/**
* Finds the look-ahead set for a production pattern. The 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 pattern the production pattern
* @param length the maximum look-ahead length
* @param stack the call stack used for loop detection
* @param filter the look-ahead set filter
*
* @return the look-ahead set for the production pattern
*
* @throws ParserCreationException if an infinite loop was found
* in the grammar
*/
private LookAheadSet FindLookAhead(ProductionPattern pattern,
int length,
CallStack stack,
LookAheadSet filter)
{
LookAheadSet result;
LookAheadSet temp;
// Check for infinite loop
if (stack.Contains(pattern.Name, length))
{
throw new ParserCreationException(
ParserCreationException.ErrorType.INFINITE_LOOP,
pattern.Name,
(String)null);
}
// Find pattern look-ahead
stack.Push(pattern.Name, length);
result = new LookAheadSet(length);
for (int i = 0; i < pattern.Count; i++)
{
temp = FindLookAhead(pattern[i],
length,
0,
stack,
filter);
result.AddAll(temp);
}
stack.Pop();
return(result);
}
/**
* Finds the look-ahead set for a production pattern element. The
* maximum look-ahead length must be specified. This method takes
* the element repeats into consideration when creating the
* look-ahead set, but does NOT include an empty sequence even if
* the minimum count is zero (0). It is also possible to specify a
* look-ahead set filter, which will make sure that unnecessary
* token sequences will be avoided.
*
* @param elem the production pattern element
* @param length the maximum look-ahead length
* @param stack the call stack used for loop detection
* @param filter the look-ahead set filter
*
* @return the look-ahead set for the pattern element
*
* @throws ParserCreationException if an infinite loop was found
* in the grammar
*/
private LookAheadSet FindLookAhead(ProductionPatternElement elem,
int length,
CallStack stack,
LookAheadSet filter)
{
LookAheadSet result;
LookAheadSet first;
LookAheadSet follow;
int max;
// Find initial element look-ahead
first = FindLookAhead(elem, length, 0, stack, filter);
result = new LookAheadSet(length);
result.AddAll(first);
if (filter == null || !filter.IsOverlap(result))
{
return(result);
}
// Handle element repetitions
if (elem.MaxCount == Int32.MaxValue)
{
first = first.CreateRepetitive();
}
max = elem.MaxCount;
if (length < max)
{
max = length;
}
for (int i = 1; i < max; i++)
{
first = first.CreateOverlaps(filter);
if (first.Size() <= 0 || first.GetMinLength() >= length)
{
break;
}
follow = FindLookAhead(elem,
length,
0,
stack,
filter.CreateFilter(first));
first = first.CreateCombination(follow);
result.AddAll(first);
}
return(result);
}
/**
* Returns the union of all alternative look-ahead sets in a
* production pattern.
*
* @param pattern the production pattern
*
* @return a unified look-ahead set
*/
private LookAheadSet FindUnion(ProductionPattern pattern)
{
LookAheadSet result;
int length = 0;
int i;
for (i = 0; i < pattern.Count; i++)
{
result = pattern[i].LookAhead;
if (result.GetMaxLength() > length)
{
length = result.GetMaxLength();
}
}
result = new LookAheadSet(length);
for (i = 0; i < pattern.Count; i++)
{
result.AddAll(pattern[i].LookAhead);
}
return(result);
}
/**
* Returns a look-ahead set with all conflics between
* alternatives in a production pattern.
*
* @param pattern the production pattern
* @param maxLength the maximum token sequence length
*
* @return a look-ahead set with the conflicts found
*
* @throws ParserCreationException if an inherent ambiguity was
* found among the look-ahead sets
*/
private LookAheadSet FindConflicts(ProductionPattern pattern,
int maxLength)
{
LookAheadSet result = new LookAheadSet(maxLength);
LookAheadSet set1;
LookAheadSet set2;
for (int i = 0; i < pattern.Count; i++)
{
set1 = pattern[i].LookAhead;
for (int j = 0; j < i; j++)
{
set2 = pattern[j].LookAhead;
result.AddAll(set1.CreateIntersection(set2));
}
}
if (result.IsRepetitive())
{
ThrowAmbiguityException(pattern.Name, null, result);
}
return(result);
}
/**
* Creates a new look-ahead set that is the combination of
* this set with another set. The combination is created by
* creating new token sequences that consist of appending all
* elements from the specified set onto all elements in this
* set. This is sometimes referred to as the cartesian
* product.
*
* @param set the set to combine with
*
* @return a new look-ahead set containing the combination
*/
public LookAheadSet CreateCombination(LookAheadSet set)
{
LookAheadSet result = new LookAheadSet(maxLength);
Sequence first;
Sequence second;
// Handle special cases
if (this.Size() <= 0)
{
return(set);
}
else if (set.Size() <= 0)
{
return(this);
}
// Create combinations
for (int i = 0; i < elements.Count; i++)
{
first = (Sequence)elements[i];
if (first.Length() >= maxLength)
{
result.Add(first);
}
else if (first.Length() <= 0)
{
result.AddAll(set);
}
else
{
for (int j = 0; j < set.elements.Count; j++)
{
second = (Sequence)set.elements[j];
result.Add(first.Concat(maxLength, second));
}
}
}
return(result);
}
/**
* Calculates the look-ahead needed for the specified production
* pattern. This method attempts to resolve any conflicts and
* stores the results in the pattern look-ahead object.
*
* @param pattern the production pattern
*
* @throws ParserCreationException if the look-ahead set couldn't
* be determined due to inherent ambiguities
*/
private void CalculateLookAhead(ProductionPattern pattern)
{
ProductionPatternAlternative alt;
LookAheadSet result;
LookAheadSet[] alternatives;
LookAheadSet conflicts;
LookAheadSet previous = new LookAheadSet(0);
int length = 1;
int i;
CallStack stack = new CallStack();
// Calculate simple look-ahead
stack.Push(pattern.Name, 1);
result = new LookAheadSet(1);
alternatives = new LookAheadSet[pattern.Count];
for (i = 0; i < pattern.Count; i++)
{
alt = pattern[i];
alternatives[i] = FindLookAhead(alt, 1, 0, stack, null);
alt.LookAhead = alternatives[i];
result.AddAll(alternatives[i]);
}
if (pattern.LookAhead == null)
{
pattern.LookAhead = result;
}
conflicts = FindConflicts(pattern, 1);
// Resolve conflicts
while (conflicts.Size() > 0)
{
length++;
stack.Clear();
stack.Push(pattern.Name, length);
conflicts.AddAll(previous);
for (i = 0; i < pattern.Count; i++)
{
alt = pattern[i];
if (alternatives[i].Intersects(conflicts))
{
alternatives[i] = FindLookAhead(alt,
length,
0,
stack,
conflicts);
alt.LookAhead = alternatives[i];
}
if (alternatives[i].Intersects(conflicts))
{
if (pattern.DefaultAlternative == null)
{
pattern.DefaultAlternative = alt;
}
else if (pattern.DefaultAlternative != alt)
{
result = alternatives[i].CreateIntersection(conflicts);
ThrowAmbiguityException(pattern.Name,
null,
result);
}
}
}
previous = conflicts;
conflicts = FindConflicts(pattern, length);
}
// Resolve conflicts inside rules
for (i = 0; i < pattern.Count; i++)
{
CalculateLookAhead(pattern[i], 0);
}
}