Speed up display of RTL text with many character compositions

* src/bidi.c (bidi_level_start): New function.
* src/dispextern.h (bidi_level_start): Add prototype.
* src/xdisp.c (compute_stop_pos, set_iterator_to_next)
(get_visually_first_element, next_element_from_buffer): Call
'bidi_level_start' when looking for composed characters backwards,
to set limit of searching back, instead of looking all the way to
BOB.  (Bug#69385)

3 files changed, 25 insertions, 9 deletions

diff --git a/src/bidi.c b/src/bidi.c
index 36d1a0496b8..bdf60001781 100644
--- a/src/bidi.c
+++ b/src/bidi.c
@@ -754,6 +754,19 @@ bidi_cache_find_level_change (int level, int dir, bool before)
   return -1;
 }
 
+/* Find the previous character position where LEVEL changes to a lower
+   one.  Return -1 if not found (which really shouldn't happen if this
+   function is called on a backward scan).  */
+ptrdiff_t
+bidi_level_start (int level)
+{
+  ptrdiff_t slot = bidi_cache_find_level_change (level, -1, true);
+
+  if (slot >= 0)
+    return bidi_cache[slot].charpos;
+  return -1;
+}
+
 static void
 bidi_cache_ensure_space (ptrdiff_t idx)
 {
diff --git a/src/dispextern.h b/src/dispextern.h
index 5387cb45603..1c3232fae3d 100644
--- a/src/dispextern.h
+++ b/src/dispextern.h
@@ -3438,6 +3438,7 @@ extern void bidi_pop_it (struct bidi_it *);
 extern void *bidi_shelve_cache (void);
 extern void bidi_unshelve_cache (void *, bool);
 extern ptrdiff_t bidi_find_first_overridden (struct bidi_it *);
+extern ptrdiff_t bidi_level_start (int);
 
 /* Defined in xdisp.c */
 
diff --git a/src/xdisp.c b/src/xdisp.c
index d03769e2a31..140d71129f3 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -4353,7 +4353,7 @@ compute_stop_pos (struct it *it)
          an automatic composition, limit the search of composable
          characters to that position.  */
       if (it->bidi_p && it->bidi_it.scan_dir < 0)
-	stoppos = -1;
+	stoppos = bidi_level_start (it->bidi_it.resolved_level) - 1;
       else if (!STRINGP (it->string)
 	       && it->cmp_it.stop_pos <= IT_CHARPOS (*it)
 	       && cmp_limit_pos > 0)
@@ -8712,9 +8712,8 @@ set_iterator_to_next (struct it *it, bool reseat_p)
 	      ptrdiff_t stop = it->end_charpos;
 
 	      if (it->bidi_it.scan_dir < 0)
-		/* Now we are scanning backward and don't know
-		   where to stop.  */
-		stop = -1;
+		/* Now we are scanning backward; figure out where to stop.  */
+		stop = bidi_level_start (it->bidi_it.resolved_level) - 1;
 	      composition_compute_stop_pos (&it->cmp_it, IT_CHARPOS (*it),
 					    IT_BYTEPOS (*it), stop, Qnil, true);
 	    }
@@ -8745,7 +8744,7 @@ set_iterator_to_next (struct it *it, bool reseat_p)
 		     re-compute the stop position for composition.  */
 		  ptrdiff_t stop = it->end_charpos;
 		  if (it->bidi_it.scan_dir < 0)
-		    stop = -1;
+		    stop = bidi_level_start (it->bidi_it.resolved_level) - 1;
 		  composition_compute_stop_pos (&it->cmp_it, IT_CHARPOS (*it),
 						IT_BYTEPOS (*it), stop, Qnil,
 						true);
@@ -9190,7 +9189,9 @@ get_visually_first_element (struct it *it)
 	  bytepos = IT_BYTEPOS (*it);
 	}
       if (it->bidi_it.scan_dir < 0)
-	stop = -1;
+	stop = STRINGP (it->string)
+	       ? -1
+	       : bidi_level_start (it->bidi_it.resolved_level) - 1;
       composition_compute_stop_pos (&it->cmp_it, charpos, bytepos, stop,
 				    it->string, true);
     }
@@ -9694,9 +9695,10 @@ next_element_from_buffer (struct it *it)
 		    && PT < it->end_charpos) ? PT : it->end_charpos;
 	}
       else
-	stop = it->bidi_it.scan_dir < 0 ? -1 : it->end_charpos;
-      if (CHAR_COMPOSED_P (it, IT_CHARPOS (*it), IT_BYTEPOS (*it),
-			   stop)
+	stop = it->bidi_it.scan_dir < 0
+	       ? bidi_level_start (it->bidi_it.resolved_level) - 1
+	       : it->end_charpos;
+      if (CHAR_COMPOSED_P (it, IT_CHARPOS (*it), IT_BYTEPOS (*it), stop)
 	  && next_element_from_composition (it))
 	{
 	  return true;