; CRC Simulation
; written by Crystal Barchet and Teresa Carrigan, 2004



globals [ original original-message final-answer number crystal generator-list num-yellow realOriginal x y column column1 columnDiv padded toprow botrow dividend lengthG newDividend zeroes pad step praise myDigits generator  new-generator  doneyet? start-x number-of-digits ]
patches-own [ name ]
breeds [ divisbar equalbar top divisor digit dividend-all dividend-bit divisor-bit working ]
digit-own [ digit-value  ]
turtles-own [ state tname goname]


;runs setup when program is first loaded
to startup
  setup
end



;initializes variables
to setup
  locals [here-y]
  ca
  set praise ["You got it!" "Correct!" "Awesome!" "Perfect!" "Right!" "Bravo!" "Splendid!"]
  set myDigits [ "0" "1" ]
  set generator-list ["101" "111" "1011" "1101"]
  set original ""
  set number 0
  set x 0
  set y 0
  
  set generator random-one-of generator-list    
  set lengthG length generator
  
  ifelse lengthG = 4
  ;make a random 8-bit "original" pattern
  [
  repeat 11
  [
    set original (word original random-one-of myDigits)]
    set number-of-digits length original
    set crystal add-space original
   ] 
  [ 
  repeat 10
  [
    set original (word original random-one-of myDigits)]
    set number-of-digits length original
    set crystal add-space original
   ] 
 
 
; create explanation bars at top and bottom
    ask patches with [ pycor < -25 or pycor > 25]
        [ set pcolor blue ]
    ask patch-at 25 34
      [ set plabel "Cyclic Redundancy Check (CRC) Error" 
        set plabel-color white
        set name -1
      ]
    ask patch-at 45 28
      [ set plabel "number here"
        set plabel-color white
        set name -2
      ]
    ask patch-at 45 -30
        [ set plabel ""
          set plabel-color white
          set name 1
        ]
    ask patch-at 45 -35
        [ set plabel "" 
          set name 2
        ]
    
    
    explain -2 "Message Received: " + add-space original + "       Generator= " + generator
    set step 0
    set realOriginal original
    set doneyet? false
end




to explain [ which what ]
  ask patches with [ name = which ]
    [  set plabel what ]
end





; clears the black area and the dividend and divisor go to their relatives spots & creates the division bar
to step0

  locals [ here-y here-x  ]
  set here-y 15
  set here-x ( - 20 )
  explain 1 "First divide, using the generator as the divisor"
  explain 2 "Dividend: " + original
  
  wait slow-motion * 5

  
        ; creates the division bar
    cct-divisbar 1 [
      setxy here-x here-y
      set color white
      set shape "divisbar"
      set size 7
      set tname ""
    ]

            ; this is the "spot" where the divisor wanders to
cct-divisor 1 [
          setxy 42 28
          set shape "circle"
          set color black
          set label-color cyan
          set label generator + " "
;          set size 0
          set tname "moving"
          ]
  
     ; moves the divisor to its spot
ask divisor 
          [ wander-to-divisor
          ]



      ; this is the "spot" where the dividend wanders to
cct-dividend-all 1 [
          setxy 46 -35
          set shape "circle"
          set color black
          set label-color yellow
          set label original + " "
;          set size 0
          set tname "moving"
          ]

          
          
          ; moves the dividend to its spot
ask dividend-all 
          [ wander-to-dividend
          ]

   
  repeat length original 
  [
    ; creates the top of the division bar
    cct-top 1 [
      setxy ( here-x + 2 ) here-y
      set color white
      set shape "top"
      set size 7
      set here-x here-x + 3
    ]   
  ]
;]    
   
  set step (step + 1)
end

to scroll
  locals [ turtle-set top-line offset]
  set offset 15
  ask dividend-bit
    [ set top-line ycor ]
  set turtle-set turtles with [ ycor < top-line ]
  ask turtle-set with [ycor > (top-line - offset )]
    [  die
    ]
  set offset ycor-of max-one-of turtle-set [ ycor ]
  set offset ( top-line - offset - 5)
  ask turtle-set
    [  set heading 0
       repeat offset
         [  fd 1
            wait slow-motion / 8
         ]
       set ycor round ycor
       set heading 180
    ]
  
end

  ;  if dividend is all zeros, then the message we want to send is all zeros
to-report check-for-all-zeros
  
  locals [ result ]
  set result true

  ask dividend-bit 
  [
    if label = "1"
      [
      set result false
      ]
  ]
  if result
    [ set step 13 ]
  report result
end

to bring-down-digits
  locals [ div-bit-ycor curr-div-bit div-bit-set working-line working-y curr-working done? 
           num-working num-to-bring-down all-zeros? ]
  ask random-one-of dividend-bit
    [ set div-bit-ycor ycor ]
  ifelse not any? turtles with [ ycor < div-bit-ycor ]
    [  ; first time
       if check-for-all-zeros
         [ 
           stop  
         ]
         
       set div-bit-set dividend-bit with [ label-color = yellow ]
       set curr-div-bit min-one-of div-bit-set [ xcor ]
       while [label-of curr-div-bit = "0"]
         [  ask curr-div-bit 
               [ set label-color cyan ]
            set div-bit-set dividend-bit with [ label-color = yellow ]
            set curr-div-bit min-one-of div-bit-set [ xcor ]
         ]
       if count div-bit-set < lengthG
         [
           explain 1 "After dropping leading zeros, there are too few"
           explain 2 "bits left. Since they aren't 0, there was an error"
           set step 20
           stop
         ]
       repeat length generator
         [  ask curr-div-bit
               [ set label-color cyan ]
            set div-bit-set dividend-bit with [ label-color = yellow ]
            set curr-div-bit min-one-of div-bit-set [ xcor ]
         ]
    ]
    [  ; not first time
       ; drop leading zeroes
       ask min-one-of working [ ycor ]
         [ set working-y ycor ]
       if working-y < -18
         [ scroll 
           ask min-one-of working [ ycor ]
             [ set working-y ycor ]
         ]
       set working-line working with [ ycor = working-y ] 
       set curr-working min-one-of working-line [ xcor ]

       set done? (label-of curr-working != "0")
       set all-zeros? false
       while [ not done? ]
         [  ask curr-working
               [  set label-color red
               ]
            set working-line working with [ ycor = working-y and label-color != red ]
            ifelse count working-line > 0
              [ set curr-working min-one-of working-line [ xcor ] 
                set done? (label-of curr-working != "0")                
              ]
              [ set done? true 
                set all-zeros? true
              ]
         ]
      ;  count digits in bottom working line, bring down to match size of generator
      set working-line working with [ ycor = working-y and label-color != red ]
      set num-working count working-line
      set num-to-bring-down (length generator - num-working)
      bring-down-n working-y num-to-bring-down all-zeros?
    ]
end

to bring-down-n [ to-y num-to-bring-down all-zeros? ]
  locals [ div-bit-set curr-div-bit working-line done? ]
  set done? false
  set div-bit-set dividend-bit with [ label-color = yellow ]
  set num-yellow count div-bit-set
  if all-zeros?
    [  
      ; special case here, when num-yellow = 0, CRC = zeros
       if num-yellow = 0
         [  
           set step 15
           stop
         ]
         
       ; add one to num-to-bring-down for every leading zero
       set curr-div-bit min-one-of div-bit-set [ xcor ]
       while [ not done? ]
        [
           ask curr-div-bit
             [  ifelse label = "0"
                   [ set label-color cyan 
                     hatch 1
                        [  set breed working
                           set heading 180
                           set shape "box"
                           set color black
                           set state "too high"
                           set label-color red
                        ]
                  ]
                  [ set done? true ]
                  
            ]  ; end ask curr-div-bit
            set div-bit-set dividend-bit with [ label-color = yellow ]
            if count div-bit-set = 0
              [ set done? true 
                set step 15
                ;;;;; call a new procedure and stop
                bring-down-zeros to-y
                stop
              ]
            set curr-div-bit min-one-of div-bit-set [ xcor ]
         ]         
      ;  end if all-zeros?
    ]

  set num-yellow count div-bit-set
  if num-to-bring-down > num-yellow
    [ set num-to-bring-down num-yellow ]
  set curr-div-bit min-one-of div-bit-set [ xcor ]
 
  repeat num-to-bring-down
         [  ask curr-div-bit
               [ set label-color cyan ]
            ask curr-div-bit
              [
                  hatch 1
                    [  set breed working
                       set heading 180
                       set shape "box"
                       set color black
                       set state "too high"
                   ]
             ]
            set div-bit-set dividend-bit with [ label-color = yellow ]
            set curr-div-bit min-one-of div-bit-set [ xcor ]
     ]         
  ask working with [ state = "too high" ]
    [
             while [ycor > to-y]
               [  fd 1
                  wait slow-motion / 4
               ]
             set state "done"
    ]                

end



to bring-down-zeros [to-y]

  locals [ num-to-kill working-line curr-working ]
  
  ask working with [ state = "too high" ]
    [
             while [ycor > to-y]
               [  fd 1
                  wait slow-motion / 4
               ]
             set state "done"
    ]               

  set working-line working with [ ycor = to-y ]
  set num-to-kill ( count working-line - lengthG + 1 )
  
  repeat num-to-kill
    [
      set curr-working min-one-of working-line [ xcor ]
      ask curr-working [ die ]  
    ]

  ask working-line 
    [
      set label-color white
    ]

end


  ; brings the divisor to the right spot under the dividend
to step1
  locals [ working-line working-y ]
  explain 1 "Dividing using XOR"
  explain 2 "Bring down the divisor"
  
  bring-down-digits
  if step > 10
    [ 
      stop 
    ]
  
    ; brings the divisor under the dividend
  ask divisor
  [
    wander-to-under
  ]
  
  ask min-one-of working [ ycor ]
    [ set working-y ycor ]
  set working-line working with [ ycor = working-y ]
  
  ask working-line
    [  hatch 1
          [  set breed equalbar
             set color white
             set shape "equalbar"
             set size 4
             set label ""
             set heading 180
             fd 4
             showturtle
          ]
    ]
  set step (step + 1)
end

  ;  does the XOR 
to step2
  locals [ working-y working-line ]
  explain 2 "Take the XOR"
  ask min-one-of working [ ycor ]
    [ set working-y ycor ]
  set working-line working with [ ycor = working-y]
  
  ask working-line [ do-xor ]
  wait slow-motion * 5
  ifelse check-for-done
    [ set step 3 ]
    [ set step 1 ]
end


  ;  if this step is reached, it is finished dividing (standard case)
  ;  check for needed padding
to step3

 locals [ working-y to-y curr-x working-line one-less] 

  bring-down-digits
  set one-less lengthG - 1
  set to-y 14
      
  ask min-one-of working [ ycor ]
    [ set working-y ycor]

  set working-line working with [ (ycor = working-y) and (label-color != red) ]
  
  if count working-line > 0 
    [
      ask working with [ label-color = red and ycor = working-y ]
        [ 
          die
        ] 
    ]
     
  
  set working-line working with [ (ycor = working-y) and (label-color != red) and (label-color != white) ]
  set curr-x min-one-of working-line [ xcor ]
  
    ifelse count working-line = 0
     [
       explain 1 "Since the remainder is 0, there is no error"
       ifelse lengthG = 3
         [
            repeat 2
              [
                set original-message but-last original
                set original but-last original
              ]
             explain 2 "The original message sent was: " + add-space original-message
          ]
          [
            repeat 3
              [
                set original-message but-last original
                set original but-last original
              ]
             explain 2 "The original message sent was: " + add-space original-message
          ]
          
       set final-answer false
       set step 20
     ]
     [
       explain 1 "Since the remainder is not 0, an error has occured."
       explain 2 "Original message can not be recovered."
       set final-answer true
       set step 20
     ]
   

end


to step13

  explain 1 "The message was all zeros, so there was no error"

   ifelse lengthG = 3
         [
            repeat 2
              [
                set original-message but-last original
                set original but-last original
              ]
             explain 2 "The original message sent was: " + add-space original-message
          ]
          [
            repeat 3
              [
                set original-message but-last original
                set original but-last original
              ]
             explain 2 "The original message sent was: " + add-space original-message
          ]
          
       set final-answer false
       set step 20

end




to step15

  locals [ working-line working-y]

  ask min-one-of working [ ycor ]
    [ set working-y ycor]
  set working-line working with [ (ycor = working-y) and (label-color != red) and (label-color != white) ]

  ifelse count working-line = 0
     [
       explain 1 "Since the remainder is 0, there is no error"
       ifelse lengthG = 3
         [
            repeat 2
              [
                set original-message but-last original
                set original but-last original
              ]
             explain 2 "The original message sent was: " + add-space original-message
          ]
          [
            repeat 3
              [
                set original-message but-last original
                set original but-last original
              ]
             explain 2 "The original message sent was: " + add-space original-message
          ]
          
       set final-answer false
       set step 20
     ]
     [
       explain 1 "Since the remainder is not 0, an error has occured"
       explain 2 ""
       set final-answer true
       set step 20
     ]
   

end



to do-xor
  locals [ one-above me]
  set heading 0
  set one-above turtles-at 0 5
  set me self
  hatch 1
    [  set heading 180
       fd 6
       set label calc-xor me one-above
    ]
  
end


  ;  this method makes the divisor go to the correct place from the blue bars
to wander-to-divisor
  locals [ to-x to-y n offset]
  set to-x ( - 24 )
  set to-y 14

  while [ (ycor != to-y) or (xcor != to-x) ]
    [ set color pcolor
      set heading towardsxy-nowrap to-x to-y
      set n (distancexy to-x to-y)
      ifelse n > 1
        [ fd 1 
          set heading towardsxy-nowrap to-x to-y
        ] 
        [ setxy to-x to-y ]
      wait (slow-motion / 8)
    ]
  set tname "done1"
  set new-generator generator
  set offset 0        
  set column 0     
          
  ask patch-at -1 0 
  [
    repeat length generator
    [
      sprout 1 
      [
         set breed divisor-bit
         set shape "box"
         set color black
         set label last new-generator 
         set label-color cyan
         set heading -90
         fd offset
         set offset offset + 3     
         set state "done1" 
         set botrow 2
         set column column + 1
         hideturtle
      ]
        set new-generator but-last new-generator
    ]
  ]
  

end


   ; this makes the dividend go to the right place from the blue bars
to wander-to-dividend
  locals [ to-x to-y n offset ]
  set to-x ( 7 )
  set to-y 14
  while [ (ycor != to-y) or (xcor != to-x) ]
    [ set color pcolor
      set heading towardsxy-nowrap to-x to-y
      set n (distancexy to-x to-y)
      ifelse n > 1
        [ fd 1 ] 
        [ setxy to-x to-y ]
      wait (slow-motion / 8)
    ]
  set tname "done"
  set label ""
  
  set offset 0
  set newDividend original
  set column1 0 
  
 ask patch-at -25 0 
  [
    repeat length original
    [
      sprout 1 
      [
         set breed dividend-bit
         set shape "box"
         set color black
         set label first newDividend 
         set label-color yellow
         set heading -270
         fd offset
         set offset offset + 3   
         set column1 column1 + 1 
         set toprow 1
             
      ]
        set newDividend but-first newDividend
    ]
  ] 
  
  die
end


  ; this procedure makes the divisor go under the dividend when doing the first division step
to wander-to-under    
  locals [ to-y to-x cyan-turtles gen-turtles now-x ]
  set cyan-turtles turtles with  [ label-color = cyan ]
  ask max-one-of divisor-bit [ xcor ]
    [ set now-x xcor ]
  ask min-one-of turtles [ ycor ]
    [ set to-y (ycor - 5) ]
  ask max-one-of cyan-turtles [ xcor ]
    [ set to-x xcor ]
  ask divisor-bit
     [ hatch 1
         [   set breed working 
             set heading 180
             set shape "box"
             set color black
             showturtle
         ]
     ]         
  ask working with [ xcor <= now-x ]
    [
             while [ycor > to-y]
               [  fd 1
                  wait slow-motion / 4
               ]
             set heading 90
             repeat (to-x - now-x )
               [  fd 1
                  wait slow-motion / 4
               ]
    ]
end


   ; this is the XOR method
to-report calc-xor [ a b ]
  locals [ label-a label-b ]
  set label-a label-of a
  set label-b label-of random-one-of b
  ifelse label-a = label-b
  [
    report "0"
  ]
  [
    report "1"
  ]

end


  ;  reports true when the CRC division is finished
to-report check-for-done
  locals [ div-bit-set working-line working-y num-working curr-x done?]
  set div-bit-set dividend-bit with [ label-color = yellow ]
  set num-yellow count div-bit-set
  
  ask min-one-of working [ ycor ]
    [ set working-y ycor]

  set working-line working with [ (ycor = working-y) and (label-color != red) ]
  
  set done? false
  set curr-x min-one-of working-line [ xcor ]
  while [ label-of curr-x = "0" and not done? ]
    [  ask curr-x
          [ set label-color red ]
       set working-line working-line with [ label-color != red ]
       ifelse count working-line > 0
           [  set curr-x min-one-of working-line [ xcor ]  ]
           [  set done? true ]
    ]

  set num-working count working-line
  ifelse (num-yellow + num-working) < length generator
    [
      report true
    ]
    [
      report false
    ]
end

; do all remaining steps
to go
  ifelse step < 16
    [ one-step ]
    [ stop ]
end

;  do a single step (whichever comes next)
to one-step
  locals [which-step]
  set which-step "step" + step
  if ( step < 16)
    [ run which-step ]
end



to show-again

  set number 0
  set x 0
  set y 0
  
  ask turtles
    [
      die
    ]  
  
  ask patch-at -20 -35
    [
       set plabel ""
    ]
  
  explain 1 ""
  explain 2 ""
  
  set doneyet? false
  
  set step 0
  
  set original clean-input crystal

end



to ask-other 
  locals [ guess target question ]
  set guess user-yes-or-no? ("Did an error occur during transmission?")
  while [ step < 16 ]
    [
       go
    ]
  set target final-answer
  ifelse guess = target 
    [ user-message random-one-of praise ]
    [ user-message "I'm sorry, but the correct answer is " +  target]
end




;  ask a quiz question
to quiz
  locals [ save-slow ]
  without-interruption [ setup ]
  set save-slow slow-motion
  set slow-motion 0
  wait .5
  ask-other
  set slow-motion save-slow
end




;  cleans up the user input
to-report clean-input [guess]
  locals [ pos k upper lowerList]
  set guess remove " " guess
  set guess remove "," guess
  report guess  
end



; adds a space in the number so it can be read easier
to-report add-space [ space-number ]
  locals [ save k ]
  set save ""
  set space-number reverse clean-input space-number
  set k 0
  while [ (length space-number) > 0 ]
    [
      set save (word last space-number save )
      set space-number butlast space-number
      set k (k + 1)
      if (k = 4) and (length space-number > 0)
        [ set save (word " " save )
          set k 0
        ]
    ]
  set space-number save  
  report reverse space-number
end

; *** NetLogo Model Copyright Notice ***
;
; Copyright 2004 by Teresa W. Carrigan.  All rights reserved.
;
; Permission to use, modify or redistribute this model is hereby granted,
; provided that both of the following requirements are followed:
; a) this copyright notice is included.
; b) this model will not be redistributed for profit without permission
;    from Teresa W. Carrigan.
; Contact Teresa W. Carrigan for appropriate licenses for redistribution 
; for profit.
;
; To refer to this model in academic publications, please use:
; Barchet, C and Carrigan, T. (2004). Cyclic Redundancy Check Error Detection Simulation model.
; Blackburn College, Carlinville IL.
;
; In other publications, please use:
; Copyright 2004 by Crystal R. Barchet and Teresa W. Carrigan.  All rights reserved.  
;
; *** End of NetLogo Model Copyright Notice ***
@#$#@#$#@
GRAPHICS-WINDOW
5
10
667
341
81
37
4.0
1
18
1
1
1

CC-WINDOW
168
446
448
588
Command Center

BUTTON
13
346
76
379
NIL
setup
NIL
1
T
OBSERVER
T

BUTTON
79
347
160
380
step
one-step
NIL
1
T
OBSERVER
T

SLIDER
489
348
661
381
slow-motion
slow-motion
0
1
0.0
0.1
1
NIL

BUTTON
163
347
226
380
NIL
go
T
1
T
OBSERVER
T

BUTTON
230
347
328
380
NIL
show-again
NIL
1
T
OBSERVER
T

BUTTON
332
348
395
381
quiz
quiz
NIL
1
T
OBSERVER
T

@#$#@#$#@
WHAT IS IT?
-----------
This model demonstrates receiving a message using Cyclic Redundancy Check Error Detection. 

HOW IT WORKS
------------
A generator is chosen randomly from a list of allowed generators, and a random message is created.  This  message is divided by the generator by using modulo 2 arithmetic, which is the same as XOR. If the remainder is 0, then no errors occured during transmission, and then one bit less than the number of bits in the generator is stripped from the end of the transmitted message. If the remainder is not 0, then an error occured during transmission and the original message can not be recovered.


HOW TO USE IT
-------------
The setup button generates a random message and selects a random generator.

The slow-motion slider is an easy way to adjust the speed of the demonstration.  Set it to zero if you want to just see the answer quickly.  0.2 is a good setting for most purposes.

The step button does whatever step comes next, and then stops so you can take notes.

The go button finishes the entire problem, at a speed set by the slow-motion slider.

The show-again button starts the exact problem from the beginning.  You may then click either the step button or the go button to see the same demonstration.

The quiz button generates a random problem and then asks you to determine the answer.  Please note that you can use the show-again button after a quiz to see all the steps in getting the correct answer. 


THINGS TO NOTICE
----------------
When the transmitted message is all zeroes, then there was no error during transmission.

When the remainder is not all zeroes, an error occurred and the original message can not be recovered.

We do <b>not</b> pad with zeroes before division, nor do we stip bits before division.

THINGS TO TRY
-------------
Set the slow-motion slider to about .2 seconds (or slower) and press the step button  several times.  Watch the demonstration of each step.

Set the slow-motion slider to about .2 seconds and press the go button. Watch the entire demonstration.

Do each step by hand, then press step to check your work.


EXTENDING THE MODEL
-------------------
Allow the user to input the beginning message.

Allow the user to input the generator.



NETLOGO FEATURES
----------------
"min-one-of" reports the agent in the agentset that reports the lowest value for the given reporter. 

"max-one-of" reports the agent in the agentset that reports the highest value for the given reporter. 

"count" reports the number of agents in the given agentset. 

"hatch 1" tells each turtle to create one turtle identical to itself.

RELATED MODELS
--------------
Cyclic Redundancy Check

CREDITS AND REFERENCES
----------------------
This model was written by Crystal Barchet and Teresa Carrigan, 2004.  

Permission to use, modify or redistribute this model is hereby granted,
provided that both of the following requirements are followed:
	a) this copyright notice is included.
	b) this model will not be redistributed for profit without permission
	   from Teresa Carrigan.
Contact Teresa Carrigan for appropriate licenses for redistribution 
for profit.

To refer to this model in academic publications, please use: 
Barchet, C. and Carrigan, T. (2004). Cyclic Redundancy Check Error Detection Simulation
Blackburn College, Carlinville, IL.

In other publications, please use: 
Copyright 2004 by Crystal R. Barchet and Teresa Carrigan.  All rights reserved.  


FOR MORE INFORMATION 
-------------------- 
For more information about Cyclic Redundancy Check Error Detection, see one of the following textbooks:
[1] Null, L. and Lobur, J. "Essentials of Computer Organization and Architecture", First Edition, Jones & Bartlett,  pages 73-76.
[2] Murdocca, M. and Heuring, V. "Principles of Computer Architecture", First Edition, Prentice Hall, page 365-366
[3] Forouzan, B. Data Communications and Networking, Third Edition, McGrawHill, pages 249-253.
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