From a38d0b914507d2c9163eef84240bf1a059a0659f Mon Sep 17 00:00:00 2001
From: Akim Demaille <akim.demaille@gmail.com>
Date: Sun, 8 Nov 2020 17:50:46 +0100
Subject: [PATCH] multistart: introduce and use rule_is_initial

* src/gram.h (rule_is_initial): New.
* src/graphviz.c, src/print-xml.c, src/print.c, src/lalr.c: Use it.
Some of these occurrences were incorrect (checking whether this is
rule 0), and not behaving properly in the case of multistart.
---
 TODO            | 19 -------------------
 src/gram.h      | 15 +++++++++++++++
 src/graphviz.c  |  2 +-
 src/lalr.c      |  2 +-
 src/print-xml.c |  3 +--
 src/print.c     |  5 ++---
 6 files changed, 20 insertions(+), 26 deletions(-)

diff --git a/TODO b/TODO
index 8b009d08..c6a71a34 100644
--- a/TODO
+++ b/TODO
@@ -685,25 +685,6 @@ participate in the count.
 Do we want to disallow terminal start symbols?  The limitation is not
 technical.  Can it be useful to someone to "parse" a token?
 
-*** Fix default_reduction_only_for_accept
-In src/lalr.c:
-
-  /* We need a lookahead either to distinguish different reductions
-     (i.e., there are two or more), or to distinguish a reduction from a
-     shift.  Otherwise, it is straightforward, and the state is
-     'consistent'.  However, do not treat a state with any reductions as
-     consistent unless it is the accepting state (because there is never
-     a lookahead token that makes sense there, and so no lookahead token
-     should be read) if the user has otherwise disabled default
-     reductions.  */
-  s->consistent =
-    !(reds->num > 1
-      || (reds->num == 1 && trans->num && TRANSITION_IS_SHIFT (trans, 0))
-      || (reds->num == 1 && reds->rules[0]->number != 0
-          && default_reduction_only_for_accept));
-
-We should have a test for "is accepting rule".
-
 ** %include
 This is a popular demand.  We already made many changes in the parser that
 should make this reasonably easy to implement.
diff --git a/src/gram.h b/src/gram.h
index 95fec9be..db874e24 100644
--- a/src/gram.h
+++ b/src/gram.h
@@ -233,9 +233,24 @@ item_rule (item_number const *item)
 void item_print (item_number *item, rule const *previous_rule,
                  FILE *out);
 
+/*--------.
+| Rules.  |
+`--------*/
+
 /* A function that selects a rule.  */
 typedef bool (*rule_filter) (rule const *);
 
+/* Whether is an accepting rule (i.e., its reduction terminates
+   parsing with success). */
+static inline bool
+rule_is_initial (rule const *r)
+{
+  /* In the case of multistart, we need to check whether the LHS is
+     $accept.  In the case of "unistart", it would suffice to
+     check whether this is rule number 0.  */
+  return r->lhs == acceptsymbol->content;
+}
+
 /* Whether the rule has a 'number' smaller than NRULES.  That is, it
    is useful in the grammar.  */
 bool rule_useful_in_grammar_p (rule const *r);
diff --git a/src/graphviz.c b/src/graphviz.c
index 799d5bef..da4db481 100644
--- a/src/graphviz.c
+++ b/src/graphviz.c
@@ -135,7 +135,7 @@ conclude_red (struct obstack *out, int source, rule_number ruleno,
       /* Build the associated diamond representation of the target rule. */
       fprintf (fout, " \"%dR%d%s\" [label=\"",
                source, ruleno, ed);
-      bool final = rules[ruleno].lhs->symbol == acceptsymbol;
+      bool const final = rule_is_initial (&rules[ruleno]);
       if (final)
         fprintf (fout, "Acc");
       else
diff --git a/src/lalr.c b/src/lalr.c
index 941a1d68..8a89d49e 100644
--- a/src/lalr.c
+++ b/src/lalr.c
@@ -473,7 +473,7 @@ state_lookaheads_count (state *s, bool default_reduction_only_for_accept)
   s->consistent =
     !(reds->num > 1
       || (reds->num == 1 && trans->num && TRANSITION_IS_SHIFT (trans, 0))
-      || (reds->num == 1 && reds->rules[0]->number != 0
+      || (reds->num == 1 && !rule_is_initial (reds->rules[0])
           && default_reduction_only_for_accept));
 
   return s->consistent ? 0 : reds->num;
diff --git a/src/print-xml.c b/src/print-xml.c
index 7548fb66..b1f728d1 100644
--- a/src/print-xml.c
+++ b/src/print-xml.c
@@ -216,8 +216,7 @@ static void
 print_reduction (FILE *out, int level, char const *lookahead,
                  rule *r, bool enabled)
 {
-  const bool final = r->lhs->symbol == acceptsymbol;
-  if (final)
+  if (rule_is_initial (r))
     xml_printf (out, level,
                 "<reduction symbol=\"%s\" rule=\"accept\" enabled=\"%s\"/>",
                 xml_escape (lookahead),
diff --git a/src/print.c b/src/print.c
index 4555ba98..233d257a 100644
--- a/src/print.c
+++ b/src/print.c
@@ -198,8 +198,7 @@ print_reduction (FILE *out, size_t width,
     fputc (' ', out);
   if (!enabled)
     fputc ('[', out);
-  const bool final = r->lhs->symbol == acceptsymbol;
-  if (final)
+  if (rule_is_initial (r))
     fprintf (out, _("accept"));
   else
     fprintf (out, _("reduce using rule %d (%s)"), r->number,
@@ -318,7 +317,7 @@ print_reductions (FILE *out, const state *s)
       aver (STREQ (default_reductions, "most")
             || (STREQ (default_reductions, "consistent")
                 && default_reduction_only)
-            || (reds->num == 1 && reds->rules[0]->number == 0));
+            || (reds->num == 1 && rule_is_initial (reds->rules[0])));
       (void) default_reduction_only;
       free (default_reductions);
     }