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Lookahead.javaAPI DocGlassfish v2 API6467Wed Aug 30 15:34:08 BST 2006persistence.antlr

Lookahead

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
persistence.antlr.Lookahead#combineWith(Lookahead)

Fields Summary
BitSet
fset
actual bitset of the lookahead
String
cycle
is this computation part of a computation cycle?
BitSet
epsilonDepth
What k values were being computed when end of rule hit?
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.
Constructors Summary
public Lookahead()


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

        fset = p;
public Lookahead(String c)
create an empty lookahead set, but with cycle

        this();
        cycle = c;
Methods Summary
public java.lang.Objectclone()
Make a deep copy of everything in this object

        Lookahead p = null;
        try {
            p = (Lookahead)super.clone();
            p.fset = (BitSet)fset.clone();
            p.cycle = cycle; // strings are immutable
            if (epsilonDepth != null) {
                p.epsilonDepth = (BitSet)epsilonDepth.clone();
            }
        }
        catch (CloneNotSupportedException e) {
            throw new InternalError();
        }
        return p;
public voidcombineWith(persistence.antlr.Lookahead q)

        if (cycle == null) {	// track at least one cycle
            cycle = q.cycle;
        }

        if (q.containsEpsilon()) {
            hasEpsilon = true;
        }

        // combine epsilon depths
        if (epsilonDepth != null) {
            if (q.epsilonDepth != null) {
                epsilonDepth.orInPlace(q.epsilonDepth);
            }
        }
        else if (q.epsilonDepth != null) {
            epsilonDepth = (BitSet)q.epsilonDepth.clone();
        }
        fset.orInPlace(q.fset);
public booleancontainsEpsilon()

        return hasEpsilon;
public persistence.antlr.Lookaheadintersection(persistence.antlr.Lookahead q)
What is the intersection of two lookahead depths? Only the Epsilon "bit" and bitset are considered.

        Lookahead p = new Lookahead(fset.and(q.fset));
        if (this.hasEpsilon && q.hasEpsilon) {
            p.setEpsilon();
        }
        return p;
public booleannil()

        return fset.nil() && !hasEpsilon;
public static persistence.antlr.Lookaheadof(int el)

        Lookahead look = new Lookahead();
        look.fset.add(el);
        return look;
public voidresetEpsilon()

        hasEpsilon = false;
public voidsetEpsilon()

        hasEpsilon = true;
public java.lang.StringtoString()

        String e = "",b,f = "",d = "";
        b = fset.toString(",");
        if (containsEpsilon()) {
            e = "+<epsilon>";
        }
        if (cycle != null) {
            f = "; FOLLOW(" + cycle + ")";
        }
        if (epsilonDepth != null) {
            d = "; depths=" + epsilonDepth.toString(",");
        }
        return b + e + f + d;
public java.lang.StringtoString(java.lang.String separator, persistence.antlr.CharFormatter formatter)

        String e = "",b,f = "",d = "";
        b = fset.toString(separator, formatter);
        if (containsEpsilon()) {
            e = "+<epsilon>";
        }
        if (cycle != null) {
            f = "; FOLLOW(" + cycle + ")";
        }
        if (epsilonDepth != null) {
            d = "; depths=" + epsilonDepth.toString(",");
        }
        return b + e + f + d;
public java.lang.StringtoString(java.lang.String separator, persistence.antlr.CharFormatter formatter, persistence.antlr.Grammar g)

        if (g instanceof LexerGrammar) {
            return toString(separator, formatter);
        }
        else {
            return toString(separator, g.tokenManager.getVocabulary());
        }
public java.lang.StringtoString(java.lang.String separator, persistence.antlr.collections.impl.Vector vocab)

        String b,f = "",d = "";
        b = fset.toString(separator, vocab);
        if (cycle != null) {
            f = "; FOLLOW(" + cycle + ")";
        }
        if (epsilonDepth != null) {
            d = "; depths=" + epsilonDepth.toString(",");
        }
        return b + f + d;