//Creates closure items with "spontaneously generated" lookaheads
private bool AddClosureItems(ParserState state)
{
bool result = false;
//note that we change collection while we iterate thru it, so we have to use "for i" loop
for (int i = 0; i < state.Items.Count; i++)
{
LRItem item = state.Items[i];
NonTerminal nextNT = item.Core.NextElement as NonTerminal;
if (nextNT == null)
{
continue;
}
//1. Add normal closure items
foreach (Production prod in nextNT.Productions)
{
LR0Item core = prod.LR0Items[0]; //item at zero index is the one that starts with dot
LRItem newItem = TryFindItem(state, core);
if (newItem == null)
{
newItem = new LRItem(state, core);
state.Items.Add(newItem);
result = true;
}
#region Comments on lookaheads processing
// The general idea of generating ("spontaneously") the lookaheads is the following:
// Let's the original item be in the form
// [A -> alpha . B beta , lset]
// where <B> is a non-terminal and <lset> is a set of lookaheads,
// <beta> is some string (B's tail in our terminology)
// Then the closure item on non-teminal B is an item
// [B -> x, firsts(beta + lset)]
// (the lookahead set is expression after the comma).
// To generate lookaheads on a closure item, we simply take "firsts"
// from the tail <beta> of the NonTerminal <B>.
// Normally if tail <beta> is nullable we would add ("propagate")
// the <lset> lookaheads from <A> to <B>.
// We dont' do it right here - we simply add a propagation link.
// We propagate all lookaheads later in a separate process.
#endregion
newItem.NewLookaheads.AddRange(item.Core.TailFirsts);
if (item.Core.TailIsNullable && !item.PropagateTargets.Contains(newItem))
{
item.PropagateTargets.Add(newItem);
}
} //foreach prod
} //for i (LRItem)
return(result);
}
} //method
#endregion
#region Creating parser states
private void CreateParserStates()
{
Data.States.Clear();
_stateHash = new ParserStateTable();
//Create initial state
//there is always just one initial production Root' -> Root + LF, and we're interested in LR item at 0 index
LR0ItemList itemList = new LR0ItemList();
itemList.Add(Data.AugmentedRoot.Productions[0].LR0Items[0]);
Data.InitialState = FindOrCreateState(itemList); //it is actually create
Data.InitialState.Items[0].NewLookaheads.Add(Grammar.Eof.Key);
//create final state - we need to create it explicitly to assign to _data.FinalState property
// final state is based on the same initial production, but different LRItem - the one with dot AFTER the root nonterminal.
// it is item at index 1.
itemList = new LR0ItemList();
itemList.Add(Data.AugmentedRoot.Productions[0].LR0Items[1]);
Data.FinalState = FindOrCreateState(itemList);
// Iterate through states (while new ones are created) and create shift transitions and new states
for (int index = 0; index < Data.States.Count; index++)
{
ParserState state = Data.States[index];
AddClosureItems(state);
//Get keys of all possible shifts
ShiftTable shiftTable = GetStateShifts(state);
//Each key in shifts dict is an input element
// Value is LR0ItemList of shifted LR0Items for this input element.
foreach (string input in shiftTable.Keys)
{
LR0ItemList shiftedCoreItems = shiftTable[input];
ParserState newState = FindOrCreateState(shiftedCoreItems);
state.Actions[input] = new ActionRecord(input, ParserActionType.Shift, newState, null);
//link original LRItems in original state to derived LRItems in newState
foreach (LR0Item coreItem in shiftedCoreItems)
{
LRItem fromItem = FindItem(state, coreItem.Production, coreItem.Position - 1);
LRItem toItem = FindItem(newState, coreItem.Production, coreItem.Position);
if (!fromItem.PropagateTargets.Contains(toItem))
{
fromItem.PropagateTargets.Add(toItem);
}
} //foreach coreItem
} //foreach input
} //for index
} //method
private void CreateParserStates()
{
Data.States.Clear();
_stateHash = new ParserStateTable();
CreateInitialAndFinalStates();
string augmRootKey = Data.AugmentedRoot.Key;
// Iterate through states (while new ones are created) and create shift transitions and new states
for (int index = 0; index < Data.States.Count; index++)
{
ParserState state = Data.States[index];
AddClosureItems(state);
//Get keys of all possible shifts
ShiftTable shiftTable = GetStateShifts(state);
//Each key in shifts dict is an input element
// Value is LR0ItemList of shifted LR0Items for this input element.
foreach (string input in shiftTable.Keys)
{
LR0ItemList shiftedCoreItems = shiftTable[input];
ParserState newState = FindOrCreateState(shiftedCoreItems);
state.Actions[input] = new ActionRecord(input, ParserActionType.Shift, newState, null);
//link original LRItems in original state to derived LRItems in newState
foreach (LR0Item coreItem in shiftedCoreItems)
{
LRItem fromItem = FindItem(state, coreItem.Production, coreItem.Position - 1);
LRItem toItem = FindItem(newState, coreItem.Production, coreItem.Position);
if (!fromItem.PropagateTargets.Contains(toItem))
{
fromItem.PropagateTargets.Add(toItem);
}
} //foreach coreItem
} //foreach input
} //for index
Data.FinalState = Data.InitialState.Actions[Data.AugmentedRoot.Key].NewState;
} //method
public ParserState(string name, LRItem item)
{
Name = name;
Items.Add(item);
}
