//*
// * Finds the look-ahead set for a production pattern alternative.
// * The pattern position and maximum look-ahead length must be
// * specified. It is also possible to specify a look-ahead set
// * filter, which will make sure that unnecessary token sequences
// * will be avoided.
// *
// * @param alt the production pattern alternative
// * @param length the maximum look-ahead length
// * @param pos the pattern element position
// * @param stack the call stack used for loop detection
// * @param filter the look-ahead set filter
// *
// * @return the look-ahead set for the pattern alternative
// *
// * @throws ParserCreationException if an infinite loop was found
// * in the grammar
//
private LookAheadSet FindLookAhead(ProductionPatternAlternative alt, int length, int pos, CallStack stack, LookAheadSet filter)
{
LookAheadSet first = default(LookAheadSet);
LookAheadSet follow = default(LookAheadSet);
LookAheadSet overlaps = default(LookAheadSet);
// Check trivial cases
if (length <= 0 || pos >= alt.Count)
{
return(new LookAheadSet(0));
}
// Find look-ahead for this element
first = FindLookAhead(alt[pos], length, stack, filter);
if (alt[pos].MinCount == 0)
{
first.AddEmpty();
}
// Find remaining look-ahead
if (filter == null)
{
length -= first.GetMinLength();
if (length > 0)
{
follow = FindLookAhead(alt, length, pos + 1, stack, null);
first = first.CreateCombination(follow);
}
}
else if (filter.IsOverlap(first))
{
overlaps = first.CreateOverlaps(filter);
length -= overlaps.GetMinLength();
filter = filter.CreateFilter(overlaps);
follow = FindLookAhead(alt, length, pos + 1, stack, filter);
first.RemoveAll(overlaps);
first.AddAll(overlaps.CreateCombination(follow));
}
return(first);
}