/* File: dfa4.pl     System: SWI-Prolog  5.2.8
   Author: Mihaela Malita
   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

    ?- check_word(0,[b,a,b,b]).
    No
    
    ?- start.
    
    A DFA that accepts only words starting with a
    Input a word as a list(Ex: [a,b,b]) :[a,b,a].
    Word accepted by DFA(Yes or No): 
    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). % if finished wrod and state is favorable
    check_word(S,[H|T]) :- t(S,H,Q),check_word(Q,T). % if word not finisehd go next state

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

start:- write('\nA DFA that accepts only words starting with a\n'),
        write('\nInput a word as a list(Ex: [a,b,b]) :'),
        read(W),
        write('\nWord accepted by DFA(Yes or No): '),        
        start(S),check_word(S,W).