Credit Crunch and Bank Failure

breed [banks bank]
breed [consumers consumer]

undirected-link-breed [loanlinks loanlink]

directed-link-breed [interbankloans interbankloan]

globals [totalloss]

banks-own [
  bank-reserves ;;the amount of money a bank has
  initial-reserves ;;initial reserves a bank has - used to calculate total losses of money from bank failure
  bank-prestige ;;this is a simplified property that denotes multiple characteristics of a bank. First of all, it denotes 
                ;;a bank's standards for borrowers, basically whether or not a bank is willing to take on a high risk customer. The higher the value, the less likely.
                ;;Secondly, it is a measure of the bank's desirability to borrowers. They will seek a bank that is known to be more stable. 
                ;;Finally, it denotes the level of connectivity the bank has with other banks, both in terms of loans taken and loans granted. This is because
                ;;prestigious banks tend to be well-connected with others. 
]

consumers-own [
  reliability ;;a consumer's ability to pay back loans
  loan ;;the amount a consumer has taken out in loans from a bank
  consumer-standard ;;the higher this value, the more prestigious a bank the borrower will attempt to acquire a loan from. This value is presumed to be independent of one's ability to pay
                    ;;back a loan, thus it is not correlated with reliability in this model. 
]

to setup
  ca
  setup-banks
  setup-consumers
  network-banks
end 

to setup-banks
  create-banks bank-count [set bank-reserves random reserve-amount + 10
    set shape "circle" 
    set color red 
    set bank-prestige random 100 
    move-to one-of patches
    set initial-reserves bank-reserves] ;;Although the banks are randomly placed spatially, this is only for the sake of appearance. It does not affect the model's performance as everything is done
                            ;;with a network topology.
end 

to setup-consumers
  create-consumers 100 + bank-count [set reliability random 100 - neg-reliability-mod ;;this is here to simulate the notion that the pool of lenders as a whole has gotten more unreliable, which is the case according to some economists
    set consumer-standard random 100
    set shape "sheep" 
    set color blue
    move-to one-of patches] ;;Same comment with banks applies here. For the sake of simplicity, spatial distance is not a consideration for consumers when choosing a bank.
                            ;;Given that the banks heavily implicated in the financial crisis were large and ubiquitous, this is not an unreasonable assumption.
end 

to network-banks
ask banks [create-interbankloan-to one-of other banks]
repeat bank-connectivity [ask banks [if random 100 < bank-prestige [create-interbankloan-to one-of other banks]]] 
;;Every bank has at least one loan with another bank. An overall measure of connectivity is then used to determine how many additional loans exist between banks. 
end 

to go
if not any? banks [stop]
if not any? consumers [stop]
ask banks [if bank-reserves <= 0 [bank-failure]] ;;banks fail when they're out of reserves
ask consumers [seek-loan]
ask consumers [check-loan]
tick
make-plot
make-plot2
if layout? [layout]
end 

to seek-loan
  if count my-loanlinks = 0 [network-with-bank]
end 

to network-with-bank ;;this is the command that will make a consumer try to create a loan with a bank, depending on their preferences
  if count banks > 0 [
  if loan = 0 [ask my-links [die]
  ifelse any? banks with [bank-prestige >= ([consumer-standard] of myself)] 
  [ask one-of banks with [bank-prestige >= ([consumer-standard] of myself) ] [if random 100 + ([reliability] of myself) > (bank-prestige - bank-risk-taking) [create-loanlink-with myself]]
  set loan loan-amount]
  [set consumer-standard consumer-standard - 2]]] ;;standards lower over time as consumers realize they will not be able to find their ideal bank
end 

to check-loan
  if count my-links > 0 [if random 100 > 75 [ifelse random 100 > reliability + 30 [if count my-links > 0 [ask one-of loanlink-neighbors [loseloan]set loan 0 die]]
    [ask loanlink-neighbors [regainloan] ask my-links [die] if random 100 > 50 [die]]]] ;;the random chance of dying is to gradually remove repeat-borrowers from the pool. 
                                                                                        ;;it isn't realistic that someone would take out repeated loans
end 

to regainloan
  set bank-reserves bank-reserves + loan-amount + 1 ;;extra 1 here is to represent repaid interest on a loan
end 

to loseloan
set bank-reserves bank-reserves - loan-amount
end    

to bank-failure
ask out-link-neighbors [set bank-reserves bank-reserves - interbank-loan-amount set totalloss totalloss + initial-reserves] ;;As a bank fails, all banks it was taking loans from suffer as the loan                                                                                                                ;;is defaulted upon
  die
end 

to make-plot
  set-current-plot "bank count"
  set-current-plot-pen "default"
  plot count banks
end 

to make-plot2
  set-current-plot "total losses"
  set-current-plot-pen "default2"
  plot totalloss
end 

;;Following code is cannibalized from Preferential Attachment for the sake of making the network here look more aesthetically pleasing.   
;;

to layout
  ;; the number 3 here is arbitrary; more repetitions slows down the
  ;; model, but too few gives poor layouts
  repeat 3 [
    ;; the more turtles we have to fit into the same amount of space,
    ;; the smaller the inputs to layout-spring we'll need to use
    let factor sqrt count turtles
    ;; numbers here are arbitrarily chosen for pleasing appearance
    layout-spring turtles links (1 / factor) (7 / factor) (1 / factor)
    display  ;; for smooth animation
  ]
  ;; don't bump the edges of the world
  let x-offset max [xcor] of turtles + min [xcor] of turtles
  let y-offset max [ycor] of turtles + min [ycor] of turtles
  ;; big jumps look funny, so only adjust a little each time
  set x-offset limit-magnitude x-offset 0.1
  set y-offset limit-magnitude y-offset 0.1
  ask turtles [ setxy (xcor - x-offset / 2) (ycor - y-offset / 2) ]
end 

to-report limit-magnitude [number limit]
  if number > limit [ report limit ]
  if number < (- limit) [ report (- limit) ]
  report number
end