Erosion Benchmark

globals [
  result
  show-water?     ;; whether the water is visible
  drains          ;; agentset of all edge patches where the water drains off
  land            ;; agentset of all non-edge patches
]

patches-own [
  elevation       ;; elevation here (may be negative)
  water           ;; depth of water here
  drain?          ;; is this an edge patch?
]

to benchmark
  random-seed 19192
  reset-timer
  setup
  repeat 250 [ go ]
  set result timer
end 

to setup
  clear-all
  set show-water? true
  ask patches [
    ifelse bumpy?
       [ ifelse hill?
           [ set elevation -100 * (distancexy 0 0 / max-pxcor) + 100 + random 100 ]
           [ set elevation random 125 ] ]
       [ set elevation 100 ]
    set water 0
    set drain? false
  ]
  ;; the DIFFUSE command is useful for smoothing out the terrain
  if bumpy? [
    repeat terrain-smoothness [ diffuse elevation 0.5 ]
  ]
  ;; make the drain around the edge
  ask patches with [count neighbors != 8]
    [ set drain? true
      set elevation -10000000 ]
  set drains patches with [drain?]
  set land patches with [not drain?]
  ;; display the terrain
  ask land [ recolor ]
end 

to recolor  ;; patch procedure
  ifelse water = 0 or not show-water?
    [ set pcolor scale-color white elevation -250 100 ]
    [ set pcolor scale-color blue (min list water 75) 100 -10 ]
end 

to show-water
  set show-water? true
  ask land [ recolor ]
end 

to hide-water
  set show-water? false
  ask land [ recolor ]
end 

to go
  ;; first do rainfall
  ask land [
    if random-float 1.0 < rainfall [
      set water water + 1
    ]
  ]
  ;; then do flow;  we don't want to bias the flow in any
  ;; particular direction, so we need to shuffle the execution
  ;; order of the patches each time; using an agentset does
  ;; this automatically.
  ask land [ if water > 0 [ flow ] ]

  ;; reset the drains to their initial state
  ask drains [
    set water 0
    set elevation -10000000
  ]
  ;; update the patch colors
  ask land [ recolor ]
  ;; advance to next tick
  tick
end 

to flow  ;; patch procedure
  ;; find the neighboring patch where the water is lowest
  let target min-one-of neighbors [elevation + water]
  ;; the amount of flow is half the level difference, unless
  ;; that much water isn't available
  let amount min list water (0.5 * (elevation + water - [elevation] of target - [water] of target))
  ;; don't flow unless the water is higher here
  if amount > 0 [
    ;; first erode
    let erosion amount * (1 - soil-hardness)
    set elevation elevation - erosion
    ;; but now the erosion has changed the amount of flow needed to equalize the level,
    ;; so we have to recalculate the flow amount
    set amount min list water (0.5 * (elevation + water - [elevation] of target - [water] of target))
    set water water - amount
    ask target [ set water water + amount ]
  ]
end