From a774839ca873d1082f79ba3c4eecc1e242a28ce1 Mon Sep 17 00:00:00 2001
From: Akim Demaille <akim.demaille@gmail.com>
Date: Sun, 7 Mar 2021 08:19:36 +0100
Subject: [PATCH] tables: fix again the handling of useless tokens
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The right-shift added in c22902e360e0fbbe9fd5657dcf107e03166da309
("tables: fix handling for useless tokens") is incorrect.  In
particular, we need to reset the "new" bits.

Reported by Balázs Scheidler.
https://github.com/akimd/bison/issues/74

* src/tables.c (pos_set_set): Fix the right-shift.
---
 NEWS         |  6 ++++++
 src/tables.c | 15 ++++++++++-----
 2 files changed, 16 insertions(+), 5 deletions(-)

diff --git a/NEWS b/NEWS
index 2ce2ad51..9598c0c2 100644
--- a/NEWS
+++ b/NEWS
@@ -2,6 +2,12 @@ GNU Bison NEWS
 
 * Noteworthy changes in release ?.? (????-??-??) [?]
 
+** Bug fixes
+
+*** Fix Table Generation
+
+  In some very rare conditions, when there are many useless tokens, it was
+  possible to generate incorrect parsers.
 
 * Noteworthy changes in release 3.7.5 (2021-01-24) [stable]
 
diff --git a/src/tables.c b/src/tables.c
index 6c8fc1cc..23a879ca 100644
--- a/src/tables.c
+++ b/src/tables.c
@@ -180,21 +180,26 @@ pos_set_set (int pos)
   int bitno = pos - pos_set_base;
   if (bitno < 0)
     {
+      // Need more room on the left.
+      // DELTA is positive.  Run 'pos_set >> delta'.
       const int delta = pos_set_base - pos;
       const int old_size = bitset_size (pos_set);
       const int new_size = old_size + delta;
       bitset_resize (pos_set, new_size);
-      // Shift all the bits by DELTA.
+      // Right-shift all the bits by DELTA.  Be sure to reset the new
+      // bits on the left.
+      //
       // FIXME: add bitset_assign, and bitset_shift?
-      for (int i = new_size - 1; delta <= i ; --i)
-        if (bitset_test (pos_set, i))
-          bitset_set (pos_set, i + delta);
+      for (int i = new_size - 1; 0 <= i ; --i)
+        if (delta <= i && bitset_test (pos_set, i - delta))
+          bitset_set (pos_set, i);
         else
-          bitset_reset (pos_set, i + delta);
+          bitset_reset (pos_set, i);
       pos_set_base = pos;
       bitno = 0;
     }
   else if (bitset_size (pos_set) <= bitno)
+    // Need more room on the right.
     bitset_resize (pos_set, bitno + 1);
   bitset_set (pos_set, bitno);
 }