Package antlr

Class Lookahead

java.lang.Object
antlr.Lookahead
All 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:
  • Field Details

    • fset

      BitSet fset
      actual bitset of the lookahead
    • cycle

      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 Details

    • Lookahead

      public Lookahead()
    • Lookahead

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

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

    • clone

      public Object clone()
      Make a deep copy of everything in this object
      Overrides:
      clone in class 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()
      Overrides:
      toString in class Object
    • 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)