antlr
Class Lookahead

java.lang.Object
  |
  +--antlr.Lookahead

All Implemented Interfaces:

java.lang.Cloneable

public class Lookahead

extends java.lang.Object

implements java.lang.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) java.lang.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(BitSet p)
create a new lookahead set with the LL(1) set to the parameter

Lookahead(java.lang.String c)
create an empty lookahead set, but with cycle

Method Summary

java.lang.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()
java.lang.String	toString()
java.lang.String	toString(java.lang.String separator, CharFormatter formatter)
java.lang.String	toString(java.lang.String separator, CharFormatter formatter, Grammar g)
java.lang.String	toString(java.lang.String separator, Vector vocab)

Methods inherited from class java.lang.Object

equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Field Detail

fset

BitSet fset

actual bitset of the lookahead

cycle

java.lang.String cycle

is this computation part of a computation cycle?

epsilonDepth

BitSet epsilonDepth

What k values were being computed when end of rule hit?

hasEpsilon

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 Detail

Lookahead

public Lookahead()

Lookahead

public Lookahead(BitSet p)

create a new lookahead set with the LL(1) set to the parameter

Lookahead

public Lookahead(java.lang.String c)

create an empty lookahead set, but with cycle

Method Detail

clone

public java.lang.Object clone()

Make a deep copy of everything in this object

Overrides:

clone in class java.lang.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 java.lang.String toString()

Overrides:

toString in class java.lang.Object

toString

public java.lang.String toString(java.lang.String separator,
                                 CharFormatter formatter)

toString

public java.lang.String toString(java.lang.String separator,
                                 CharFormatter formatter,
                                 Grammar g)

toString

public java.lang.String toString(java.lang.String separator,
                                 Vector vocab)