public void StateToStringShouldCreateCorrectFormat()
{
var state = new PredictionState(
new DottedRule(
Production.From(
NonTerminal.From("A"), NonTerminal.From("B"), NonTerminal.From("C")),
1),
0,
null);
Assert.AreEqual("A -> B\u25CFC\t\t(0)", state.ToString());
}
/// <summary>
/// Implements a check for leo quasi complete items
/// </summary>
/// <param name="state">the state to check for quasi completeness</param>
/// <returns>true if quasi complete, false otherwise</returns>
private bool IsNextStateQuasiComplete(PredictionState state)
{
var production = state.DottedRule.Production;
var symbolCount = production.Count;
if (symbolCount == 0)
{
return(true);
}
var nextDot = state.DottedRule.Dot + 1;
var isComplete = nextDot == production.Count;
if (isComplete)
{
return(true);
}
// if all subsequent symbols are nullable
for (var i = nextDot; i < production.Count; i++)
{
var nextSymbol = production[nextDot];
var isSymbolNullable = IsSymbolNullable(nextSymbol);
if (!isSymbolNullable)
{
return(false);
}
// From Page 4 of Leo's paper:
//
// "on a non-empty deterministic reduction path there always
// exists a topmost item if S =+> S is impossible.
// The easiest way to avoid problems in this respect is to augment
// the grammar with a new start symbol S'.
// this means adding the rule S'=>S as the start."
//
// to fix this, check if S can derive S. Basically if we are in the StartState state
// and the StartState state is found and is nullable, exit with false
if (Grammar.Start.Is(production) && Grammar.Start.Is(nextSymbol))
{
return(false);
}
}
return(true);
}
private void Predict(PredictionState evidence, int location)
{
var nonTerminal = evidence.DottedRule.PostDotSymbol as NonTerminal;
Debug.Assert(nonTerminal != null);
var rulesForNonTerminal = Grammar.ProductionsFor(nonTerminal);
foreach (var production in rulesForNonTerminal)
{
PredictProduction(location, production);
}
var isNullable = Grammar.IsTransitiveNullable(nonTerminal);
if (isNullable)
{
PredictAycockHorspool(evidence, location);
}
}