// Goes recursively, decreasing currentLevel until 0.
// Enumerator returns the same object for every yield, and it is the same object as its parameter 'path'.
// We do it to reuse a single StackPath instance in all iterations
private IEnumerable<GlrStackSlice> EnumerateTopStackSlices(GlrStackItem current, int currentLevel, GlrStackSlice path) {
path.Items[currentLevel] = current;
if (currentLevel == 0) { // we reach the start of the path
path.PrecedingItems = current.Previous;
yield return path;
} else {
foreach (var prev in current.Previous)
foreach (var subPath in EnumerateTopStackSlices(prev, currentLevel - 1, path)) {
yield return path; //pass path from child
}
}
}
public void ExecuteReduces(GlrStep step, GlrStackItem stackTop) {
var inputTerm = step.Input.Term;
GlrParserAction actionInfo;
if (!stackTop.State.GlrActions.TryGetValue(inputTerm, out actionInfo)) {
step.Discarded.Add(stackTop);
return;
}
// if this top is elligible for shift on input, add it to step.ShiftBases
if (actionInfo.Shifts.Count > 0)
step.ShiftBases.Add(stackTop);
foreach (var reduceItem in actionInfo.Reduces) {
if (!reduceItem.Lookaheads.Contains(inputTerm)) continue;
//execute reduce
var prod = reduceItem.Core.Production;
var prodLen = prod.RValues.Count;
var sliceBuffer = new GlrStackSlice(prodLen);
foreach (var slice in EnumerateTopStackSlices(stackTop, sliceBuffer)) {
// the slice contains top stack elements, we can now try to reduce
var newNode = prod.ExecuteReduce(slice.Items.Select(el => el.ParseNode));
//Shift over newNode from all stack elements preceding reduced segment
foreach (var prev in sliceBuffer.PrecedingItems) {
GlrParserAction actionFromPrev;
if (!prev.State.GlrActions.TryGetValue(newNode.Term, out actionFromPrev) || actionFromPrev.Shifts.Count == 0)
// TODO: possible it never happens in LALR, investigate!
continue; // no shifts over newNode;
// find next state after shifting node
var stateAfterShift = actionFromPrev.Shifts[0].ShiftedItem.State;
// do not check here if this state is compatible with input; if not it will be discarded inside the ExeduceReduces call that follows
var itemAfterShift = new GlrStackItem(stateAfterShift, newNode, prev);
ExecuteReduces(step, itemAfterShift);
}// foreach prev
}//foreach path
}//foreach reduce
}//method
private IEnumerable<GlrStackSlice> EnumerateTopStackSlices(GlrStackItem current, GlrStackSlice slice) {
if (slice.Length == 0) {
// special case, no child elements - empty production; we generate a single path consisting of 0 elements and PrecedingItems with only 'current' element.
// Note that PrecedingItems are those items (states) that will be used as bases (popped states)
// from which we will shift over created non-terminal. In case of empty production the base is the top element itself.
slice.PrecedingItems = new List<GlrStackItem>();
slice.PrecedingItems.Add(current);
return new GlrStackSlice[] {slice};
} else
return EnumerateTopStackSlices(current, slice.Length - 1, slice);
}