public bool Add(Lr1Item <T> item)
{
// See if there already exists an item with the same core
var oldItem = Items.FirstOrDefault(f => f.ProductionRule == item.ProductionRule && f.DotLocation == item.DotLocation);
if (oldItem != null)
{
// There might be lookaheads that needs adding
bool addedLookahead = false;
foreach (var lookahead in item.Lookaheads)
{
addedLookahead |= oldItem.Lookaheads.Add(lookahead);
}
return(addedLookahead);
}
// There's no old item. Add the item and return true to indicate that we've added stuff
Items.Add(item);
return(true);
}
private Lr1ItemSet <T> Closure(IEnumerable <Lr1Item <T> > items, TerminalSet <T> first, ISet <NonTerminal <T> > nullable)
{
// The items themselves are always in their own closure set
Lr1ItemSet <T> closure = new Lr1ItemSet <T>();
foreach (Lr1Item <T> lr1Item in items)
{
closure.Add(lr1Item);
}
// This needs to be a normal for loop since we add to the underlying collection
// as we go along. This avoids investigating the same rule twice
for (int currentItem = 0; currentItem < closure.Count(); ++currentItem)
{
Lr1Item <T> item = closure[currentItem];
ISymbol <T> symbolRightOfDot = item.SymbolRightOfDot;
if (symbolRightOfDot != null)
{
// Generate the lookahead items
HashSet <Terminal <T> > lookaheads = new HashSet <Terminal <T> >();
bool nonNullableFound = false;
for (int i = item.DotLocation + 1; i < item.ProductionRule.Symbols.Length; ++i)
{
ISymbol <T> symbol = item.ProductionRule.Symbols[i];
// If symbol is terminal, just add it
if (symbol is Terminal <T> )
{
lookaheads.Add((Terminal <T>)symbol);
// Terminals are not nullable, break out of loop
nonNullableFound = true;
break;
}
foreach (Terminal <T> terminal in first[(NonTerminal <T>)symbol])
{
lookaheads.Add(terminal);
}
if (!nullable.Contains(symbol))
{
nonNullableFound = true;
break;
}
}
if (!nonNullableFound)
{
// Add each of the lookahead symbols of the generating rule to the new lookahead set
foreach (Terminal <T> lookahead in item.Lookaheads)
{
lookaheads.Add(lookahead);
}
}
// Create new Lr1 items from all rules where the resulting symbol of the production rule
// matches the symbol that was to the right of the dot.
foreach (Lr1Item <T> lr1Item in from f in grammar.ProductionRules
where f.ResultSymbol == symbolRightOfDot
select new Lr1Item <T>(f, 0, lookaheads))
{
closure.Add(lr1Item);
}
}
}
return(closure);
}
