private LookAheadSet FindLookAhead(ProductionPatternAlternative alt,
int length,
int pos,
CallStack stack,
LookAheadSet filter)
{
LookAheadSet follow;
// Check trivial cases
if (length <= 0 || pos >= alt.Count)
{
return(new LookAheadSet(0));
}
// Find look-ahead for this element
var 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))
{
var 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);
}
private void CalculateLookAhead(ProductionPatternAlternative alt,
int pos)
{
LookAheadSet previous = new LookAheadSet(0);
int length = 1;
// Check trivial cases
if (pos >= alt.Count)
{
return;
}
// Check for non-optional element
var pattern = alt.Pattern;
var elem = alt[pos];
if (elem.MinCount == elem.MaxCount)
{
CalculateLookAhead(alt, pos + 1);
return;
}
// Calculate simple look-aheads
var first = FindLookAhead(elem, 1, new CallStack(), null);
var follow = FindLookAhead(alt, 1, pos + 1, new CallStack(), null);
// Resolve conflicts
var location = "at position " + (pos + 1);
var conflicts = FindConflicts(pattern.Name,
location,
first,
follow);
while (conflicts.Size() > 0)
{
length++;
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);
}
