tax

turtles-own [ wealth ]                                        ; A economic inequality ABM
globals [ exc tax totaltax gini counter sumexc ]              ; © Al Pugh, 2016

to setup
  ca    set gini 0    set counter 0    set totaltax 0
  ask patches [ set pcolor green ]                            ; set green background for contrast
  ask n-of 400 patches [                                      ; ask some patches to create agents
    sprout 1 [set shape "person" set wealth (90 + random 21)
      set heading random 360 ] ]
  ask turtles [ set color yellow ]                            ; initialize agent color
  update-gini         reset-ticks
end 

to go
  set sumexc 0
  ask turtles [
    set heading heading + random 10 fd 1                      ; move agent randomly
    wealthtax                                                 ; tax wealthy
    encounter ]                                               ; resolve agent encounters
  redistribute                                                ; redistribute taxes
  update-gini                                                 ; update Gini coefficient
  ; checkwealth                                               ; (for debugging only)
  tick
end                                                            ; (plot set up commands in plots)

to wealthtax
  if ( wealth > 1000 ) [                                      ; tax those with 10x mean starting wealth
    set tax ( wealthtaxrate / 100. * wealth )
    set wealth wealth - tax
    set totaltax totaltax + tax ]
end 

to encounter
  if ( count turtles-here = 2 ) [                              ; for every interaction of 2 agents:
    let n2 one-of other turtles-here
    set exc ( min [ wealth ] of turtles-here / 5.0 )           ; determine exchange amount
    set sumexc sumexc + exc                                    ; add to sum of exchanges
    set tax ( exc * salestaxrate / 100. )                      ; extract sales tax
    set totaltax totaltax + 2 * tax
    ifelse ( random 2 ) = 0                                    ; determine winner
      [ set wealth ( wealth - exc - tax )
        ask n2 [set wealth ( [wealth] of n2 + exc - tax ) ] ]
      [ set wealth ( wealth + exc - tax )
        ask n2 [set wealth ( [wealth] of n2 - exc - tax ) ] ]
    set color scale-color yellow                               ; color agents by wealth
      wealth 0 int max [ wealth ] of turtles ]
end 

to redistribute
  set counter counter + 1
  if ( totaltax > 0 ) [                                        ; distribute tax receipts
    ifelse ( poordist? )
      [ let share ( totaltax / count turtles ) / 2.0
        ask turtles [ set wealth  wealth + share ]             ; all agents get reduced share
        let npoor count turtles with [ wealth < 100.0 ]        ; define poor as < mean starting wealth
        let poorshare share * count turtles / npoor            ; poor get additional share
        ask turtles with [ wealth < 100.0 ] [
          set wealth wealth + poorshare ]
        set totaltax 0  ]
      [ let share ( totaltax / count turtles )                 ; agents share tax redistribution equally
        ask turtles [ set wealth wealth + share ]
        set totaltax 0 ]  ]
end 

to update-gini                                                  ; find Gini coefficient
  let sorted-wealths sort [wealth] of turtles
  let total-wealth sum sorted-wealths     let wealth-sum-so-far 0
  let index 0          let gini-index-reserve 0
  repeat count turtles [
    set wealth-sum-so-far (wealth-sum-so-far + item index sorted-wealths)
    set index (index + 1)
    set gini-index-reserve gini-index-reserve + (index / count turtles)
      - (wealth-sum-so-far / total-wealth) ]
  set gini (gini-index-reserve / count turtles) / 0.5
end 

to checkwealth                                                  ; check total wealth remains unchanged
  print sum [ wealth ] of turtles + totaltax
end