Package antlr

Class 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

      Fields 
      Modifier and Type Field Description
      (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

      Constructors 
      Constructor Description
      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

    • 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)