import java.util.*;
aspect Follow {
  
  // *** API ***

  // A nonterminal has an attribute *follow*
  // - the set of terminal symbols that can follow this nonterminal in a derivation
  syn Set NDecl.follow() circular [new HashSet()];
  
  
  // *** Implementation ***
  
  eq NDecl.follow() {
    Set f = new HashSet();
    for (Iterator iter = occurences().iterator(); iter.hasNext();) {
      NUse u = (NUse) iter.next();
      f.addAll(u.follow());
    }
    return f;
  }
  
  syn Set NUse.follow() circular [new HashSet()]{
    if (nullableSuffix()) {
      Set s = new HashSet();
      s.addAll(firstSuffix());
      s.addAll(enclosingRule().getNDecl().follow());
      return s;
    }
    else {
      return firstSuffix();
    }
  }
  
  // Help attributes and methods

  syn lazy Set NDecl.occurences() = findUses(getID());
  inh boolean NUse.nullableSuffix(); // Returns true if all symbols following NUse are nullable
  inh Set NUse.firstSuffix(); // Returns the FIRST set of the substring following NUse
  inh Rule NUse.enclosingRule();

  eq Rule.getProd(int index).enclosingRule() = this;
  eq Prod.getSymbol(int index).nullableSuffix() {
    for (int i=index+1; i< getNumSymbol(); i++) {
      if (!getSymbol(i).nullable()) return false;
    }
    return true;
  }
  eq Prod.getSymbol(int index).firstSuffix() {
    Set s = new HashSet();
    for (int i=index+1; i< getNumSymbol(); i++) {
      s.addAll(getSymbol(i).first());
      if (!getSymbol(i).nullable()) return s;
    }
    return s;
  }
  
  inh Set NDecl.findUses(String name);
  eq CFG.getRule(int index).findUses(String name) {
     Set result = new HashSet();
     collectUses(result, name);
     return result;
  }
  protected void ASTNode.collectUses(Set s, String name) {
     for (int i=0; i<getNumChild();i++) {
        getChild(i).collectUses(s, name);
     }
  }
  protected void NUse.collectUses(Set s, String name) {
     if (name.equals(getID())) s.add(this);
     super.collectUses(s, name);
  }
      
}