/* File: dfa5.pl     System: SWI-Prolog  5.2.8
   Author: MM
   Date: 10/02/2003
   Title: Check if a word is accepted by the DFA 
   
   DFA accepts words that start with a.
   Alphabet={a,b}, States={0,1,2}   Favorable={1} start is state 0.
   Transition function is:
                t(0,a)=1    t(0,b)=2
                t(1,a)=1    t(1,b)=1
                t(2,a)=2    t(2,b)=2
   
    ?- check_word(0,[a,b,b]).
    Yes
    ?- start.
    A DFA that accepts only words starting with a
    Input a word as a list(Ex: [a,b,b]) :[b,a,a,a].
    Word accepted by DFA(Yes or No): 0b2a2a2a
    No
    ?- start.
    A DFA that accepts only words starting with a
    Input a word as a list(Ex: [a,b,b]) :[a,b,b,b].
    Word accepted by DFA(Yes or No): 0a1b1b1b1
    Yes
*************************************************/
% This is the definition of our DFA in Prolog.

    t(0,a,1).   t(0,b,2).
    t(1,a,1).   t(1,b,1).
    t(2,a,2).   t(2,b,2).  
    
    start(0).       % 0 is the starting state
    
    favorable(1). % 1 is a favorable state
       
%   Check if a word is accepted.
   
    check_word(S,[]):- favorable(S),write(S).
    check_word(S,[H|T]) :- write(S),write(H),delta(S,H,Q),check_word(Q,T).

%    Ask for a word and check if it is accepted (yes/no).

    start:- write('A DFA that accepts only words starting with a\n'),
        write('Input a word as a list(Ex: [a,b,b]) :'),
        read(W),write('Path followed by DFA: '),        
        start(S),check_word(S,W).