private TransitionList CreateLookbackTransitions(LRItemSet sourceItems)
{
var newTransitions = new TransitionList();
//Build set of initial cores - this is optimization for performance
//We need to find all initial items in all states that shift into one of sourceItems
// Each such initial item would have the core from the "initial" cores set that we build from source items.
var iniCores = new LR0ItemSet();
foreach (var sourceItem in sourceItems)
{
iniCores.Add(sourceItem.Core.Production.LR0Items[0]);
}
//find
foreach (var state in _data.States)
{
foreach (var iniItem in state.BuilderData.InitialItems)
{
if (!iniCores.Contains(iniItem.Core))
{
continue;
}
var iniItemNt = iniItem.Core.Production.LValue; // iniItem's non-terminal (left side of production)
Transition lookback = null; // local var for lookback - transition over iniItemNt
var currItem = iniItem; // iniItem is initial item for all currItem's in the shift chain.
while (currItem != null)
{
if (sourceItems.Contains(currItem))
{
// We create transitions lazily, only when we actually need them. Check if we have iniItem's transition
// in local variable; if not, get it from state's transitions table; if not found, create it.
if (lookback == null && !state.BuilderData.Transitions.TryGetValue(iniItemNt, out lookback))
{
lookback = new Transition(state, iniItemNt);
newTransitions.Add(lookback);
}
//Now for currItem, either add trans to Lookbacks, or "include" it into currItem.Transition
// We need lookbacks ONLY for final items; for non-Final items we need proper Include lists on transitions
if (currItem.Core.IsFinal)
{
currItem.Lookbacks.Add(lookback);
}
else // if (currItem.Transition != null)
// Note: looks like checking for currItem.Transition is redundant - currItem is either:
// - Final - always the case for the first run of this method;
// - it has a transition after the first run, due to the way we select sourceItems list
// in SelectNewItemsThatNeedLookback (by transitions)
{
currItem.Transition.Include(lookback);
}
}//if
//move to next item
currItem = currItem.ShiftedItem;
} //while
} //foreach iniItem
} //foreach state
return(newTransitions);
}
private TransitionList CreateLookbackTransitions(LRItemSet sourceItems)
{
var newTransitions = new TransitionList();
//Build set of initial cores - this is optimization for performance
//We need to find all initial items in all states that shift into one of sourceItems
// Each such initial item would have the core from the "initial" cores set that we build from source items.
var iniCores = new LR0ItemSet();
foreach (var sourceItem in sourceItems)
{
iniCores.Add(sourceItem.Core.Production.LR0Items[0]);
}
//find
foreach (var state in Data.States)
{
foreach (var iniItem in state.BuilderData.InitialItems)
{
if (!iniCores.Contains(iniItem.Core))
{
continue;
}
var currItem = iniItem;
while (currItem != null)
{
if (sourceItems.Contains(currItem))
{
//iniItem is initial item for currItem (one of source items)
// check if transition for iniItem's non-terminal exists
var ntLeft = iniItem.Core.Production.LValue;
Transition trans;
if (!state.BuilderData.Transitions.TryGetValue(ntLeft, out trans))
{
trans = new Transition(iniItem.State, iniItem.Core.Production.LValue);
newTransitions.Add(trans);
}
//Now for currItem, either add trans to Lookbackbacks, or "include" it into currItem.Transition
if (currItem.Core.IsFinal)
{
currItem.Lookbacks.Add(trans);
}
else if (currItem.Transition != null)
{
currItem.Transition.Include(trans);
}
}//if
//move to next items
currItem = currItem.ShiftedItem;
} //while
} //foreach iniItem
} //foreach state
return(newTransitions);
}