bug#20783: 25.0.50; [PATCH] byte-to-position has internal off-by-one bug

[Wolfgang Jenkner <wjenkner@inode.at>]

On Wed, Jun 10 2015, Eli Zaretskii wrote:

> Wouldn't it be better to handle this use case in Fbyte_to_position?
> The BYTE_TO_CHAR macro is called an awful lot in the Emacs innermost
> loops, and it's _always_ called with a byte position that's on a
> character boundary.

I see.  How about something like the patch below?  The loop could be
improved a bit by doing pointer arithmetic like in DEC_POS but it's
perhaps not worth complicating things for the case where bytepos is not
at a character boundary.

-- >8 --
Subject: [PATCH] * editfns.c (Fbyte_to_position): Fix bytepos not at char
 boundary.

The behavior now matches the description in the manual.  (Bug#20783)
---
 src/editfns.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/src/editfns.c b/src/editfns.c
index cddb0d4..94715fe 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -1025,10 +1025,18 @@ DEFUN ("byte-to-position", Fbyte_to_position, Sbyte_to_position, 1, 1, 0,
 If BYTEPOS is out of range, the value is nil.  */)
   (Lisp_Object bytepos)
 {
+  ptrdiff_t pos_byte;
+
   CHECK_NUMBER (bytepos);
-  if (XINT (bytepos) < BEG_BYTE || XINT (bytepos) > Z_BYTE)
+  pos_byte = XINT (bytepos);
+  if (pos_byte < BEG_BYTE || pos_byte > Z_BYTE)
     return Qnil;
-  return make_number (BYTE_TO_CHAR (XINT (bytepos)));
+  if (Z != Z_BYTE)
+    /* There are multibyte characters in the buffer.
+       Search for the start of the current character. */
+    while (!CHAR_HEAD_P (FETCH_BYTE (pos_byte)))
+      pos_byte--;
+  return make_number (BYTE_TO_CHAR (pos_byte));
 }
 \f
 DEFUN ("following-char", Ffollowing_char, Sfollowing_char, 0, 0, 0,
-- 
2.4.2