private (Item?, ParseAction) FindTransitionOfItemOnUnchecked(Item item, Symbol symbol)
{
var productionLen = item.Production.Ingredients.Length;
if (item.DotPos < productionLen && symbol == item.Production.Ingredients[item.DotPos])
{
return(item.ShiftedByOne(), ParseAction.MakeShift());
}
if (item.DotPos >= productionLen && symbol == item.Follow)
{
if (symbol == Symbol.EndOfInput && item.Production.Product == Symbol.ExtendedStartSymbol)
{
return(null, ParseAction.MakeAccept());
}
return(null, ParseAction.MakeReduce(item.Production));
}
return(null, ParseAction.MakeDiscard());
}
public ParseAction TransitionedOn(Symbol symbol, out ParserState?targetState)
{
var items = Expand();
var transitions = new HashSet <Transition>();
foreach (var item in items)
{
var(target, action) = mCfg.FindTransitionOfItemOn(item, symbol);
if (action.Type == ParseActionType.Discard)
{
continue;
}
var transition = new Transition(action, item, target);
transitions.Add(transition);
}
if (transitions.Count == 0)
{
targetState = null;
return(ParseAction.MakeReject());
}
if (transitions.Any(t => t.Action.Type == ParseActionType.Accept))
{
targetState = null;
return(ParseAction.MakeAccept());
}
if (transitions.Any(t => t.Action.Type == ParseActionType.Reduce && (
mCfg.AssociativityOf(t.Action.Production) == Associativity.Left ||
mCfg.PrecedenceOf(t.Action.Production) > mCfg.PrecedenceOf(symbol))))
{
transitions.RemoveWhere(t => t.Action.Type != ParseActionType.Reduce);
if (transitions.Count > 1)
{
throw new Exception("There is a reduce-reduce conflict.");
}
targetState = null;
return(ParseAction.MakeReduce(transitions.First().Action.Production));
}
if (transitions.All(t => t.Action.Type == ParseActionType.Reduce))
{
if (transitions.Count > 1)
{
throw new Exception("There is a reduce-reduce conflict.");
}
targetState = null;
return(ParseAction.MakeReduce(transitions.First().Action.Production));
}
transitions.RemoveWhere(t => t.Action.Type != ParseActionType.Shift);
if (transitions.Count == 0)
{
targetState = null;
return(ParseAction.MakeReject());
}
targetState = new ParserState(
mCfg,
transitions
.Select(t => t.Dest ?? throw new NullReferenceException())
.ToHashSet());
return(ParseAction.MakeShift());
}