/// <summary>
/// Generates a CNF for a given grammar or returns null if the Grammar doesn't produce any words.
/// </summary>
/// <param name="g">the original grammar</param>
/// <returns>the CNF or null</returns>
public static ContextFreeGrammar getEquivalentCNF(ContextFreeGrammar g)
{
if (g == null)
{
return(null);
}
if (g.IsInCNF())
{
return(g);
}
try
{
ContextFreeGrammar res = ContextFreeGrammar.MkCNF(g);
//handle empty string
res.setAcceptanceForEmptyString(g.acceptsEmptyString());
return(res);
}
catch (AutomataException e)
{
if (g.acceptsEmptyString())
{
var res = new ContextFreeGrammar(new Nonterminal("S"), new Production[] { new Production(new Nonterminal("S"), new GrammarSymbol[] { new Nonterminal("S"), new Nonterminal("S") }) });
res.setAcceptanceForEmptyString(true);
return(res);
}
return(null);
}
}
/// <summary>
/// Generates a CFG that accepts the prefix closure of a given grammar.
/// </summary>
/// <param name="g">the original grammar</param>
/// <returns>the prefix closure</returns>
public static ContextFreeGrammar getPrefixClosure(ContextFreeGrammar g)
{
Func <Nonterminal, Nonterminal> prefixFor = delegate(Nonterminal x)
{
return(new Nonterminal(x.Name + "PREFIX"));
};
if (g == null)
{
return(g);
}
if (!g.IsInCNF())
{
g = getEquivalentCNF(g);
}
if (g == null)
{
return(g);
}
Nonterminal prefixStart = prefixFor(g.StartSymbol);
var prefixProductions = new List <Production>();
foreach (Production p in g.GetProductions())
{
//add original
prefixProductions.Add(p);
Nonterminal prefixNT = prefixFor(p.Lhs);
if (p.Rhs.Length == 2) // case: X->AB ==> X' ->A' | AB'
{
prefixProductions.Add(new Production(prefixNT, new GrammarSymbol[] { p.Rhs[0], prefixFor((Nonterminal)p.Rhs[1]) }));
prefixProductions.Add(new Production(prefixNT, new GrammarSymbol[] { prefixFor((Nonterminal)p.Rhs[0]) }));
}
else // case: X->a ==> X'->a
{
prefixProductions.Add(new Production(prefixNT, new GrammarSymbol[] { p.Rhs[0] }));
}
}
var res = new ContextFreeGrammar(prefixStart, prefixProductions);
res.setAcceptanceForEmptyString(true);
return(res);
}