- * [A ::= B * C d E , {a,b,c}]
- *
- * 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:
- * [C ::= * X e Z, {d}]
- * [X ::= * f, {e}]
- *
- * 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.
- *
- * 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 Symbol 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: 7/3/96
- * @author Frank Flannery
- *
- */
-
-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
- *
- * 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.
- * 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;
- terminal_set conflict_set = new terminal_set();
-
- /* 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 */
- terminal term = terminal.find(t);
- other_act = our_act_row.under_term[t];
-
- /* if the other act was not a shift */
- if ((other_act.kind() != parse_action.SHIFT) &&
- (other_act.kind() != parse_action.NONASSOC))
- {
- /* 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;
- }
- } else {
- /* Check precedences,see if problem is correctable */
- if(fix_with_precedence(itm.the_production(),
- t, our_act_row, act)) {
- term = null;
- }
- }
- if(term!=null) {
-
- conflict_set.add(term);
- }
- }
- }
- }
- }
-
- /* 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 */
- production p = ((reduce_action)our_act_row.under_term[sym.index()]).reduce_with();
-
- /* shift always wins */
- if (!fix_with_precedence(p, sym.index(), our_act_row, act)) {
- our_act_row.under_term[sym.index()] = act;
- conflict_set.add(terminal.find(sym.index()));
- }
- }
- }
- 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 (!conflict_set.empty())
- report_conflicts(conflict_set);
- }
-
- /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
-
-
- /** Procedure that attempts to fix a shift/reduce error by using
- * precedences. --frankf 6/26/96
- *
- * if a production (also called rule) or the lookahead terminal
- * has a precedence, then the table can be fixed. if the rule
- * has greater precedence than the terminal, a reduce by that rule
- * in inserted in the table. If the terminal has a higher precedence,
- * it is shifted. if they have equal precedence, then the associativity
- * of the precedence is used to determine what to put in the table:
- * if the precedence is left associative, the action is to reduce.
- * if the precedence is right associative, the action is to shift.
- * if the precedence is non associative, then it is a syntax error.
- *
- * @param p the production
- * @param term_index the index of the lokahead terminal
- * @param parse_action_row a row of the action table
- * @param act the rule in conflict with the table entry
- */
-
- protected boolean fix_with_precedence(
- production p,
- int term_index,
- parse_action_row table_row,
- parse_action act)
-
- throws internal_error {
-
- terminal term = terminal.find(term_index);
-
- /* if the production has a precedence number, it can be fixed */
- if (p.precedence_num() > assoc.no_prec) {
-
- /* if production precedes terminal, put reduce in table */
- if (p.precedence_num() > term.precedence_num()) {
- table_row.under_term[term_index] =
- insert_reduce(table_row.under_term[term_index],act);
- return true;
- }
-
- /* if terminal precedes rule, put shift in table */
- else if (p.precedence_num() < term.precedence_num()) {
- table_row.under_term[term_index] =
- insert_shift(table_row.under_term[term_index],act);
- return true;
- }
- else { /* they are == precedence */
-
- /* equal precedences have equal sides, so only need to
- look at one: if it is right, put shift in table */
- if (term.precedence_side() == assoc.right) {
- table_row.under_term[term_index] =
- insert_shift(table_row.under_term[term_index],act);
- return true;
- }
-
- /* if it is left, put reduce in table */
- else if (term.precedence_side() == assoc.left) {
- table_row.under_term[term_index] =
- insert_reduce(table_row.under_term[term_index],act);
- return true;
- }
-
- /* if it is nonassoc, we're not allowed to have two nonassocs
- of equal precedence in a row, so put in NONASSOC */
- else if (term.precedence_side() == assoc.nonassoc) {
- table_row.under_term[term_index] = new nonassoc_action();
- return true;
- } else {
- /* something really went wrong */
- throw new internal_error("Unable to resolve conflict correctly");
- }
- }
- }
- /* check if terminal has precedence, if so, shift, since
- rule does not have precedence */
- else if (term.precedence_num() > assoc.no_prec) {
- table_row.under_term[term_index] =
- insert_shift(table_row.under_term[term_index],act);
- return true;
- }
-
- /* otherwise, neither the rule nor the terminal has a precedence,
- so it can't be fixed. */
- return false;
- }
-
- /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
-
-
- /* given two actions, and an action type, return the
- action of that action type. give an error if they are of
- the same action, because that should never have tried
- to be fixed
-
- */
- protected parse_action insert_action(
- parse_action a1,
- parse_action a2,
- int act_type)
- throws internal_error
- {
- if ((a1.kind() == act_type) && (a2.kind() == act_type)) {
- throw new internal_error("Conflict resolution of bogus actions");
- } else if (a1.kind() == act_type) {
- return a1;
- } else if (a2.kind() == act_type) {
- return a2;
- } else {
- throw new internal_error("Conflict resolution of bogus actions");
- }
- }
-
- /* find the shift in the two actions */
- protected parse_action insert_shift(
- parse_action a1,
- parse_action a2)
- throws internal_error
- {
- return insert_action(a1, a2, parse_action.SHIFT);
- }
-
- /* find the reduce in the two actions */
- protected parse_action insert_reduce(
- parse_action a1,
- parse_action a2)
- throws internal_error
- {
- return insert_action(a1, a2, parse_action.REDUCE);
- }
-
- /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
-
- /** Produce warning messages for all conflicts found in this state. */
- protected void report_conflicts(terminal_set conflict_set)
- throws internal_error
- {
- lalr_item itm, compare;
- symbol shift_sym;
-
- 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 */
-
- /* 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);
- }
- }
- }
- /* 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;
- }
-
- /*-----------------------------------------------------------*/
-}
- * [A ::= a b X * c, {d,e}]
- * [B ::= a b X * Y, {a,b}]
- *
- * 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).