//*
// * Calculates the look-aheads needed for the specified pattern
// * alternative. This method attempts to resolve any conflicts in
// * optional elements by recalculating look-aheads for referenced
// * productions.
// *
// * @param alt the production pattern alternative
// * @param pos the pattern element position
// *
// * @throws ParserCreationException if the look-ahead set couldn't
// * be determined due to inherent ambiguities
//
private void CalculateLookAhead(ProductionPatternAlternative alt, int pos)
{
ProductionPattern pattern = default(ProductionPattern);
ProductionPatternElement elem = default(ProductionPatternElement);
LookAheadSet first = default(LookAheadSet);
LookAheadSet follow = default(LookAheadSet);
LookAheadSet conflicts = default(LookAheadSet);
LookAheadSet previous = new LookAheadSet(0);
string location = null;
int length = 1;
// Check trivial cases
if (pos >= alt.Count)
{
return;
}
// Check for non-optional element
pattern = alt.Pattern;
elem = alt[pos];
if (elem.MinCount == elem.MaxCount)
{
CalculateLookAhead(alt, pos + 1);
return;
}
// Calculate simple look-aheads
first = FindLookAhead(elem, 1, new CallStack(), null);
follow = FindLookAhead(alt, 1, pos + 1, new CallStack(), null);
// Resolve conflicts
location = "at position " + (pos + 1);
conflicts = FindConflicts(pattern.Name, location, first, follow);
while (conflicts.Size() > 0)
{
length += 1;
conflicts.AddAll(previous);
first = FindLookAhead(elem, length, new CallStack(), conflicts);
follow = FindLookAhead(alt, length, pos + 1, new CallStack(), conflicts);
first = first.CreateCombination(follow);
elem.LookAhead = first;
if (first.Intersects(conflicts))
{
first = first.CreateIntersection(conflicts);
ThrowAmbiguityException(pattern.Name, location, first);
}
previous = conflicts;
conflicts = FindConflicts(pattern.Name, location, first, follow);
}
// Check remaining elements
CalculateLookAhead(alt, pos + 1);
}