If conflict resolution makes states unreachable, remove those states,

report rules that are then unused, and don't report conflicts in those
states.
* src/conflicts.c, src/conflicts.h (conflicts_update_state_numbers):
New global function.
* src/lalr.c, src/lalr.h (lalr_update_state_numbers): New global
function.
* src/main.c (main): After conflict resolution, remove the unreachable
states and update all data structures that reference states by number.
* src/state.c (state_new): Initialize each state's reachable member to
false.
(state_mark_reachable_states): New static function.
(state_remove_unreachable_states): New global function.
* src/state.h (struct state): Add member bool reachable.
(state_remove_unreachable_states): Prototype.
* tests/conflicts.at (Unreachable States After Conflict Resolution):
New test case.
* tests/existing.at (GNU pic Grammar): Update test case output now that
an unused rule is discovered.

src/conflicts.c

@@ -327,6 +327,16 @@ conflicts_solve (void)
 }
 
 
+void
+conflicts_update_state_numbers (state_number old_to_new[],
+                                state_number nstates_old)
+{
+  for (state_number i = 0; i < nstates_old; ++i)
+    if (old_to_new[i] != nstates_old)
+      conflicts[old_to_new[i]] = conflicts[i];
+}
+
+
 /*---------------------------------------------.
 | Count the number of shift/reduce conflicts.  |
 `---------------------------------------------*/

src/conflicts.h

@@ -23,6 +23,19 @@
 # include "state.h"
 
 void conflicts_solve (void);
+
+/**
+ * Update state numbers recorded in internal arrays such that:
+ *   - \c nstates_old is the old number of states.
+ *   - Where \c i is the old state number, <tt>old_to_new[i]</tt> is either:
+ *     - \c nstates_old if state \c i is removed because it is unreachable.
+ *     - The new state number.
+ *   - The highest new state number is the number of remaining states - 1.
+ *   - The numerical order of the remaining states has not changed.
+ */
+void conflicts_update_state_numbers (state_number old_to_new[],
+                                     state_number nstates_old);
+
 void conflicts_print (void);
 int conflicts_total_count (void);
 void conflicts_output (FILE *out);

src/lalr.c

@@ -453,6 +453,37 @@ lalr (void)
 }
 
 
+void
+lalr_update_state_numbers (state_number old_to_new[], state_number nstates_old)
+{
+  goto_number ngotos_reachable = 0;
+  symbol_number nonterminal = 0;
+  aver (nsyms == nvars + ntokens);
+  for (goto_number i = 0; i < ngotos; ++i)
+    {
+      while (i == goto_map[nonterminal])
+        goto_map[nonterminal++] = ngotos_reachable;
+      /* If old_to_new[from_state[i]] = nstates_old, remove this goto
+         entry.  */
+      if (old_to_new[from_state[i]] != nstates_old)
+        {
+          /* from_state[i] is not removed, so it and thus to_state[i] are
+             reachable, so to_state[i] != nstates_old.  */
+          aver (old_to_new[to_state[i]] != nstates_old);
+          from_state[ngotos_reachable] = old_to_new[from_state[i]];
+          to_state[ngotos_reachable] = old_to_new[to_state[i]];
+          ++ngotos_reachable;
+        }
+    }
+  while (nonterminal <= nvars)
+    {
+      aver (ngotos == goto_map[nonterminal]);
+      goto_map[nonterminal++] = ngotos_reachable;
+    }
+  ngotos = ngotos_reachable;
+}
+
+
 void
 lalr_free (void)
 {

src/lalr.h

@@ -46,6 +46,18 @@
 */
 void lalr (void);
 
+/**
+ * Update state numbers recorded in #goto_map, #from_state, and #to_state such
+ * that:
+ *   - \c nstates_old is the old number of states.
+ *   - Where \c i is the old state number, <tt>old_to_new[i]</tt> is either:
+ *     - \c nstates_old if state \c i is removed because it is unreachable.
+ *       Thus, remove all goto entries involving this state.
+ *     - The new state number.
+ */
+void lalr_update_state_numbers (state_number old_to_new[],
+                                state_number nstates_old);
+
 
 /** Release the information related to lookahead tokens.
 

src/main.c

@@ -115,6 +115,13 @@ main (int argc, char *argv[])
      declarations.  */
   timevar_push (TV_CONFLICTS);
   conflicts_solve ();
+  {
+    state_number old_to_new[nstates];
+    state_number nstates_old = nstates;
+    state_remove_unreachable_states (old_to_new);
+    lalr_update_state_numbers (old_to_new, nstates_old);
+    conflicts_update_state_numbers (old_to_new, nstates_old);
+  }
   conflicts_print ();
   timevar_pop (TV_CONFLICTS);
 

src/state.c

@@ -145,6 +145,7 @@ state_new (symbol_number accessing_symbol,
   res->errs = NULL;
   res->consistent = 0;
   res->solved_conflicts = NULL;
+  res->reachable = false;
 
   res->nitems = nitems;
   memcpy (res->items, core, items_size);
@@ -352,6 +353,44 @@ state_hash_lookup (size_t nitems, item_number *core)
   return entry;
 }
 
+
+/*------------------------------------------------------.
+| Mark S and all states reachable from S as reachable.  |
+`------------------------------------------------------*/
+
+static void
+state_mark_reachable_states (state *s)
+{
+  if (s->reachable)
+    return;
+  s->reachable = true;
+  for (int i = 0; i < s->transitions->num; ++i)
+    if (!TRANSITION_IS_DISABLED (s->transitions, i))
+      state_mark_reachable_states (s->transitions->states[i]);
+}
+
+void
+state_remove_unreachable_states (state_number old_to_new[])
+{
+  state_number nstates_reachable = 0;
+  state_mark_reachable_states (states[0]);
+  for (state_number i = 0; i < nstates; ++i)
+    {
+      if (states[i]->reachable)
+        {
+          states[nstates_reachable] = states[i];
+          states[nstates_reachable]->number = nstates_reachable;
+          old_to_new[i] = nstates_reachable++;
+        }
+      else
+        {
+          state_free (states[i]);
+          old_to_new[i] = nstates;
+        }
+    }
+  nstates = nstates_reachable;
+}
+
 /* All the decorated states, indexed by the state number.  */
 state **states = NULL;
 

src/state.h

@@ -209,6 +209,11 @@ struct state
      a human readable description of the resolution.  */
   const char *solved_conflicts;
 
+  /* Conflict resolution sometimes makes states unreachable.  Initialized to 0
+     in state_new and then used by state_remove_unreachable_states after
+     conflicts_solve.  */
+  bool reachable;
+
   /* Its items.  Must be last, since ITEMS can be arbitrarily large.
      */
   size_t nitems;
@@ -248,9 +253,16 @@ state *state_hash_lookup (size_t core_size, item_number *core);
 /* Insert STATE in the state hash table.  */
 void state_hash_insert (state *s);
 
+/* Remove unreachable states, renumber remaining states, update NSTATES, and
+   write to OLD_TO_NEW a mapping of old state numbers to new state numbers such
+   that the old value of NSTATES is written as the new state number for removed
+   states.  The size of OLD_TO_NEW must be the old value of NSTATES.  */
+void state_remove_unreachable_states (state_number old_to_new[]);
+
 /* All the states, indexed by the state number.  */
 extern state **states;
 
 /* Free all the states.  */
 void states_free (void);
+
 #endif /* !STATE_H_ */