package java_cup;

/** This class represents a transition in an LALR viable prefix recognition 
 *  machine.  Transitions can be under terminals for non-terminals.  They are
 *  internally linked together into singly linked lists containing all the 
 *  transitions out of a single state via the _next field.
 *
 * @see     java_cup.lalr_state
 * @version last updated: 11/25/95
 * @author  Scott Hudson
 *
 */
public class lalr_transition {

  /*-----------------------------------------------------------*/
  /*--- Constructor(s) ----------------------------------------*/
  /*-----------------------------------------------------------*/

  /** Full constructor.
   * @param on_sym  symbol we are transitioning on.
   * @param to_st   state we transition to.
   * @param nxt     next transition in linked list.
   */
  public lalr_transition(symbol on_sym, lalr_state to_st, lalr_transition nxt)
    throws internal_error
    {
      /* sanity checks */
      if (on_sym == null)
	throw new internal_error("Attempt to create transition on null symbol");
      if (to_st == null)
	throw new internal_error("Attempt to create transition to null state");

      /* initialize */
      _on_symbol = on_sym;
      _to_state  = to_st;
      _next      = nxt;
    }

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Constructor with null next. 
   * @param on_sym  symbol we are transitioning on.
   * @param to_st   state we transition to.
   */
  public lalr_transition(symbol on_sym, lalr_state to_st) throws internal_error
    {
      this(on_sym, to_st, null);
    }

  /*-----------------------------------------------------------*/
  /*--- (Access to) Instance Variables ------------------------*/
  /*-----------------------------------------------------------*/

  /** The symbol we make the transition on. */
  protected symbol _on_symbol;

  /** The symbol we make the transition on. */
  public symbol on_symbol() {return _on_symbol;}

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** The state we transition to. */
  protected lalr_state _to_state;

  /** The state we transition to. */
  public lalr_state to_state() {return _to_state;}

  /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/

  /** Next transition in linked list of transitions out of a state */
  protected lalr_transition _next;

  /** Next transition in linked list of transitions out of a state */
  public lalr_transition next() {return _next;}

  /*-----------------------------------------------------------*/
  /*--- General Methods ---------------------------------------*/
  /*-----------------------------------------------------------*/

  /** Convert to a string. */
  public String toString()
    {
      String result;

      result = "transition on " + on_symbol().name() + " to state [";
      result += _to_state.index();
      result += "]";

      return result;
    }

  /*-----------------------------------------------------------*/
};