private void SolveLeftRecursion(IEnumerable <Symbol>[] alphas)
{
if (alphas.Length == 0)
{
return;
}
var primeSymbol = Grammer.AddRule();
primeSymbol.RHSList.Add(Expression.E);
foreach (var alpha in alphas)
{
primeSymbol.RHSList.Add(Expression.Create(alpha.Append(primeSymbol)));
}
var betas = RHSList.Where(e => e.Value[0] != this).ToArray();
RHSList.Clear();
foreach (var beta in betas)
{
if (beta.IsEpsilon)
{
RHSList.Add(Expression.Create(primeSymbol));
}
else
{
RHSList.Add(beta.Append(primeSymbol));
}
}
//here we should apply left factoring on the prime symbol but the Grammer will do that.
}
public static Grammer Create(IEnumerable <char> alphabet, IEnumerable <string> rules, bool eleminateExtraRules)
{
var result = new Grammer(alphabet, rules);
result.Fix();
if (eleminateExtraRules)
{
result.EleminateExtralRules();
}
return(result);
}
internal void ApplyLeftFactoring()
{
var trie = TrieNode.CreateRoot();
foreach (var exp in RHSList)
{
if (exp.IsEpsilon)
{
continue;
}
trie.AddExpression(exp);
}
var alphas = trie.GetMinCommonExpressions();
alphas.Add(Expression.E);
var groupedExpressions = alphas.ToDictionary(a => a, a => new List <Expression>());
//Group them by there alpha and if non then epsilon is its alpha
foreach (var exp in RHSList)
{
var expAlpha = alphas.FirstOrDefault(a => exp.StartsWith(a)) ?? Expression.E;
groupedExpressions[expAlpha].Add(exp);
}
RHSList.Clear();
var primes = new List <NonTerminalSymbol>();
foreach (var grp in groupedExpressions)
{
if (grp.Key == Expression.E)
{
RHSList.AddRange(grp.Value);
}
else
{
var primeSymbol = Grammer.AddRule();
primeSymbol.RHSList.AddRange(grp.Value.Select(e => e.SubExpression(grp.Key.Count)));
RHSList.Add(Expression.Create(grp.Key.Value.Append(primeSymbol)));
primes.Add(primeSymbol);
}
}
foreach (var prime in primes)
{
prime.ApplyLeftFactoring();
}
}
internal NonTerminalSymbol(char val, bool isStartSymbol, Grammer grammer) : base(val)
{
Grammer = grammer;
IsStartSymbol = isStartSymbol;
}