; DisDB (Distributed Dynamic BAcktracking)  for the n-queens problem
; by Ionel Muscalagiu - mionel@fih.utt.ro 
;
breeds [queens]

globals [no-more-messages  tmp nr_cicluri gata]

;rows, and cols go from [0..num-queens-1]
;
;message-queue contains the incoming messages. We take new ones out from the head.
;col is the position of the queen on it's row.
;MyContext (current-view in the ABT) is a list indexed by queen number [col0 col1 col2...] col = -1 if unknown.
;nogoods is a list of inconsistent positions [0 1 1 0 ... ] where 0 is good and 1 is no-good.
;messages-recieved is the number of messages this queen has received.
queens-own [message-queue MyValue_col MyContext nogoods messages-received_ok messages-received_nogood nr_constraintc
AgentC_Cost parent parent-agent children consistent ]

to ComputeParent_Children
locals [i]
set Children []
set i who + 1
 while [i < num-queens]
    [
    set Children lput i Children
    set i i + 1
    ]
set Parent []
set i 0
 while [i < who]
    [
    set Parent lput i Parent
    set i i + 1
    ]
end

to setup-patches
  ask patches with [(abs (pxcor + pycor)) mod 2 = 0 and
                    pxcor < num-queens - screen-edge-x and
                    pycor > screen-edge-y - num-queens] [
    set pcolor 8] 
end

; n is length of list
; el is the element
to-report get-list [n el]
  locals [i lst]
  set i 0
  set lst []
  while [i < n] [
    set lst fput el lst
    set i i + 1]
  report lst
end

to setup-queens
  set nr_cicluri 0
  create-custom-queens num-queens [
    set MyValue_col 0 ;initial value is 0 for all.
    setxy (MyValue_col - screen-edge-x) (screen-edge-y - who)
    ;set color red
    set heading 0
    set messages-received_ok 0
    set messages-received_nogood 0
    set nr_constraintc 0
    set AgentC_Cost 0
    set MyContext get-list num-queens -1
    set nogoods get-list num-queens 0
    set message-queue []
    set-current-plot "Messages"
    create-temporary-plot-pen "q" + who
    set-current-plot-pen "q" + who
    set-plot-pen-color color]
end




to setup
  ca
  setup-patches
  setup-queens
  set gata false
  ask queens [ComputeParent_Children
  initialize
  ]
end

to update
  set no-more-messages true
  set nr_cicluri nr_cicluri + 1
  if (nr_cicluri > Max_cycle) 
   [Stop]
  ask queens [
      if (not empty? message-queue)[
      set no-more-messages false
        ]
  ]
  if (no-more-messages) [
  ask queens
    [show MyValue_col] 
  stop]
  ask queens [
  handle-message
  ]   
   if (gata) [
    show "no solution"
    ask queens
    [show MyValue_col] 
    stop]
  ask queens [
    set-current-plot "Messages"
    create-temporary-plot-pen "q" + who
    plot messages-received_nogood
    ;plot messages-received_ok
    
    ]
  if (show-trace)[
    print "=============="
    ask queens [
      show "view=" + MyContext
      show "nogood=" + nogoods]]
end


to receive-message [msg]
  without-interruption [
    set message-queue lput msg message-queue]
end

to-report retrieve-message
  locals [msg]
  without-interruption [
    set msg first message-queue
    set message-queue butfirst message-queue
    ]
  report msg
  
end


to initialize
 locals [msg]
  set consistent true
  ;ask queens with [who > who-of myself]
   ; [set message-queue lput (list "info" (list (who-of myself) (MyValue_col-of myself))) message-queue
   ; ]
  foreach children
    [
     set msg (list "info" (list who MyValue_col) )
     ask turtles with [who = ? ]
         [receive-message msg]
    ] 
end

;this is the DisDB procedure in the paper
to handle-message
  locals [msg xj dj]
  if (empty? message-queue) [stop]
  set msg retrieve-message
  if (first msg = "stop")
   [set gata true
   stop]
  if (first msg = "info")
    [set messages-received_ok messages-received_ok + 1
    GoAHead msg]
  if (first msg = "back")
    [ 
    set messages-received_nogood messages-received_nogood + 1
    ResolveConflict item 1 msg] 
end

 ;this is the CheckAgentView procedure in the ABT algorithm  
 to GoAHead  [msg ]
    locals [xj dj  i j TempValue ]
    if (msg != "" )
     [
     set xj item 0 (item 1 msg)
     set dj item 1 (item 1 msg)
     ;set nr_constraintc nr_constraintc + who * (num-queens + 1) 
     UpdateContextInfo xj dj 
     ]
    if (not consistent ) 
       [
       set TempValue ChooseValue
       ifelse (TempValue > -1 )
        [ 
        set MyValue_col TempValue
        foreach children
         [
          set msg (list "info" (list who MyValue_col) )
          ask turtles with [who = ? ]
              [receive-message msg]
         ]
         ]
         [ResolveNogoods]
      ]
  end       

to UpdateContextInfo [Qj Vj ]
 locals [i j ]
    set MyContext replace-item Qj MyContext Vj
    ;set i Qj 
    set consistent true
    ;show MyValue_col + " " + current-view
    set i 0
    while [ i < who ] ;for each queen
    [
      if ( abs (who - i)  =  abs (MyValue_col - (item i MyContext)));diagonal match
             [  
             set consistent false
             ]   
      if ( MyValue_col = (item i MyContext) );verticle match
             [ 
             set consistent false
             ]   
      set i i + 1
     
    ]         
    set nogoods get-list num-queens 0
    set i 0
    while [ i < who ] ;for each queen
        [set j 0
         while [ j < num-queens ] ;for each column
            [
             if ( abs (who - i)  =  abs (j - (item i MyContext)));diagonal match
                [set nogoods replace-item j nogoods 1]
             if ( j = (item i MyContext) );verticle match
                [set nogoods replace-item j nogoods 1]   
             set j (j + 1)]
         set i (i + 1)]    
      
end

to ResolveConflict [msgNogood ]
    locals [ i j msg TempValue]

    ;if it is consistent with the agent view, otherwise it is discarded due to obsolescence.
    set i 0
    while [i < who]
        [if ((item i msgNogood) != (item i MyContext)) [stop]            
         set i (i + 1)]
    ;An accepted nogood is used to update the agent view of agents not in PArent
     ;Update MyContext
    ;if (MyValue_col > -1)
    set nogoods replace-item MyValue_col nogoods 1 ;mark current position as no good
    set consistent false
    set TempValue ChooseValue
    ifelse (TempValue > -1 )
     [ 
        set MyValue_col TempValue
        set msg (list "info" (list who MyValue_col) )
        foreach children
         [
           ask turtles with [who = ? ]
              [receive-message msg]
         ]
     ]
     [ResolveNogoods]
 end 


;this is the BackTrack procedure in the ABT algorithm    
 to ResolveNogoods
    locals [msg i j ]     
    ifelse (who = 0);newNogood=empty; 
        [set gata true 
        set msg "stop"
        receive-message msg
        ]
       [ set msg (list "back" MyContext )
         ask turtles with [who = who-of myself - 1]
            [receive-message msg]
      ]
    ;set nr_constraintc nr_constraintc + who * num-queens   
    UpdateContextConflict
end


to UpdateContextConflict 
   locals [i j msg]
  
   set nogoods get-list num-queens 0
   set i 0
   while [ i < who ] ;for each queen
        [set j 0
         while [ j < num-queens ] ;for each column
            [
             if ( abs (who - i)  =  abs (j - (item i MyContext)));diagonal match
                [set nogoods replace-item j nogoods 1]
             if ( j = (item i MyContext) );verticle match
                [set nogoods replace-item j nogoods 1]   
             set j (j + 1)]
         set i (i + 1)]    
    set i 0
    set j 0
    while [ i < num-queens and j = 0]
        [if ( item i nogoods = 0 )
            [set Myvalue_col i
             setxy (MyValue_col - screen-edge-x) (screen-edge-y - who)
             set msg list "info" (list who MyValue_col)
             ;ask turtles with [who > who-of myself]
             ;   [receive-message msg]
             foreach children
              [
              ask turtles with [who = ? ]
                 [receive-message msg]
             ]    
             set j 1]
         set i (i + 1)] 
   end

 
to-report ChooseValue
 locals [i j ]
   set i 0
   set j 0
   while [ i < num-queens and j = 0]
        [if ( item i nogoods = 0 )
            [
             setxy (i - screen-edge-x) (screen-edge-y - who)
             set j 1
             report i
             ]
         set i (i + 1)]
  ; setxy (i - screen-edge-x) (screen-edge-y - who)
   report -1      
end 

























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@#$#@#$#@
Title: NQueens DisDB (Distributed Dynamic Backtracking)
Author: Muscalagiu Ionel
Description:
This is the N-Queens problem solution using
Distributed Dynamic Backtracking-DisDB as outlined in <ul>
<li>
Christian Bessiere, Arnold Maestre, Pedro Meseguer.  
<a href="http://www.lirmm.fr/~bessiere/stock/ijcai01ws.ps">
Distributed Dynamic Backtracking.</a> 
C. Bessiere, A. Maestre, and P. Meseguer. 
In M.C. Silaghi, editor, Proceedings of the IJCAI'01 workshop on Distributed Constraint Reasoning, Seattle WA, pages 9-16. 
</li>
</ul>

	
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