antlr

Class Lookahead

Implemented Interfaces:
Cloneable

public class Lookahead
extends Object
implements Cloneable

This object holds all information needed to represent the lookahead for any particular lookahead computation for a single lookahead depth. Final lookahead information is a simple bit set, but intermediate stages need computation cycle and FOLLOW information.

Concerning the cycle variable. If lookahead is computed for a RuleEnd node, then computation is part of a FOLLOW cycle for this rule. If lookahead is computed for a RuleBlock node, the computation is part of a FIRST cycle to this rule.

Concerning the epsilonDepth variable. This is not the depth relative to the rule reference that epsilon was encountered. That value is

 		initial_k - epsilonDepth + 1
 
Also, lookahead depths past rule ref for local follow are:
 		initial_k - (initial_k - epsilonDepth)
 
Used for rule references. If we try to compute look(k, ruleref) and there are fewer than k lookahead terminals before the end of the the rule, epsilon will be returned (don't want to pass the end of the rule). We must track when the the lookahead got stuck. For example,
 		a : b A B E F G;
 		b : C ;
 
LOOK(5, ref-to(b)) is {} with depth = 4, which indicates that at 2 (5-4+1) tokens ahead, end of rule was reached. Therefore, the token at 4=5-(5-4) past rule ref b must be included in the set == F. The situation is complicated by the fact that a computation may hit the end of a rule at many different depths. For example,
 		a : b A B C ;
 		b : E F		// epsilon depth of 1 relative to initial k=3
 		  | G		// epsilon depth of 2
 		  ;
 
Here, LOOK(3,ref-to(b)) returns epsilon, but the depths are {1, 2}; i.e., 3-(3-1) and 3-(3-2). Those are the lookahead depths past the rule ref needed for the local follow.

This is null unless an epsilon is created.

See Also:
combineWith(Lookahead)

Field Summary

(package private) String
cycle
is this computation part of a computation cycle?
(package private) BitSet
epsilonDepth
What k values were being computed when end of rule hit?
(package private) BitSet
fset
actual bitset of the lookahead
(package private) boolean
hasEpsilon
Does this lookahead depth include Epsilon token type? This is used to avoid having a bit in the set for Epsilon as it conflicts with parsing binary files.

Constructor Summary

Lookahead()
Lookahead(String c)
create an empty lookahead set, but with cycle
Lookahead(BitSet p)
create a new lookahead set with the LL(1) set to the parameter

Method Summary

Object
clone()
Make a deep copy of everything in this object
void
combineWith(Lookahead q)
boolean
containsEpsilon()
Lookahead
intersection(Lookahead q)
What is the intersection of two lookahead depths? Only the Epsilon "bit" and bitset are considered.
boolean
nil()
static Lookahead
of(int el)
void
resetEpsilon()
void
setEpsilon()
String
toString()
String
toString(String separator, CharFormatter formatter)
String
toString(String separator, CharFormatter formatter, Grammar g)
String
toString(String separator, Vector vocab)

Field Details

cycle

(package private)  String cycle
is this computation part of a computation cycle?


epsilonDepth

(package private)  BitSet epsilonDepth
What k values were being computed when end of rule hit?


fset

(package private)  BitSet fset
actual bitset of the lookahead


hasEpsilon

(package private)  boolean hasEpsilon
Does this lookahead depth include Epsilon token type? This is used to avoid having a bit in the set for Epsilon as it conflicts with parsing binary files.

Constructor Details

Lookahead

public Lookahead()


Lookahead

public Lookahead(String c)
create an empty lookahead set, but with cycle


Lookahead

public Lookahead(BitSet p)
create a new lookahead set with the LL(1) set to the parameter

Method Details

clone

public Object clone()
Make a deep copy of everything in this object


combineWith

public void combineWith(Lookahead q)


containsEpsilon

public boolean containsEpsilon()


intersection

public Lookahead intersection(Lookahead q)
What is the intersection of two lookahead depths? Only the Epsilon "bit" and bitset are considered.


nil

public boolean nil()


of

public static Lookahead of(int el)


resetEpsilon

public void resetEpsilon()


setEpsilon

public void setEpsilon()


toString

public String toString()


toString

public String toString(String separator,
                       CharFormatter formatter)


toString

public String toString(String separator,
                       CharFormatter formatter,
                       Grammar g)


toString

public String toString(String separator,
                       Vector vocab)