tools/dasm/src/java_cup/non_terminal.java

package java_cup;

import java.util.Enumeration;
import java.util.Hashtable;

/** This class represents a non-terminal symbol in the grammar.  Each
 *  non terminal has a textual name, an index, and a string which indicates
 *  the type of object it will be implemented with at runtime (i.e. the class
 *  of object that will be pushed on the parse stack to represent it). 
 *
 * @version last updated: 11/25/95
 * @author  Scott Hudson
 */

public class non_terminal extends symbol {

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

  /** Full constructor.
   * @param nm  the name of the non terminal.
   * @param tp  the type string for the non terminal.
   */
  public non_terminal(String nm, String tp) 
    {
      /* super class does most of the work */
      super(nm, tp);

      /* add to set of all non terminals and check for duplicates */
      Object conflict = _all.put(nm,this);
      if (conflict != null)
    // can't throw an exception here because these are used in static
    // initializers, so we crash instead
    // was: 
    // throw new internal_error("Duplicate non-terminal ("+nm+") created");
    (new internal_error("Duplicate non-terminal ("+nm+") created")).crash();

      /* assign a unique index */
      _index = next_index++;
    }

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

  /** Constructor with default type. 
   * @param nm  the name of the non terminal.
   */
  public non_terminal(String nm) 
    {
      this(nm, null);
    }

  /*-----------------------------------------------------------*/
  /*--- (Access to) Static (Class) Variables ------------------*/
  /*-----------------------------------------------------------*/

  /** Table of all non-terminals -- elements are stored using name strings 
   *  as the key 
   */
  protected static Hashtable _all = new Hashtable();

  /** Access to all non-terminals. */
  public static Enumeration all() {return _all.elements();};

  /** lookup a non terminal by name string */ 
  public static non_terminal find(String with_name)
    {
      if (with_name == null)
        return null;
      else 
        return (non_terminal)_all.get(with_name);
    }

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

  /** Total number of non-terminals. */
  public static int number() {return _all.size();};

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

  /** Static counter to assign unique indexes. */
  protected static int next_index = 0;

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

  /** Static counter for creating unique non-terminal names */
  static protected int next_nt = 0;

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

  /** special non-terminal for start symbol */
  public static final non_terminal START_nt = new non_terminal("$START");

  /*-----------------------------------------------------------*/
  /*--- Static Methods ----------------------------------------*/
  /*-----------------------------------------------------------*/
     
  /** Method for creating a new uniquely named hidden non-terminal using 
   *  the given string as a base for the name (or "NT$" if null is passed).
   * @param prefix base name to construct unique name from. 
   */
  static non_terminal create_new(String prefix) throws internal_error
    {
      if (prefix == null) prefix = "NT$";
      return new non_terminal(prefix + next_nt++);
    }

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

  /** static routine for creating a new uniquely named hidden non-terminal */
  static non_terminal create_new() throws internal_error
    { 
      return create_new(null); 
    }

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

  /** Compute nullability of all non-terminals. */
  public static void compute_nullability() throws internal_error
    {
      boolean      change = true;
      non_terminal nt;
      Enumeration  e;
      production   prod;

      /* repeat this process until there is no change */
      while (change)
    {
      /* look for a new change */
      change = false;

      /* consider each non-terminal */
      for (e=all(); e.hasMoreElements(); )
        {
          nt = (non_terminal)e.nextElement();

          /* only look at things that aren't already marked nullable */
          if (!nt.nullable())
        {
          if (nt.looks_nullable())
            {
              nt._nullable = true;
              change = true;
            }
        }
        }
    }
      
      /* do one last pass over the productions to finalize all of them */
      for (e=production.all(); e.hasMoreElements(); )
    {
      prod = (production)e.nextElement();
      prod.set_nullable(prod.check_nullable());
    }
    }

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

  /** Compute first sets for all non-terminals.  This assumes nullability has
   *  already computed.
   */
  public static void compute_first_sets() throws internal_error
    {
      boolean      change = true;
      Enumeration  n;
      Enumeration  p;
      non_terminal nt;
      production   prod;
      terminal_set prod_first;

      /* repeat this process until we have no change */
      while (change)
    {
      /* look for a new change */
      change = false;

      /* consider each non-terminal */
      for (n = all(); n.hasMoreElements(); )
        {
          nt = (non_terminal)n.nextElement();

          /* consider every production of that non terminal */
          for (p = nt.productions(); p.hasMoreElements(); )
        {
          prod = (production)p.nextElement();

          /* get the updated first of that production */
          prod_first = prod.check_first_set();

          /* if this going to add anything, add it */
          if (!prod_first.is_subset_of(nt._first_set))
            {
              change = true;
              nt._first_set.add(prod_first);
            }
        }
        }
    }
    }

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

  /** Table of all productions with this non terminal on the LHS. */
  protected Hashtable _productions = new Hashtable(11);

  /** Access to productions with this non terminal on the LHS. */
  public Enumeration productions() {return _productions.elements();};

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

  /** Total number of productions with this non terminal on the LHS. */
  public int num_productions() {return _productions.size();};

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

  /** Add a production to our set of productions. */
  public void add_production(production prod) throws internal_error
    {
      /* catch improper productions */
      if (prod == null || prod.lhs() == null || prod.lhs().the_symbol() != this)
    throw new internal_error(
      "Attempt to add invalid production to non terminal production table");

      /* add it to the table, keyed with itself */
      _productions.put(prod,prod);
    }

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

  /** Nullability of this non terminal. */
  protected boolean _nullable;

  /** Nullability of this non terminal. */
  public boolean nullable() {return _nullable;}

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

  /** First set for this non-terminal. */
  protected terminal_set _first_set = new terminal_set();

  /** First set for this non-terminal. */
  public terminal_set first_set() {return _first_set;}

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

  /** Indicate that this symbol is a non-terminal. */
  public boolean is_non_term() 
    {
      return true;
    }

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

  /** Test to see if this non terminal currently looks nullable. */
  protected boolean looks_nullable() throws internal_error
    {
      /* look and see if any of the productions now look nullable */
      for (Enumeration e = productions(); e.hasMoreElements(); )
    /* if the production can go to empty, we are nullable */
    if (((production)e.nextElement()).check_nullable())
      return true;

      /* none of the productions can go to empty, so we are not nullable */
      return false;
    }

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

  /** convert to string */
  public String toString()
    {
      return super.toString() + "[" + index() + "]" + (nullable() ? "*" : "");
    }

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