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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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//----------------------------------------------------------------------------
// Copyright (C) 2003  Rafael H. Bordini, Jomi F. Hubner, et al.
// 
// This library is free software; you can redistribute it and/or
// modify it under the terms of the GNU Lesser General Public
// License as published by the Free Software Foundation; either
// version 2.1 of the License, or (at your option) any later version.
// 
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// Lesser General Public License for more details.
// 
// You should have received a copy of the GNU Lesser General Public
// License along with this library; if not, write to the Free Software
// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
// 
// To contact the authors:
// http://www.inf.ufrgs.br/~bordini
// http://www.das.ufsc.br/~jomi
//
//----------------------------------------------------------------------------

package jason.stdlib;

import jason.asSemantics.DefaultInternalAction;
import jason.asSemantics.InternalAction;
import jason.asSemantics.TransitionSystem;
import jason.asSemantics.Unifier;
import jason.asSyntax.ListTerm;
import jason.asSyntax.NumberTerm;
import jason.asSyntax.NumberTermImpl;
import jason.asSyntax.StringTerm;
import jason.asSyntax.Term;

/**

  Internal action: .length.

  
Description: gets the length of strings or lists. 

  
Parameters:

  + argument (string or list): the term whose length is to be determined.

  
+/- length (number). 
  

  Examples:

   .length("abc",X): X unifies with 3.
  
 .length([a,b],X): X unifies with 2.
  
 .length("a",2): false.
  

  @see jason.stdlib.concat
  @see jason.stdlib.delete
  @see jason.stdlib.max
  @see jason.stdlib.member
  @see jason.stdlib.min
  @see jason.stdlib.sort
  @see jason.stdlib.nth
  @see jason.stdlib.sort
  @see jason.stdlib.reverse

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

  @see jason.functions.Length function version
  
 */
public class length extends DefaultInternalAction {

    private static InternalAction singleton = null;
    public static InternalAction create() {
        if (singleton == null) 
            singleton = new length();
        return singleton;
    }
    
    @Override public int getMinArgs() { return 2; }
    @Override public int getMaxArgs() { return 2; }

    @Override
    public Object execute(TransitionSystem ts, Unifier un, Term[] args) throws Exception {
        checkArguments(args);
        Term l1 = args[0];
        Term l2 = args[1];

        NumberTerm size = null;
        if (l1.isList()) {
            ListTerm lt = (ListTerm) l1;
            size = new NumberTermImpl(lt.size());
        } else if (l1.isString()) {
            StringTerm st = (StringTerm) l1;
            size = new NumberTermImpl(st.getString().length());
        }
        if (size != null) {
            return un.unifies(l2, size);
        }
        return false;
    }
}