
  ]
  create-chickens chicken-number [
    set color red
    setxy random-xcor random-ycor
  ]
end 

to go
  if all? patches [pcolor = 69.9] [stop]  ;; when all the grass is down to lowest level of richness
  if mean [ energy ] of patches <= 0 [stop] ;; when the grass is on average at or below 0 energy, stop
  ask cows [
    eat-grass-cows
    move-turtles
  ]
  ask sheep [
    eat-grass-sheep
    move-turtles
  ]
  ask chickens [
    eat-bugs
    move-chickens
  ]
  update-patches
  tick
end 

to move-turtles
  uphill-energy  ;; turn towards richest grass in front three patches
  forward 1
  if not can-move? 1 [ rt random 150 ]  ;; if stuck against one edge turn around
end 

to move-chickens ;; move towards most manure in front three patches
  uphill-nutrient
  forward 1
  if not can-move? 1 [ rt random 150 ]  ;; if stuck against one edge
end 

to eat-grass-cows
  ifelse energy >= 5 [          ;; if there's enough energy/grass for a full meal
  set energy energy - 5         ;; eat grass
  set manure manure + 6         ;; deposit manure
  ]
  [                                     ;; if there isn't enough energy/grass for a full meal
    set manure manure + energy * 1.2    ;; deposit manure in proportion to how much energy/grass there is
    set energy 0                        ;; use up all the energy
  ]
end 

to eat-grass-sheep
  ifelse (energy >= 1) [         ;; if there's enough energy/grass for a full meal
  set energy energy - 1          ;; eat grass
  set manure manure + 1.2        ;; deposit manure
  ]
  [                                     ;; if there isn't enough energy/grass for a full meal
    set manure manure + energy * 1.2    ;; deposit manure in proportion to how much energy/grass there is
    set energy 0                        ;; use up all the energy
  ]
end 

to eat-bugs
    if manure >= .05 and energy > 0 [         ;; scratch through manure, adding to decomposition
    set manure manure - .05                   ;; I have no idea what this number should be, if even > 0
    if (type-of-change = "linear") [set energy energy + .05]
    if (type-of-change = "nonlinear") [set energy energy + .16 / sqrt energy]
  ]
  if energy >= .2 [                            ;; if there's enough energy/grass for a full meal
    set energy energy - .2                     ;; eat grass
    set chicken-manure chicken-manure + .25    ;; deposit manure
  ]
  if energy < .2 [                             ;; if there isn't enough energy/grass for a full meal
    set chicken-manure chicken-manure + energy * 1.2  ;; deposit manure in proportion to how much energy/grass there is
    set energy 0                                      ;; use up all the energy
  ]
end 

to update-patches
  ask patches [
    if  manure >= .25 and energy > 0 [    ;; manure decays on its own, enriching grass, but only if there's still grass left
      set manure manure - .25
      if (type-of-change = "linear") [set energy energy + .25]
      if (type-of-change = "nonlinear") [set energy energy + .8 / sqrt energy]
  ]
    if manure < .25 and energy > 0 [      ;; if there's very little manure
      set energy energy + manure         ;; increase energy by amount of manure (use manure up)
      set manure 0
    ]
    if chicken-manure >= .25 and energy > 0 [
      set chicken-manure chicken-manure - .25
      if (type-of-change = "linear") [set energy energy + .25]
      if (type-of-change = "nonlinear")[set energy energy + .8 / sqrt energy]
  ]
    if chicken-manure < .25 and energy > 0 [   ;; if there's very little chicken-manure
      set energy energy + chicken-manure      ;; increase energy by amount of manure (use manure up)
      set chicken-manure 0
    ]
  set nutrient manure + chicken-manure
  recolor-patch
  ]
end 

;; set colour of patch paler or darker according to richness, but not to black or white, so we can still see the livestock

to recolor-patch
  if energy < 1 [
    set pcolor 69.9
  ]
  if energy > 1 [
    if energy <= 3 [
    set pcolor 69
  ]]
  if energy > 3 [
    if energy <= 5 [
    set pcolor 68
  ]]
  if energy > 5 [
    if energy <= 7 [
    set pcolor 67
  ]]
  if energy > 7 [
   if energy <= 9 [
    set pcolor 66
  ]]
  if energy > 9 [
    if energy <= 11 [
    set pcolor 65
  ]]
  if energy > 11 [
    if energy <= 13 [
    set pcolor 64
  ]]
  if energy > 13 [
    if energy <= 15 [
    set pcolor 63
  ]]
  if energy > 15 [
    set pcolor 62
  ]
end 

;; move towards patch ahead with most manure

to uphill-nutrient
  let manure-ahead nutrient-scent-at-angle   0
  let manure-right nutrient-scent-at-angle  45
  let manure-left  nutrient-scent-at-angle -45
  ;; if all patches ahead are equally rich, wiggle
  if (manure-right = manure-ahead) and (manure-left = manure-ahead) [ wiggle ]
  ;; if patches ahead vary in richness, go to richest
  if (manure-right > manure-ahead) or (manure-left > manure-ahead)
  [ ifelse manure-right > manure-left
    [ rt 45 ]
    [ lt 45 ] ]
end 

;; move randomly ahead

to wiggle
  right random 90
  left random 90
end 

;; move towards richest patch of grass ahead

to uphill-energy
  let energy-ahead energy-scent-at-angle   0
  let energy-right energy-scent-at-angle  45
  let energy-left  energy-scent-at-angle -45
  ;; if all patches ahead are equally manured, wiggle
  if (energy-right = energy-ahead) and (energy-left = energy-ahead) [ wiggle ]
  ;; if patches ahead vary in richness of manure, go to richest
  if (energy-right > energy-ahead) or (energy-left > energy-ahead)
  [ ifelse energy-right > energy-left
    [ rt 45 ]
    [ lt 45 ] ]
end 

;; check out which patch ahead has the most manure (cribbed from ants)

to-report nutrient-scent-at-angle [angle]
  let p patch-right-and-ahead angle 1
  if p = nobody [ report 0 ]
  report [manure] of p
end 

;; check out which patch ahead has the richest grass (cribbed from ants)

to-report energy-scent-at-angle [angle]
  let p patch-right-and-ahead angle 1
  if p = nobody [ report -5 ]
  report [energy] of p
end