network supply chain

globals [
  demand-today           ;; the demand occurs at all retailers everyday
  colors                 ;; colors for plot
  ]

breed [players player]

;; There are 2 types of directed links in the model
;; demand links for order placement, and supply links for shipment pipeline

directed-link-breed [supply-links supply-link]
supply-links-own [orders-filled pair-demand-link]

directed-link-breed [demand-links demand-link]
demand-links-own [orders-placed back-orders]

players-own [
  role             ;; 3 roles in the model: supplier, distributor, and retailer
  base-stock       ;; we assume players are operating under base-stock policy   
  on-hand
  backlog
  inventory-position   ;; inventory equals to on-hand inventory + pipeline inventory - back-orders
  last-received
  pen-color
  cost
  id-number         ;; id-number is used to distinguish players with the same role
  demand-history    ;; a list consisting of the demands in a few past days
  revenue           ;; revenue from selling the products
  safety-factor     ;; parameter determining to what extent the player want to keep safety inventory against demand uncertainty
  current-supplier  
]

to setup
  ca
  set colors [5 15 25 35 45 55 65 75 85 95 105 115 125 135 67 117 28 18 37]  ;; a list of 20 colors
  set colors shuffle colors   
  layout
  initialize
  resize-shape
  reset-plots
  reset-ticks
end 

to go
  if ticks >= days-of-simulation [stop]
  set demand-today daily-demand          ;; generate today's demand
  place-order-to-up
  receive-order-from-up
  process-order-from-down
  summarize
  update-policy
  resize-shape
  tick
end 

to update-policy             ;; the players can update their inventory policy according to their demand record
  ask players with [role = "distributor"][
    set base-stock cal-base-stock-level demand-history lead-time-supplier-distributor
  ]
  ask players with [role = "retailer"][
    set base-stock cal-base-stock-level demand-history lead-time-distributor-retailer
  ]
end 

;; Inventory operations
;; **************************

to place-order-to-up
  ask players with [role = "retailer" or role = "distributor"][
                                                                       ;; in this model, we assume all players are operating under base-stock policy
    let amount-to-order max list (base-stock - inventory-position) 0   ;; that is once the inventory position is below the base stock level, 
                                                                       ;; we order the quantity equal to the difference between inventory position and base stock level
                                                                       ;; otherwise, don't place any order

    ask my-out-demand-links [set orders-placed 0]                      ;; since in a network supply chain, a player can order from multiple upstream nodes
    ask who-to-order [set orders-placed amount-to-order]               ;; choose one node each time
  ]
end 

to receive-order-from-up 
  ask players [
    if role = "distributor" or role = "retailer" [                       
      set last-received sum [first orders-filled] of my-in-supply-links     ;; take out the first item in the supply-link pipeline
      ask my-in-supply-links [set orders-filled but-first orders-filled]    ;; remove it from the pipeline
      set on-hand on-hand + last-received                                   ;; add it to the current on-hand inventory
      
    ]
    if role = "supplier" [set on-hand 10000]                                ;; we assume the supplier has unlimited supply
  ]
end 

to process-order-from-down
  ask players [
    let new-orders 0                                        ;; for distributors and suppliers, new orders equal to the sum of the orders-placed of all in-demand-links
    if role = "distributor" or role = "supplier" [set new-orders sum [orders-placed] of my-in-demand-links]   
    if role = "retailer" [set new-orders demand-today]      ;; for retailers, new orders simply equal to today's demand
 
    set demand-history lput new-orders demand-history       ;; record the new-order in the demand history    
    set demand-history but-first demand-history             ;; delete the earliest demand history on the record, in order to keep the record length the same
    
    let orders-requested new-orders + backlog               ;; besides new orders, back-orders also need to be satisfied
    let orders-to-ship min list orders-requested on-hand    ;; if there is sufficient inventory, ship the requested amount
                                                            ;; if not sufficient inventory, ship whatever on-hand
    
    if role = "distributor" [set revenue revenue + 2 * orders-to-ship]    ;; revenue for distributors is 2 for each unit shipped
    if role = "retailer" [set revenue revenue + 3 * orders-to-ship]       ;; revenue for is 3 for each unit shipped
    
    set backlog max list 0 (backlog - on-hand + new-orders)    ;; the unsatisfied demand is counted as backlog (or back-orders)
    
    let rest-amount orders-to-ship       ;; allocate total shipping amount to each downstream node
    
    foreach sort my-out-supply-links [
      ask ? [                                    ;; quota to each supply link is proportional the sum of back-orders and new orders of the pair demand link
        let quota sum [back-orders] of pair-demand-link + sum [orders-placed] of pair-demand-link   
        let ship-to-this-link 0                  ;; if no order, ship nothing, and put 0 in the supply link
        if orders-requested > 0 [                ;; if positive order, ship according to the quota
          set ship-to-this-link min list ceiling (quota * orders-to-ship / orders-requested) rest-amount
        ]                                        ;; note that we use ceiling to guarantee the integrity of the shipping quantity
        set rest-amount rest-amount - ship-to-this-link  
        set orders-filled lput ship-to-this-link orders-filled    ;; put the ship quantity at the last place of the supply pipeline
        ask pair-demand-link [set back-orders max list 0 (quota - ship-to-this-link)]  ;; update the back-orders in the pair demand link
      ]
    ]
    
    set on-hand on-hand - orders-to-ship    ;; reduce the shipped quantity from the on-hand inventory
  ]  
end 

to summarize
  ask players [
    let pipeline sum [sum orders-filled] of my-in-supply-links    ;; calculate the pipeline inventory (inventory in-transit) for each player
    set inventory-position on-hand + pipeline - backlog           ;; recalculate the inventory position
    let cost-add (0.5 * on-hand + 2 * backlog)                    ;; calculate inventory holding cost and backlog penalty
    set cost cost + cost-add                                      ;; update the cost
  ]
  
  ask players with [role = "distributor" or role = "retailer"] [  ;; plots
    plot-on-hand-inventory
    plot-profit
  ]
end 

;; ****************************

to-report y-locations [number]         ;; report a list of y-coordinates that would evenly distribute the "number" players on y-axis
  let y-list []

  let interval round ((2 * (max-pycor - 3) + 1) / (number + 1))
  
  let current-pos (- max-pycor + 3 + interval)
  
  repeat number [
    set y-list lput current-pos y-list
    set current-pos current-pos + interval
  ]
  report y-list
end 

to layout
  set-default-shape players "circle"
  set-default-shape links "arc"

  create-players 1 [
    setxy -10 0
    set color red
    set role "supplier"
    set size 5                ;; the size of the supplier won't be updated in the simulation
  ] 
  
  ask patches with [pxcor = 0 and (member? pycor y-locations num-of-distributors)] [  ;; evenly layout the distributors in the middle
    sprout-players 1 [
      set color blue
      set role "distributor"      
    ]
  ]
  
  ask patches with [pxcor = 10 and (member? pycor y-locations num-of-retailers)] [ ;; evenly layout the retailers on the right
    sprout-players 1 [
      set color green
      set role "retailer"
    ]
  ]
  
  
  let d-number 1
  foreach sort players with [role = "distributor"][   ;; assign each distributor an id-number, and label them with the id-number
    ask ?[
      set label word "D-" d-number
      set id-number d-number
      set d-number d-number + 1
    ]
  ]
  
  let r-number 1
  foreach sort players with [role = "retailer"][    ;; assign each retailer an id-number, and label them with the id-number
    ask ?[
      set label word "R-" r-number
      set id-number r-number
      set r-number r-number + 1
    ]
  ]
  
  let index 0
  foreach sort players with [role = "distributor" or role = "retailer"][
    ask ?[
      set pen-color item index colors    ;; assign each distributor and retailer a different plot pen color
      set index index + 1
    ]
  ]
  
  
  
  ask players with [role = "retailer"] [
    create-demand-links-to players with [role = "distributor"]     ;; create demand and supply links between distributors and retailers
    create-supply-links-from players with [role = "distributor"]   
  ]
  ask players with [role = "distributor"] [
    create-demand-links-to players with [role = "supplier"]        ;; create demand and supply links between supplier and distributors
    create-supply-links-from players with [role = "supplier"] 
  ]
  
  ask supply-links [                                  ;; define the pair demand link of each supply link, the link with the same ends
    set pair-demand-link demand-links with [end1 = [end2] of myself and end2 = [end1] of myself]
  ]
end 

to initialize
  
  ask players [
    if role = "distributor" [
      set base-stock initial-stock-distributor   ;; set the initial base-stock level and on-hand inventory to "initial-stock-distributor"
      set on-hand initial-stock-distributor
      ask my-out-demand-links [
        set orders-placed 0 
        set back-orders 0
        ]
      
      ;; create a list with all elements equal to 0, with the length equals to "lead-time-supplier-distributor"
      ;; representing the supply pipeline between the supplier and the distributor
      
      ask my-in-supply-links [set orders-filled n-values lead-time-supplier-distributor [0]]  
    ]
    if role = "retailer" [
      set base-stock initial-stock-retailer
      set on-hand initial-stock-retailer
      
      ask my-out-demand-links [
        set orders-placed 0
        set back-orders random 0
        ]
      
      ;; create a list with all elements equal to 0, with the length equals to "lead-time-distributor-retailer"
      ;; representing the supply pipeline between the distributor and the retailer
      
      ask my-in-supply-links [set orders-filled n-values lead-time-distributor-retailer [0]]
    ]
    if role = "supplier"[
      set on-hand 10000
    ]
    set backlog 0
    set inventory-position on-hand - backlog
  ]
  
  ask players [    ;; randomly assign the safety-factor to players
    set cost 0     ;; the higher safety factor  means that the player is willing to keep higher safety inventory against the demand uncertainty
    set revenue 0                                    
    set safety-factor 1.5 + random-float 1
    set demand-history n-values record-length [""]  ;; set the demand history as a list with empty elements, with the length equals to "record-length"
    ]
end 

to resize-shape    ;; visualize the on-hand stock via size of the turtle
  ask players with [role = "distributor" or role = "retailer"][
    set size 0.5 * (sqrt on-hand)          
  ]
end 

to reset-plots          ;; clear all plots and create plot pens for each player
  clear-all-plots
  ask players with [role = "distributor" or role = "retailer"][
    create-plot-pens
  ]
end 

to create-plot-pen [my-plot]
  set-current-plot my-plot
  create-temporary-plot-pen label  ;; name the plot pen with the player's label
  set-plot-pen-color pen-color
end 

to create-plot-pens
  create-plot-pen "total profit"       ;; one plot for total profit
  create-plot-pen "on-hand inventory"  ;; one plot for on-hand inventory level
end 

to plot-profit
  set-current-plot "total profit"
  set-current-plot-pen label
  plot revenue - cost
end 

to plot-on-hand-inventory
  set-current-plot "on-hand inventory"
  set-current-plot-pen label
  plot on-hand
end 



;; Demand distribution generation
;; **************************************

to-report daily-demand                ;; we design three mechanisms for generating the daily demand, which can be chosen in the chooser
  if distribution = "deterministic"
    [report deterministic-demand]     ;; deterministic demand means the demand is constant, there is no uncertainty      
  
  if distribution = "poisson"         ;; poisson demand means that the daily demand follows Poisson distribution
    [report random-poisson mean-for-poisson]             
  
  if distribution = "normal"          ;; normal demand means that the daily demand follows truncated normal distribution (modified in this model)
    [report truncated-normal mean-for-normal std-for-normal lower-bound-for-normal upper-bound-for-normal]   
end 

to-report truncated-normal [mean-value std-value min-value max-value]    ;; there are 4 parameters for the truncated normal distribution
  let random-num random-normal mean-value std-value                      ;; we first generate a random-normal number according to the mean value and standard-deviation value
  ifelse random-num > max-value or random-num < min-value                
  [report round (min-value + random-float (max-value - min-value))]      ;; if the value is beyond the min-value and max-value, report a random number within the range
  [report round random-num]                                              ;; if the value is within the min-value and max-value, report the rounding of this number
end 
;; ***************************************************************

to-report cal-base-stock-level [demand-list delay]     ;; calculate base-stock based on demand-history
  let numbers filter is-number? demand-list            ;; during the first few days, not all the elements in the demand history are numbers, but ""
  let mean-value mean numbers                          ;; calculate mean value
  
  let std 0
  if length numbers >= 2[                              ;; calculate the standard deviation of the demand history
    set std standard-deviation numbers
  ]
                                                       ;; according to inventory theories, the base-stock level is usually calculate according to mean and std and supply delays                                  
  report round (mean-value * (delay + 1) + safety-factor * std * (sqrt (delay + 1)))  ;; "+1" because of the order processing delay
end 

to-report who-to-order  ;; report the demand link that has the lowest back-orders, so that the player will choose to order from this one
  let min-back-order min [back-orders] of my-out-demand-links  ;; find out the minimum back-order
  let sorted-links []                         ;; prepare an empty list
  foreach sort my-out-demand-links [          ;; if the back-orders of the demand links equals to the minimum back-order
    ask ? [                                   ;; add the demand link to sorted-links list
      if back-orders = min-back-order [set sorted-links lput self sorted-links]
    ]
  ]
  ifelse member? current-supplier sorted-links [   ;; if the current-supplier (actually the corresponding demand link) is among the demand links with the minimum back-order 
    report current-supplier                        ;; choose the current supplier due to customer loyalty
  ][
  let chosen-one one-of sorted-links               ;; if not, choose one from the demand links with the minimum back-order
  set current-supplier chosen-one                  ;; then transfer the supplier to this one
  report chosen-one                               
  ]
end