src/java_cup/lalr_state.java

package java_cup;

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

/** This class represents a state in the LALR viable prefix recognition machine.
 *  A state consists of an LALR item set and a set of transitions to other
 *  states under terminal and non-terminal symbols.  Each state represents
 *  a potential configuration of the parser.  If the item set of a state
 *  includes an item such as: <pre>
 *    [A ::= B * C d E , {a,b,c}]
 *  </pre>
 *  this indicates that when the parser is in this state it is currently
 *  looking for an A of the given form, has already seen the B, and would
 *  expect to see an a, b, or c after this sequence is complete.  Note that
 *  the parser is normally looking for several things at once (represented
 *  by several items).  In our example above, the state would also include
 *  items such as: <pre>
 *    [C ::= * X e Z, {d}]
 *    [X ::= * f, {e}]
 *  </pre>
 *  to indicate that it was currently looking for a C followed by a d (which
 *  would be reduced into a C, matching the first symbol in our production
 *  above), and the terminal f followed by e.<p>
 *
 *  At runtime, the parser uses a viable prefix recognition machine made up
 *  of these states to parse.  The parser has two operations, shift and reduce.
 *  In a shift, it consumes one token and makes a transition to a new state.
 *  This corresponds to "moving the dot past" a terminal in one or more items
 *  in the state (these new shifted items will then be found in the state at
 *  the end of the transition).  For a reduce operation, the parser is
 *  signifying that it is recognizing the RHS of some production.  To do this
 *  it first "backs up" by popping a stack of previously saved states.  It
 *  pops off the same number of states as are found in the RHS of the
 *  production.  This leaves the machine in the same state is was in when the
 *  parser first attempted to find the RHS.  From this state it makes a
 *  transition based on the non-terminal on the LHS of the production.  This
 *  corresponds to placing the parse in a configuration equivalent to having
 *  replaced all the symbols from the the input corresponding to the RHS with
 *  the symbol on the LHS.
 *
 * @see     java_cup.lalr_item
 * @see     java_cup.lalr_item_set
 * @see     java_cup.lalr_transition
 * @version last updated: 11/25/95
 * @author  Scott Hudson
 *
 */

public class lalr_state {
  /*-----------------------------------------------------------*/
  /*--- Constructor(s) ----------------------------------------*/
  /*-----------------------------------------------------------*/

  /** Constructor for building a state from a set of items.
   * @param itms the set of items that makes up this state.
   */
  public lalr_state(lalr_item_set itms) throws internal_error
   {
     /* don't allow null or duplicate item sets */
     if (itms == null)
       throw new internal_error(
     "Attempt to construct an LALR state from a null item set");

     if (find_state(itms) != null)
       throw new internal_error(
     "Attempt to construct a duplicate LALR state");

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

     /* store the items */
     _items = itms;

     /* add to the global collection, keyed with its item set */
     _all.put(_items,this);
   }

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

  /** Collection of all states. */
  protected static Hashtable _all = new Hashtable();

  /** Collection of all states. */
  public static Enumeration all() {return _all.elements();}

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

  /** Indicate total number of states there are. */
  public static int number() {return _all.size();}

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

  /** Hash table to find states by their kernels (i.e, the original,
   *  unclosed, set of items -- which uniquely define the state).  This table
   *  stores state objects using (a copy of) their kernel item sets as keys.
   */
  protected static Hashtable _all_kernels = new Hashtable();

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

  /** Find and return state with a given a kernel item set (or null if not
   *  found).  The kernel item set is the subset of items that were used to
   *  originally create the state.  These items are formed by "shifting the
   *  dot" within items of other states that have a transition to this one.
   *  The remaining elements of this state's item set are added during closure.
   * @param itms the kernel set of the state we are looking for.
   */
  public static lalr_state find_state(lalr_item_set itms)
    {
      if (itms == null)
      return null;
      else
      return (lalr_state)_all.get(itms);
    }

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

  /** Static counter for assigning unique state indexes. */
  protected static int next_index = 0;

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

  /** The item set for this state. */
  protected lalr_item_set _items;

  /** The item set for this state. */
  public lalr_item_set items() {return _items;}

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

  /** List of transitions out of this state. */
  protected lalr_transition _transitions = null;

  /** List of transitions out of this state. */
  public lalr_transition transitions() {return _transitions;}

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

  /** Index of this state in the parse tables */
  protected int _index;

  /** Index of this state in the parse tables */
  public int index() {return _index;}

  /*-----------------------------------------------------------*/
  /*--- Static Methods ----------------------------------------*/
  /*-----------------------------------------------------------*/

  /** Helper routine for debugging -- produces a dump of the given state
    * onto System.out.
    */
  protected static void dump_state(lalr_state st) throws internal_error
    {
      lalr_item_set itms;
      lalr_item itm;
      production_part part;

      if (st == null)
    {
      System.out.println("NULL lalr_state");
      return;
    }

      System.out.println("lalr_state [" + st.index() + "] {");
      itms = st.items();
      for (Enumeration e = itms.all(); e.hasMoreElements(); )
    {
      itm = (lalr_item)e.nextElement();
      System.out.print("  [");
      System.out.print(itm.the_production().lhs().the_symbol().name());
      System.out.print(" ::= ");
      for (int i = 0; i<itm.the_production().rhs_length(); i++)
        {
          if (i == itm.dot_pos()) System.out.print("(*) ");
          part = itm.the_production().rhs(i);
          if (part.is_action())
        System.out.print("{action} ");
          else
        System.out.print(((symbol_part)part).the_symbol().name() + " ");
        }
      if (itm.dot_at_end()) System.out.print("(*) ");
      System.out.println("]");
    }
      System.out.println("}");
    }

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

  /** Propagate lookahead sets through the constructed viable prefix
   *  recognizer.  When the machine is constructed, each item that results
      in the creation of another such that its lookahead is included in the
      other's will have a propagate link set up for it.  This allows additions
      to the lookahead of one item to be included in other items that it
      was used to directly or indirectly create.
   */
  protected static void propagate_all_lookaheads() throws internal_error
    {
      /* iterate across all states */
      for (Enumeration st = all(); st.hasMoreElements(); )
    {
      /* propagate lookaheads out of that state */
      ((lalr_state)st.nextElement()).propagate_lookaheads();
    }
    }

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

  /** Add a transition out of this state to another.
   * @param on_sym the symbol the transition is under.
   * @param to_st  the state the transition goes to.
   */
  public void add_transition(symbol on_sym, lalr_state to_st)
    throws internal_error
    {
      lalr_transition trans;

      /* create a new transition object and put it in our list */
      trans = new lalr_transition(on_sym, to_st, _transitions);
      _transitions = trans;
    }

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

  /** Build an LALR viable prefix recognition machine given a start
   *  production.  This method operates by first building a start state
   *  from the start production (based on a single item with the dot at
   *  the beginning and EOF as expected lookahead).  Then for each state
   *  it attempts to extend the machine by creating transitions out of
   *  the state to new or existing states.  When considering extension
   *  from a state we make a transition on each symbol that appears before
   *  the dot in some item.  For example, if we have the items: <pre>
   *    [A ::= a b * X c, {d,e}]
   *    [B ::= a b * X d, {a,b}]
   *  </pre>
   *  in some state, then we would be making a transition under X to a new
   *  state.  This new state would be formed by a "kernel" of items
   *  corresponding to moving the dot past the X.  In this case: <pre>
   *    [A ::= a b X * c, {d,e}]
   *    [B ::= a b X * Y, {a,b}]
   *  </pre>
   *  The full state would then be formed by "closing" this kernel set of
   *  items so that it included items that represented productions of things
   *  the parser was now looking for.  In this case we would items
   *  corresponding to productions of Y, since various forms of Y are expected
   *  next when in this state (see lalr_item_set.compute_closure() for details
   *  on closure). <p>
   *
   *  The process of building the viable prefix recognizer terminates when no
   *  new states can be added.  However, in order to build a smaller number of
   *  states (i.e., corresponding to LALR rather than canonical LR) the state
   *  building process does not maintain full loookaheads in all items.
   *  Consequently, after the machine is built, we go back and propagate
   *  lookaheads through the constructed machine using a call to
   *  propagate_all_lookaheads().  This makes use of propagation links
   *  constructed during the closure and transition process.
   *
   * @param start_prod the start production of the grammar
   * @see   java_cup.lalr_item_set#compute_closure
   * @see   java_cup.lalr_state#propagate_all_lookaheads
   */

  public static lalr_state build_machine(production start_prod)
    throws internal_error
    {
      lalr_state    start_state;
      lalr_item_set start_items;
      lalr_item_set new_items;
      lalr_item_set linked_items;
      lalr_item_set kernel;
      Stack         work_stack = new Stack();
      lalr_state    st, new_st;
      symbol_set    outgoing;
      lalr_item     itm, new_itm, existing, fix_itm;
      symbol        sym, sym2;
      Enumeration   i, s, fix;

      /* sanity check */
      if (start_prod == null)
    throw new internal_error(
       "Attempt to build viable prefix recognizer using a null production");

      /* build item with dot at front of start production and EOF lookahead */
      start_items = new lalr_item_set();
      itm = new lalr_item(start_prod);
      itm.lookahead().add(terminal.EOF);
      start_items.add(itm);

      /* create copy the item set to form the kernel */
      kernel = new lalr_item_set(start_items);

      /* create the closure from that item set */
      start_items.compute_closure();

      /* build a state out of that item set and put it in our work set */
      start_state = new lalr_state(start_items);
      work_stack.push(start_state);

      /* enter the state using the kernel as the key */
      _all_kernels.put(kernel, start_state);

      /* continue looking at new states until we have no more work to do */
      while (!work_stack.empty())
    {
      /* remove a state from the work set */
      st = (lalr_state)work_stack.pop();

      /* gather up all the symbols that appear before dots */
      outgoing = new symbol_set();
      for (i = st.items().all(); i.hasMoreElements(); )
        {
          itm = (lalr_item)i.nextElement();

          /* add the symbol before the dot (if any) to our collection */
          sym = itm.symbol_after_dot();
          if (sym != null) outgoing.add(sym);
        }

      /* now create a transition out for each individual symbol */
      for (s = outgoing.all(); s.hasMoreElements(); )
        {
          sym = (symbol)s.nextElement();

          /* will be keeping the set of items with propagate links */
          linked_items = new lalr_item_set();

          /* gather up shifted versions of all the items that have this
         symbol before the dot */
          new_items = new lalr_item_set();
          for (i = st.items().all(); i.hasMoreElements();)
        {
          itm = (lalr_item)i.nextElement();

          /* if this is the symbol we are working on now, add to set */
          sym2 = itm.symbol_after_dot();
          if (sym.equals(sym2))
            {
              /* add to the kernel of the new state */
              new_items.add(itm.shift());

              /* remember that itm has propagate link to it */
              linked_items.add(itm);
            }
        }

          /* use new items as state kernel */
          kernel = new lalr_item_set(new_items);

          /* have we seen this one already? */
          new_st = (lalr_state)_all_kernels.get(kernel);

          /* if we haven't, build a new state out of the item set */
          if (new_st == null)
        {
              /* compute closure of the kernel for the full item set */
              new_items.compute_closure();

          /* build the new state */
          new_st = new lalr_state(new_items);

          /* add the new state to our work set */
          work_stack.push(new_st);

          /* put it in our kernel table */
          _all_kernels.put(kernel, new_st);
        }
          /* otherwise relink propagation to items in existing state */
          else
        {
          /* walk through the items that have links to the new state */
          for (fix = linked_items.all(); fix.hasMoreElements(); )
            {
              fix_itm = (lalr_item)fix.nextElement();

              /* look at each propagate link out of that item */
              for (int l =0; l < fix_itm.propagate_items().size(); l++)
            {
              /* pull out item linked to in the new state */
              new_itm =
                (lalr_item)fix_itm.propagate_items().elementAt(l);

              /* find corresponding item in the existing state */
              existing = new_st.items().find(new_itm);

              /* fix up the item so it points to the existing set */
              if (existing != null)
                fix_itm.propagate_items().setElementAt(existing ,l);
            }
            }
        }

          /* add a transition from current state to that state */
          st.add_transition(sym, new_st);
        }
    }

      /* all done building states */

      /* propagate complete lookahead sets throughout the states */
      propagate_all_lookaheads();

      return start_state;
    }

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

  /** Propagate lookahead sets out of this state. This recursively
   *  propagates to all items that have propagation links from some item
   *  in this state.
   */
  protected void propagate_lookaheads() throws internal_error
    {
      /* recursively propagate out from each item in the state */
      for (Enumeration itm = items().all(); itm.hasMoreElements(); )
    ((lalr_item)itm.nextElement()).propagate_lookaheads(null);
    }

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

  /** Fill in the parse table entries for this state.  There are two
   *  parse tables that encode the viable prefix recognition machine, an
   *  action table and a reduce-goto table.  The rows in each table
   *  correspond to states of the machine.  The columns of the action table
   *  are indexed by terminal symbols and correspond to either transitions
   *  out of the state (shift entries) or reductions from the state to some
   *  previous state saved on the stack (reduce entries).  All entries in the
   *  action table that are not shifts or reduces, represent errors.    The
   *  reduce-goto table is indexed by non terminals and represents transitions
   *  out of a state on that non-terminal.<p>
   *  Conflicts occur if more than one action needs to go in one entry of the
   *  action table (this cannot happen with the reduce-goto table).  Conflicts
   *  are resolved by always shifting for shift/reduce conflicts and choosing
   *  the lowest numbered production (hence the one that appeared first in
   *  the specification) in reduce/reduce conflicts.  All conflicts are
   *  reported and if more conflicts are detected than were declared by the
   *  user, code generation is aborted.
   *
   * @param act_table    the action table to put entries in.
   * @param reduce_table the reduce-goto table to put entries in.
   */
  public void build_table_entries(
    parse_action_table act_table,
    parse_reduce_table reduce_table)
    throws internal_error
    {
      parse_action_row our_act_row;
      parse_reduce_row our_red_row;
      lalr_item        itm;
      parse_action     act, other_act;
      symbol           sym;
      boolean          conflicted = false;

      /* pull out our rows from the tables */
      our_act_row = act_table.under_state[index()];
      our_red_row = reduce_table.under_state[index()];

      /* consider each item in our state */
      for (Enumeration i = items().all(); i.hasMoreElements(); )
    {
      itm = (lalr_item)i.nextElement();

      /* if its completed (dot at end) then reduce under the lookahead */
      if (itm.dot_at_end())
        {
          act = new reduce_action(itm.the_production());

          /* consider each lookahead symbol */
          for (int t = 0; t < terminal.number(); t++)
        {
          /* skip over the ones not in the lookahead */
          if (!itm.lookahead().contains(t)) continue;

              /* if we don't already have an action put this one in */
              if (our_act_row.under_term[t].kind() ==
              parse_action.ERROR)
            {
                  our_act_row.under_term[t] = act;
            }
              else
            {
              /* we now have at least one conflict */
              conflicted = true;

              other_act = our_act_row.under_term[t];

              /* if the other act was not a shift */
              if (other_act.kind() != parse_action.SHIFT)
                {
                  /* if we have lower index hence priority, replace it*/
                  if (itm.the_production().index() <
                  ((reduce_action)other_act).reduce_with().index())
                {
                  /* replace the action */
                  our_act_row.under_term[t] = act;
                }
                }
            }
        }
        }
    }

      /* consider each outgoing transition */
      for (lalr_transition trans=transitions(); trans!=null; trans=trans.next())
    {
      /* if its on an terminal add a shift entry */
      sym = trans.on_symbol();
      if (!sym.is_non_term())
        {
          act = new shift_action(trans.to_state());

          /* if we don't already have an action put this one in */
          if ( our_act_row.under_term[sym.index()].kind() ==
           parse_action.ERROR)
        {
              our_act_row.under_term[sym.index()] = act;
        }
          else
        {
          /* we now have at least one conflict */
          conflicted = true;

          /* shift always wins */
          our_act_row.under_term[sym.index()] = act;
        }
        }
      else
        {
          /* for non terminals add an entry to the reduce-goto table */
          our_red_row.under_non_term[sym.index()] = trans.to_state();
        }
    }

      /* if we end up with conflict(s), report them */
      if (conflicted)
        report_conflicts();
    }

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

  /** Produce warning messages for all conflicts found in this state.  */
  protected void report_conflicts()
    throws internal_error
    {
      lalr_item    itm, compare;
      symbol       shift_sym;
      terminal_set conflict_set;
      boolean      after_itm;

      /* consider each element */
      for (Enumeration itms = items().all(); itms.hasMoreElements(); )
    {
      itm = (lalr_item)itms.nextElement();

      /* clear the S/R conflict set for this item */
      conflict_set = new terminal_set();

      /* if it results in a reduce, it could be a conflict */
      if (itm.dot_at_end())
        {
          /* not yet after itm */
          after_itm = false;

          /* compare this item against all others looking for conflicts */
          for (Enumeration comps = items().all(); comps.hasMoreElements(); )
        {
          compare = (lalr_item)comps.nextElement();

          /* if this is the item, next one is after it */
          if (itm == compare) after_itm = true;

          /* only look at it if its not the same item */
          if (itm != compare)
            {
              /* is it a reduce */
              if (compare.dot_at_end())
            {
              /* only look at reduces after itm */
              if (after_itm)
                            /* does the comparison item conflict? */
                            if (compare.lookahead().intersects(itm.lookahead()))
                              /* report a reduce/reduce conflict */
                              report_reduce_reduce(itm, compare);
            }
              /* must be a shift on a terminal or non-terminal */
              else
            {
                  /* is it a shift on a terminal */
              shift_sym = compare.symbol_after_dot();
              if (!shift_sym.is_non_term())
                {
                  /* does the terminal conflict with our item */
                  if (itm.lookahead().contains((terminal)shift_sym))
                    /* remember the terminal symbol in conflict */
                    conflict_set.add((terminal)shift_sym);
                }
            }
            }
        }
          /* report S/R conflicts under all the symbols we conflict under */
          for (int t = 0; t < terminal.number(); t++)
        if (conflict_set.contains(t))
          report_shift_reduce(itm,t);
        }
    }
    }

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

  /** Produce a warning message for one reduce/reduce conflict.
   *
   * @param itm1 first item in conflict.
   * @param itm2 second item in conflict.
   */
  protected void report_reduce_reduce(lalr_item itm1, lalr_item itm2)
    throws internal_error
    {
      boolean comma_flag = false;

      System.err.println("*** Reduce/Reduce conflict found in state #"+index());
      System.err.print  ("  between ");
      System.err.println(itm1.to_simple_string());
      System.err.print  ("  and     ");
      System.err.println(itm2.to_simple_string());
      System.err.print("  under symbols: {" );
      for (int t = 0; t < terminal.number(); t++)
    {
      if (itm1.lookahead().contains(t) && itm2.lookahead().contains(t))
        {
          if (comma_flag) System.err.print(", "); else comma_flag = true;
          System.err.print(terminal.find(t).name());
        }
    }
      System.err.println("}");
      System.err.print("  Resolved in favor of ");
      if (itm1.the_production().index() < itm2.the_production().index())
    System.err.println("the first production.\n");
      else
    System.err.println("the second production.\n");

      /* count the conflict */
      emit.num_conflicts++;
      lexer.warning_count++;
    }

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

  /** Produce a warning message for one shift/reduce conflict.
   *
   * @param red_itm      the item with the reduce.
   * @param conflict_sym the index of the symbol conflict occurs under.
   */
  protected void report_shift_reduce(
    lalr_item red_itm,
    int       conflict_sym)
    throws internal_error
    {
      lalr_item    itm;
      symbol       shift_sym;

      /* emit top part of message including the reduce item */
      System.err.println("*** Shift/Reduce conflict found in state #"+index());
      System.err.print  ("  between ");
      System.err.println(red_itm.to_simple_string());

      /* find and report on all items that shift under our conflict symbol */
      for (Enumeration itms = items().all(); itms.hasMoreElements(); )
    {
      itm = (lalr_item)itms.nextElement();

      /* only look if its not the same item and not a reduce */
      if (itm != red_itm && !itm.dot_at_end())
        {
          /* is it a shift on our conflicting terminal */
          shift_sym = itm.symbol_after_dot();
          if (!shift_sym.is_non_term() && shift_sym.index() == conflict_sym)
            {
          /* yes, report on it */
                  System.err.println("  and     " + itm.to_simple_string());
        }
        }
    }
      System.err.println("  under symbol "+ terminal.find(conflict_sym).name());
      System.err.println("  Resolved in favor of shifting.\n");

      /* count the conflict */
      emit.num_conflicts++;
      lexer.warning_count++;
    }

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

  /** Equality comparison. */
  public boolean equals(lalr_state other)
    {
      /* we are equal if our item sets are equal */
      return other != null && items().equals(other.items());
    }

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

  /** Generic equality comparison. */
  public boolean equals(Object other)
    {
      if (!(other instanceof lalr_state))
    return false;
      else
    return equals((lalr_state)other);
    }

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

  /** Produce a hash code. */
  public int hashCode()
    {
      /* just use the item set hash code */
      return items().hashCode();
    }

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

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

      /* dump the item set */
      result = "lalr_state [" + index() + "]: " + _items + "\n";

      /* do the transitions */
      for (tr = transitions(); tr != null; tr = tr.next())
    {
      result += tr;
      result += "\n";
    }

      return result;
    }

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