(scan_lists): Properly skip the comment-fence character

that ends a comment, when moving forward.
(Fbackward_prefix_chars): Return immediately if point is at BEGV.

(prev_char_comend_first): New function.
(back_comment): Use that.  Carefully update syntax table position
for each character that is fetched.
(Fforward_comment): Likewise.
(scan_lists): Likewise.
(prev_char_comstart_first): New function.

1 files changed, 88 insertions, 43 deletions

diff --git a/src/syntax.c b/src/syntax.c
index d043e115565..efa4bf3d271 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -130,7 +130,7 @@ update_syntax_table (charpos, count, init, object)
       invalidate = 0;
       if (NULL_INTERVAL_P (i))
         return;
-      /* interval_of () updates only ->position of the return value,
+      /* interval_of updates only ->position of the return value, so
          update the parents manually to speed up update_interval.  */
       while (!NULL_PARENT (i)) 
         {
@@ -158,7 +158,7 @@ update_syntax_table (charpos, count, init, object)
   else if (charpos < i->position)               /* Move left.  */
     {
       if (count > 0)
-        error ("Error in syntax_table logic for intervals <-.");
+        error ("Error in syntax_table logic for intervals <-");
       /* Update the interval.  */
       i = update_interval (i, charpos);
       if (oldi->position != INTERVAL_LAST_POS (i))
@@ -172,7 +172,7 @@ update_syntax_table (charpos, count, init, object)
   else if (charpos >= INTERVAL_LAST_POS (i)) /* Move right.  */
     {
       if (count < 0)
-        error ("Error in syntax_table logic for intervals ->.");
+        error ("Error in syntax_table logic for intervals ->");
       /* Update the interval.  */
       i = update_interval (i, charpos);
       if (i->position != INTERVAL_LAST_POS (oldi))
@@ -394,6 +394,38 @@ find_defun_start (pos, pos_byte)
   return find_start_value;
 }
 
+/* Return the SYNTAX_COMEND_FIRST of the character before POS, POS_BYTE.  */
+
+static int
+prev_char_comend_first (pos, pos_byte)
+     int pos, pos_byte;
+{
+  int c, val;
+
+  DEC_BOTH (pos, pos_byte);
+  UPDATE_SYNTAX_TABLE_BACKWARD (pos);
+  c = FETCH_CHAR (pos_byte);
+  val = SYNTAX_COMEND_FIRST (c);
+  UPDATE_SYNTAX_TABLE_FORWARD (pos + 1);
+  return val;
+}
+
+/* Return the SYNTAX_COMSTART_FIRST of the character before POS, POS_BYTE.  */
+
+static int
+prev_char_comstart_first (pos, pos_byte)
+     int pos, pos_byte;
+{
+  int c, val;
+
+  DEC_BOTH (pos, pos_byte);
+  UPDATE_SYNTAX_TABLE_BACKWARD (pos);
+  c = FETCH_CHAR (pos_byte);
+  val = SYNTAX_COMSTART_FIRST (c);
+  UPDATE_SYNTAX_TABLE_FORWARD (pos + 1);
+  return val;
+}
+
 /* Checks whether charpos FROM is at the end of a comment.
    FROM_BYTE is the bytepos corresponding to FROM.
    Do not move back before STOP.
@@ -444,7 +476,7 @@ back_comment (from, from_byte, stop, comstyle, charpos_ptr, bytepos_ptr)
      that determines quote parity to the comment-end.  */
   while (from != stop)
     {
-      int temp_byte;
+      int temp_byte, prev_comend_second;
 
       /* Move back and examine a character.  */
       DEC_BOTH (from, from_byte);
@@ -456,23 +488,25 @@ back_comment (from, from_byte, stop, comstyle, charpos_ptr, bytepos_ptr)
       /* If this char is the second of a 2-char comment end sequence,
          back up and give the pair the appropriate syntax.  */
       if (from > stop && SYNTAX_COMEND_SECOND (c)
-          && (temp_byte = dec_bytepos (from_byte),
-              SYNTAX_COMEND_FIRST (FETCH_CHAR (temp_byte))))
+          && prev_char_comend_first (from, from_byte))
         {
           code = Sendcomment;
           DEC_BOTH (from, from_byte);
-          /* This is apparently the best we can do: */
           UPDATE_SYNTAX_TABLE_BACKWARD (from);
           c = FETCH_CHAR (from_byte);
         }
                         
       /* If this char starts a 2-char comment start sequence,
          treat it like a 1-char comment starter.  */
-      if (from < scanstart && SYNTAX_COMSTART_FIRST (c)
-          && (temp_byte = inc_bytepos (from_byte),
-              (SYNTAX_COMSTART_SECOND (FETCH_CHAR (temp_byte))
-               && comstyle == SYNTAX_COMMENT_STYLE (FETCH_CHAR (temp_byte)))))
-        code = Scomment;
+      if (from < scanstart && SYNTAX_COMSTART_FIRST (c))
+        {
+          temp_byte = inc_bytepos (from_byte);
+          UPDATE_SYNTAX_TABLE_FORWARD (from + 1);
+          if (SYNTAX_COMSTART_SECOND (FETCH_CHAR (temp_byte))
+              && comstyle == SYNTAX_COMMENT_STYLE (FETCH_CHAR (temp_byte)))
+            code = Scomment;
+          UPDATE_SYNTAX_TABLE_BACKWARD (from);
+        }
 
       /* Ignore escaped characters, except comment-enders.  */
       if (code != Sendcomment && char_quoted (from, from_byte))
@@ -1590,6 +1624,8 @@ between them, return t; otherwise return nil.")
     {
       do
         {
+          int comstart_first;
+
           if (from == stop)
             {
               SET_PT_BOTH (from, from_byte);
@@ -1599,9 +1635,11 @@ between them, return t; otherwise return nil.")
           UPDATE_SYNTAX_TABLE_FORWARD (from);
           c = FETCH_CHAR (from_byte);
           code = SYNTAX (c);
+          comstart_first = SYNTAX_COMSTART_FIRST (c);
           INC_BOTH (from, from_byte);
+          UPDATE_SYNTAX_TABLE_FORWARD (from);
           comstyle = 0;
-          if (from < stop && SYNTAX_COMSTART_FIRST (c)
+          if (from < stop && comstart_first
               && (c1 = FETCH_CHAR (from_byte),
                   SYNTAX_COMSTART_SECOND (c1)))
             {
@@ -1649,9 +1687,10 @@ between them, return t; otherwise return nil.")
                section.  */
             break;
           if (from < stop && SYNTAX_COMEND_FIRST (c)
+              && SYNTAX_COMMENT_STYLE (c) == comstyle
               && (c1 = FETCH_CHAR (from_byte),
-                  SYNTAX_COMEND_SECOND (c1))
-              && SYNTAX_COMMENT_STYLE (c) == comstyle)
+                  UPDATE_SYNTAX_TABLE_FORWARD (from),
+                  SYNTAX_COMEND_SECOND (c1)))
             /* we have encountered a comment end of the same style
                as the comment sequence which began this comment
                section */
@@ -1668,7 +1707,8 @@ between them, return t; otherwise return nil.")
     {
       while (1)
         {
-          int quoted;
+          int quoted, comstart_second;
+
           if (from <= stop)
             {
               SET_PT_BOTH (BEGV, BEGV_BYTE);
@@ -1677,34 +1717,34 @@ between them, return t; otherwise return nil.")
             }
 
           DEC_BOTH (from, from_byte);
+          /* char_quoted does UPDATE_SYNTAX_TABLE_BACKWARD (from).  */
           quoted = char_quoted (from, from_byte);
           if (quoted)
             {
               DEC_BOTH (from, from_byte);
               goto leave;
             }
-          UPDATE_SYNTAX_TABLE_BACKWARD (from);
           c = FETCH_CHAR (from_byte);
           code = SYNTAX (c);
           comstyle = 0;
           if (code == Sendcomment)
             comstyle = SYNTAX_COMMENT_STYLE (c);
-          temp_pos = dec_bytepos (from_byte);
+          comstart_second = SYNTAX_COMSTART_SECOND (c);
           if (from > stop && SYNTAX_COMEND_SECOND (c)
-              && (c1 = FETCH_CHAR (temp_pos),
-                  SYNTAX_COMEND_FIRST (c1))
+              && prev_char_comend_first (from, from_byte)
               && !char_quoted (from - 1, temp_pos))
             {
               /* We must record the comment style encountered so that
                  later, we can match only the proper comment begin
                  sequence of the same style.  */
-              code = Sendcomment;
-              comstyle = SYNTAX_COMMENT_STYLE (c1);
               DEC_BOTH (from, from_byte);
+              code = Sendcomment;
+              /* Calling char_quoted, above, set up global syntax position
+                 at the new value of FROM.  */
+              comstyle = SYNTAX_COMMENT_STYLE (FETCH_CHAR (from_byte));
             }
-          if (from > stop && SYNTAX_COMSTART_SECOND (c)
-              && (c1 = FETCH_CHAR (temp_pos),
-                  SYNTAX_COMSTART_FIRST (c1))
+          if (from > stop && comstart_second
+              && prev_char_comstart_first (from, from_byte)
               && !char_quoted (from - 1, temp_pos))
             {
               /* We must record the comment style encountered so that
@@ -1798,14 +1838,17 @@ scan_lists (from, count, depth, sexpflag)
     {
       while (from < stop)
         {
+          int comstart_first, prefix;
           UPDATE_SYNTAX_TABLE_FORWARD (from);
           c = FETCH_CHAR (from_byte);
           code = SYNTAX (c);
+          comstart_first = SYNTAX_COMSTART_FIRST (c);
+          prefix = SYNTAX_PREFIX (c);
           if (depth == min_depth)
             last_good = from;
           INC_BOTH (from, from_byte);
           UPDATE_SYNTAX_TABLE_FORWARD (from);
-          if (from < stop && SYNTAX_COMSTART_FIRST (c)
+          if (from < stop && comstart_first
               && SYNTAX_COMSTART_SECOND (FETCH_CHAR (from_byte))
               && parse_sexp_ignore_comments)
             {
@@ -1817,10 +1860,10 @@ scan_lists (from, count, depth, sexpflag)
               code = Scomment;
               comstyle = SYNTAX_COMMENT_STYLE (FETCH_CHAR (from_byte));
               INC_BOTH (from, from_byte);
+              UPDATE_SYNTAX_TABLE_FORWARD (from);
             }
           
-          UPDATE_SYNTAX_TABLE_FORWARD (from);
-          if (SYNTAX_PREFIX (c))
+          if (prefix)
             continue;
 
           switch (SWITCH_ENUM_CAST (code))
@@ -1871,6 +1914,7 @@ scan_lists (from, count, depth, sexpflag)
                     }
                   UPDATE_SYNTAX_TABLE_FORWARD (from);
                   c = FETCH_CHAR (from_byte);
+                  INC_BOTH (from, from_byte);
                   if (code == Scomment 
                       ? (SYNTAX (c) == Sendcomment
                          && SYNTAX_COMMENT_STYLE (c) == comstyle)
@@ -1879,10 +1923,10 @@ scan_lists (from, count, depth, sexpflag)
                        as the comment sequence which began this comment
                        section */
                     break;
-                  INC_BOTH (from, from_byte);
                   if (from < stop && SYNTAX_COMEND_FIRST (c)
-                      && SYNTAX_COMEND_SECOND (FETCH_CHAR (from_byte))
                       && SYNTAX_COMMENT_STYLE (c) == comstyle
+                      && (UPDATE_SYNTAX_TABLE_FORWARD (from),
+                          SYNTAX_COMEND_SECOND (FETCH_CHAR (from_byte)))
                       && code == Scomment)
                     /* we have encountered a comment end of the same style
                        as the comment sequence which began this comment
@@ -1975,25 +2019,22 @@ scan_lists (from, count, depth, sexpflag)
           comstyle = 0;
           if (code == Sendcomment)
             comstyle = SYNTAX_COMMENT_STYLE (c);
-          temp_pos = from_byte;
-          if (! NILP (current_buffer->enable_multibyte_characters))
-            DEC_POS (temp_pos);
-          else
-            temp_pos--;
           if (from > stop && SYNTAX_COMEND_SECOND (c)
-              && (c1 = FETCH_CHAR (temp_pos), SYNTAX_COMEND_FIRST (c1))
+              && prev_char_comstart_first (from, from_byte)
               && parse_sexp_ignore_comments)
             {
-              /* we must record the comment style encountered so that
+              /* We must record the comment style encountered so that
                  later, we can match only the proper comment begin
-                 sequence of the same style */
-              code = Sendcomment;
-              comstyle = SYNTAX_COMMENT_STYLE (c1);
+                 sequence of the same style.  */
               DEC_BOTH (from, from_byte);
+              UPDATE_SYNTAX_TABLE_BACKWARD (from);
+              code = Sendcomment;
+              comstyle = SYNTAX_COMMENT_STYLE (FETCH_CHAR (from_byte));
             }
           
           /* Quoting turns anything except a comment-ender
-             into a word character.  */
+             into a word character.  Note that this if cannot be true
+             if we decremented FROM in the if-statement above.  */
           if (code != Sendcomment && char_quoted (from, from_byte))
             code = Sword;
           else if (SYNTAX_PREFIX (c))
@@ -2192,11 +2233,15 @@ This includes chars with \"quote\" or \"prefix\" syntax (' or p).")
   int pos_byte = PT_BYTE;
   int c;
 
-  if (pos > beg) 
+  if (pos <= beg) 
     {
-      SETUP_SYNTAX_TABLE (pos, -1);
+      SET_PT_BOTH (opoint, opoint_byte);
+
+      return Qnil;
     }
 
+  SETUP_SYNTAX_TABLE (pos, -1);
+
   DEC_BOTH (pos, pos_byte);
 
   while (!char_quoted (pos, pos_byte)