package tarau.jinni;

import java.util.Stack;

/**
   Implements a stack of undo actions
   for backtracking, and in particular,
   resetting a Var's val fiels to unbound (i.e. this).

   @see Prog
   @see Clause
   @see Term
   @see Var
*/
public class Trail extends Stack {

  Trail() {super();};

  public String name() {
    return "trail"+hashCode()%64;
  }

  public String pprint() {
    return name()+"\n"+super.toString()+"\n";
  }
 
  /**
    Used to undo bindings after unification,
    if we intend to leave no side effects.
  */
 
  synchronized final public void unwind(int to) {
     //IO.mes("unwind TRAIL: "+name()+": "+size()+"=>"+to);
     //if(to>size()) IO.assertion("unwind attempted from smaller to larger top");
     for(int i=size()-to; i>0; i--) {
       Term V=(Term)(pop());
       V.undo();
     }
  }

  public String stat() {
    return "Trail="+size();
  }
}