Code Golf: Finite-state machine!

Answer 1

Adam provided a reference implementation. I didn't see it before I made mine, but the logic is similar:

Edit: This is Python 2.6 code. I did not try to minimize length; I just tried to make it conceptually simple.

import sys
a = sys.stdin.read().split('\n')
states = a[0].split()
transitions = a[1:-2]
input = a[-2]
statelist = {}
for state in states:
    statelist[state] = {}

for start, char, end in [x.split() for x in transitions]:
    statelist[start][char] = end

state = states[0]
for char in input:
    if char not in statelist[state]:
        print state,char
        print "REJECT"
        exit()
    newstate = statelist[state][char]
    print state, char, '->', newstate
    state = newstate
if state[0].upper() == state[0]:
    print "ACCEPT"
else:
    print "REJECT"

Answer 2

Python 2.7+, 201 192 187 181 179 175 171 chars

PS. After the problem was relaxed (no need to output state line on empty input), here is new code that's notably shorter. If you are on version <2.7, there is no dict comprehension, so instead of {c+o:s for o,c,s in i[1:-1]} try dict((c+o,s)for o,c,s in i[1:-1]) for the price of +5.

import sys
i=map(str.split,sys.stdin)
s=i[0][0]
for c in''.join(i[-1]):
    if s:o=s;s={c+o:s for o,c,s in i[1:-1]}.get(c+s,());print o,c,'->',s
print'ARCECJEEPCTT'[s>'Z'::2]

And its test output:

# for empty input
ACCEPT

# for input '1001010'
S1 1  ->  S1
S1 0  ->  s2
s2 0  ->  S1
S1 1  ->  S1
S1 0  ->  s2
s2 1  ->  s2
s2 0  ->  S1
ACCEPT

# for input '101'
S1 1  ->  S1
S1 0  ->  s2
s2 1  ->  s2
REJECT

# for input '10X'
S1 1  ->  S1
S1 0  ->  s2
s2 X  ->  ()
REJECT

# for input 'X10'
S1 X  ->  ()
REJECT

---

Previous entry (len 201):

import sys
i=list(sys.stdin)
s=i[0].split()[0]
t={}
for r in i[1:-1]:a,b,c=r.split();t[a,b]=c
try:
    for c in i[-1]:print s,c.strip(),;s=t[s,c];print' ->',s
except:print('ACCEPT','REJECT')[s>'Z'or' '<c]

I want to apologize before someone slaps me for it: the code behavior is slightly different from the original spec - per question-comments discussion. This is my illustration for the discussion.

PS. while i like the resolution ACCEPT/REJECT on the same line with the final state, it can me moved to solitude (e.g. imagine results are to be parsed by stupid script that only cares about last line being accept or reject) by adding '\n'+ (5 chars) to the last print for the price of +5 chars.

Example output:

# for empty input
S1  ACCEPT

# for input '1001010'
S1 1  -> S1
S1 0  -> s2
s2 0  -> S1
S1 1  -> S1
S1 0  -> s2
s2 1  -> s2
s2 0  -> S1
S1  ACCEPT

# for input '101'
S1 1  -> S1
S1 0  -> s2
s2 1  -> s2
s2  REJECT

# for input '10X'
S1 1  -> S1
S1 0  -> s2
s2 X REJECT

# for input 'X10'
S1 X REJECT

Answer 3

Lua, 356

Takes any nonspace characters for states, and any non-space one characters for transition letters. Though it seems not shortest, I'll post it any way. Could save 25 chars printing tabs instead of spaces.

Readable version:

i=io.read
p=print
S={}
i():gsub("(%S+)",function (a) f=f or a S[a]={} end )
l=i"*a"
for a,t,d in l:gmatch"(%S+) (%S) (%S+)"do
    S[a][t]=d
end
I=l:match"(%S+)%s$"or"" -- fixes empty input
function F(a,i)
    t=I:sub(i,i)
    if t==""then
        p"ACCEPT"
    elseif S[a][t] then
        p(("%s %s -> %s"):format(a,t, S[a][t]))
        return F( S[a][t],i+1)
    else
        if t~=""then p(a.." "..t)end p'REJECT'
    end
end
F(f,1)

Golfed version + in- an output.

i=io.read p=print S={}i():gsub('(%S+)',function(a)f=f or a S[a]={}end)l=i'*a'for a,t,d in l:gmatch"(%S+) (%S) (%S+)"do S[a][t]=d end I=l:match'(%S+)%s$'or''function F(a,i)t=I:sub(i,i)if t==""and a:match'^%u'then p'ACCEPT'elseif S[a][t]then p(('%s %s -> %s'):format(a,t,S[a][t]))return F(S[a][t],i+1)else if t~=''then p(a.." "..t)end p'REJECT'end end F(f,1)
-- input --
A B C   
A B B
A C C
A A A
B A A 
B B B
B C C
C A A 
C B B
C C C
AABCCBCBAX
-- output --

A A -> A
A A -> A
A B -> B
B C -> C
C C -> C
C B -> B
B C -> C
C B -> B
B A -> A
REJECT

Answer 4

I'm feeling retro today, my language of choice for this task is IBM Enterprise Cobol - char count 2462 4078 (Sorry, pasted from a screen oriented device, trailing spaces are a tragic side effect):

 Identification Division.               
 Program-ID. FSM.                       
 Environment Division.                  
 Data Division.                         
 Working-Storage Section.               

 01 FSM-Storage.                        

*> The current state                    
   05 Start-State      Pic X(2).        
   05 Next-State       Pic X(2).        

*> List of valid states                 
   05 FSM-State-Cnt    Pic 9.           
   05 FSM-States       Occurs 9         
                       Pic X(2).        

*> List of valid transitions            
   05 FSM-Trans-Cnt    Pic 999.         
   05 FSM-Trans        Occurs 999.      
     10 Trans-Start    Pic X(2).        
     10 Trans-Char     Pic X.           
     10 Trans-End      Pic X(2).        

*> Input                                
   05 In-Buff          Pic X(72).      

*> Some work fields                     
   05 II               Pic s9(8) binary.
   05 JJ               Pic s9(8) binary.

   05 Wk-Start         Pic X(2).        
   05 Wk-Char          Pic X.           
   05 Wk-End           Pic X(2).        
   05 Wk-Cnt           Pic 999.         

   05                  Pic X value ' '. 
     88 Valid-Input        value 'V'.   

   05                  Pic X value ' '.                 
     88 Valid-State        value 'V'.                   

   05                  Pic X value ' '.                 
     88 End-Of-States      value 'E'.                   

   05                  Pic X value ' '.                 
     88 Trans-Not-Found    value ' '.                   
     88 Trans-Found        value 'T'.                   


 Linkage Section.                                       

 01 The-Char-Area.                                      
   05 The-Char         Pic X.                           
     88 End-Of-Input       value x'13'.                 
   05 The-Next-Char    Pic X.                           

 Procedure Division.                                    

      Perform Load-States                               
      Perform Load-Transitions                          
      Perform Load-Input                                
      Perform Process-Input                             

      Goback.                                           

*> Run the machine...                                   
 Process-Input.                                         

      Move FSM-States (1) to Start-State                
      Set address of The-Char-Area to address of In-Buff

      Perform until End-Of-Input                        

        Perform Get-Next-State                          
        Set address of The-Char-Area to address of The-Next-Char

        Move Next-State to Start-State                          

      End-Perform                                               

      If Start-State (1:1) is Alphabetic-Lower                  
        Display 'REJECT'                                        
      Else                                                      
        Display 'ACCEPT'                                        
      End-If                                                    

      Exit.                                                     

*> Look up the first valid transition...                        
 Get-Next-State.                                                

      Set Trans-Not-Found to true                               
      Perform varying II from 1 by 1                            
        until (II > FSM-State-Cnt)                              
           or Trans-Found                                       

        If Start-State = Trans-Start (II)                       
          and The-Char = Trans-Char (II)                        

          Move Trans-End (II) to Next-State                     
          Set Trans-Found to true                               

        End-If                                                  

      End-Perform                                               
      Display Start-State ' ' The-Char ' -> ' Next-State        

      Exit.                                                     

*> Read the states in...                                        
 Load-States.                                                   

      Move low-values to In-Buff                         
      Accept In-Buff from SYSIN                          

      Move 0 to FSM-State-Cnt                            
      Unstring In-Buff                                   
        delimited by ' '                                 
        into FSM-States (1) FSM-States (2) FSM-States (3)
             FSM-States (4) FSM-States (5) FSM-States (6)
             FSM-States (7) FSM-States (8) FSM-States (9)
        count in FSM-State-Cnt                           
      End-Unstring                                       

      Exit.                                              

*> Read the transitions in...                            
 Load-Transitions.                                       

      Move low-values to In-Buff                         
      Accept In-Buff from SYSIN                          

      Perform varying II from 1 by 1                     
        until End-Of-States                              

        Move 0 to Wk-Cnt                                 
        Unstring In-Buff                                 
          delimited by ' '                               
          into Wk-Start Wk-Char Wk-End                   
          count in Wk-Cnt                                
        End-Unstring                                     

        If Wk-Cnt = 3                                    

          Add 1 to FSM-Trans-Cnt                         
          Move Wk-Start to Trans-Start (FSM-Trans-Cnt)   
          Move Wk-Char  to Trans-Char  (FSM-Trans-Cnt)   
          Move Wk-End   to Trans-End   (FSM-Trans-Cnt)   

          Move low-values to In-Buff                     
          Accept In-Buff from SYSIN                           

        Else                                                  

          Set End-Of-States to true                           

        End-If                                                

      End-Perform                                             

      Exit.                                                   

*> Fix input so it has newlines...the joys of mainframes      
 Load-Input.                                                  

      Perform varying II from length of In-Buff by -1         
        until Valid-Input                                     

        If In-Buff (II:1) = ' ' or In-Buff (II:1) = low-values
          Move x'13' to In-Buff (II:1)                        
        Else                                                  
          Set Valid-Input to true                             
        End-If                                                

      End-Perform                                             

      Exit.                                                   

  End Program FSM.    

Answer 5

Python, 218 characters

import sys
T=sys.stdin.read()
P=T.split()
S=P[0]
n="\n"
for L in P[-1]if T[-2]!=n else"":
 i=T.find(n+S+" "+L)
 if i<0:print S,L;S=n;break
 S=T[i:].split()[2];print S,L,"->",S
print ("REJECT","ACCEPT")[S[0].isupper()]

Answer 6

Common Lisp - 725

(defun split (string)
  (loop for i = 0 then (1+ j)
     as j = (position #\Space string :start i)
     collect (subseq string i j)
     while j))

(defun do-fsm ()
  (let* ((lines (loop for l = (read-line *standard-input* nil)
      until (not l)
     collect (split l)))
  (cur (caar lines))
  (transitions (subseq lines 1 (- (length lines) 1))))
    (if (or (loop for c across (caar (last lines))
      do (format t "~a ~a" cur c)
        when (not (loop for tr in transitions
       when (and (equal cur (car tr))
          (equal c (char (cadr tr) 0)))
       return (progn (format t " -> ~a~%"
        (setq cur (caddr tr)))
       t)
         ))
        return t)
     (lower-case-p (char cur 0)))
 (format t "~%REJECT~%")
 (format t "ACCEPT~%"))))

No real attempt to minimize the code -- Common Lisp pays a heavy penalty in the required input processing, so I don't think there's much chance of this solution winning :-)