From: Yuan Fu <casouri@gmail.com>
Date: Sat, 18 Feb 2023 10:20:12 +0000 (-0800)
Subject: Fix comment in treesit_record_change (bug#61369)
X-Git-Tag: emacs-29.0.90~412
X-Git-Url: http://git.eshelyaron.com/gitweb/?a=commitdiff_plain;h=e985466556c;p=emacs.git

Fix comment in treesit_record_change (bug#61369)

Turns out the previous commit message and comment is not entirely
correct: the old behavior is in fact wrong, not just "correct but has
problems".

Here is why the old code is wrong:

|visible range|     -> markup for visible range

updated range       -> markup for updated range
-------------

First we have some text

|aaaaaa|

Now we insert something at the beginning, because we clip
new_end_offset to visible_end, out of eight b's inserted, only the
first six are known to tree-sitter.

|bbbbbbbbaaaa|aa  start: 0, old_end: 0, new_end: 6
 ------

In treesit_sync_visible_region, we sync up visible region, but the two
missing b's are not in the updated range.

|bbbbbbbbaaaaaa|  start: 12, old_end: 12, new_end: 14
             --

The old behavior not only is wrong, but also doesn't make much sense.

* src/treesit.c (treesit_record_change): Update comment.
---

diff --git a/src/treesit.c b/src/treesit.c
index e1d6f1ef79f..ef0f2407840 100644
--- a/src/treesit.c
+++ b/src/treesit.c
@@ -797,12 +797,10 @@ treesit_record_change (ptrdiff_t start_byte, ptrdiff_t old_end_byte,
 					   max (visible_beg, old_end_byte))
 				      - visible_beg);
 	  /* We don't clip new_end_offset under visible_end, because
-             inserting in narrowed region always extends the visible
-             region.  If we clip new_end_offset here, and re-add the
-             clipped "tail" in treesit_sync_visible_region later,
-             while it is technically equivalent, tree-sitter's
-             incremental parsing algorithm doesn't seem to like it
-             (bug#61369).  */
+	     otherwise we would miss updating the clipped part.  Plus,
+	     when inserting in narrowed region, the narrowed region
+	     will grow to accommodate the new text, so this is the
+	     correct behavior.  (Bug#61369).  */
 	  ptrdiff_t new_end_offset = (max (visible_beg, new_end_byte)
 				      - visible_beg);
 	  eassert (start_offset <= old_end_offset);