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Jason is a fully-fledged interpreter for an extended version of AgentSpeak, a BDI agent-oriented logic programming language.
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package jason.stdlib;

import jason.JasonException;
import jason.asSemantics.DefaultInternalAction;
import jason.asSemantics.InternalAction;
import jason.asSemantics.TransitionSystem;
import jason.asSemantics.Unifier;
import jason.asSyntax.ListTerm;
import jason.asSyntax.Term;

import java.util.Iterator;

/**

  Internal action: .member(T,L).
  
  
Description: checks if some term T is in a list L. If
  T is a free variable, this internal action backtracks all
  possible values for T.

  
Parameters:

  
  +/- member (term): the term to be checked.
  + list (list): the list where the term should be in.
  
  
  
  Examples:
 

   .member(c,[a,b,c]): true.
   .member(3,[a,b,c]): false.
   .member(X,[a,b,c]): unifies X with any member of the list.

  

  @see jason.stdlib.concat
  @see jason.stdlib.length
  @see jason.stdlib.sort
  @see jason.stdlib.nth
  @see jason.stdlib.max
  @see jason.stdlib.min
  @see jason.stdlib.reverse

  @see jason.stdlib.difference
  @see jason.stdlib.intersection
  @see jason.stdlib.union

*/
public class member extends DefaultInternalAction {
    
    private static InternalAction singleton = null;
    public static InternalAction create() {
        if (singleton == null) 
            singleton = new member();
        return singleton;
    }
    
    @Override public int getMinArgs() { return 2; }
    @Override public int getMaxArgs() { return 2; }

    @Override protected void checkArguments(Term[] args) throws JasonException {
        super.checkArguments(args); // check number of arguments
        if (!args[1].isList())
            throw JasonException.createWrongArgument(this,"second argument must be a list");
    }
    
    
    
    @Override
    public Object execute(TransitionSystem ts, final Unifier un, Term[] args) throws Exception {
        checkArguments(args);

        final Term member = args[0];
        final Iterator i = ((ListTerm)args[1]).iterator();

        return new Iterator() {
            Unifier c = null; // the current response (which is an unifier)
            
            public boolean hasNext() {
                if (c == null) // the first call of hasNext should find the first response 
                    find();
                return c != null; 
            }

            public Unifier next() {
                if (c == null) find();
                Unifier b = c;
                find(); // find next response
                return b;
            }
            
            void find() {
                while (i.hasNext()) {
                    c = un.clone();
                    if (c.unifiesNoUndo(member, i.next()))
                        return; // member found in the list, c is the current response
                }
                c = null; // no member is found, 
            }

            public void remove() {}
        };
        
        /* -- old version of the implementation
         * -- problem: even if the user wants only the first member, if search all
        List answers = new ArrayList();
        Unifier newUn = (Unifier)un.clone(); // clone un so as not to change it
        for (Term t: lt) {
            if (newUn.unifies(member, t)) {
                // add this unification to the  answers
                answers.add(newUn);
                newUn = (Unifier)un.clone(); // creates a new clone of un
            }
        }                
        return answers.iterator();
        */
    }
}