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

From: Wolfgang Jenkner <[email-masked]>
Date: Tue, 16 Jun 2015 17:40:38 +0200

On Thu, Jun 11 2015, Wolfgang Jenkner wrote:

> The loop could be improved a bit by doing pointer arithmetic like in
> DEC_POS

I wondered whether such a change (to avoid unnecessary buffer gap
considerations while in the middle of a multibyte character) would
actually make a measurable difference, so, silly as that may be, I wrote
a simple benchmark for byte-to-position, using the tutorials as data
samples, and passed the results to ministat(1)[*], please see the
attached btp-ministat.el and ministat.sh for details.

[*] https://www.freebsd.org/cgi/man.cgi?query=ministat&sektion=1&manpath=FreeBSD+10.1-RELEASE

The result is that ministat reports statistical differences for several
of the tutorials (but not generally for the same languages at each run,
system load apparently generating too much statistical noise) and I find
that the version with the DEC_POS like loop is _always_ faster in those
cases (judging from the average values or just by taking a quick look at
the histograms).

So, while this is not really very important, it seems that it would be
safe to use the following patch with the improved loop instead:

>From be2adf5b7b427ee5d84c9ae011d8d11d452c2f4e Mon Sep 17 00:00:00 2001
From: Wolfgang Jenkner <[email-masked]>
Date: Thu, 11 Jun 2015 16:21:21 +0200
Subject: [PATCH] * src/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 | 22 ++++++++++++++++++++--
 1 file changed, 20 insertions(+), 2 deletions(-)

diff --git a/src/editfns.c b/src/editfns.c
index cddb0d4..ff54e73 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -1025,10 +1025,28 @@ 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.
+       The argument of BYTE_TO_CHAR must be a byte position at
+       a character boundary, so search for the start of the current
+       character.  */
+    {
+      unsigned char *chp = BYTE_POS_ADDR (pos_byte);
+
+      while (!CHAR_HEAD_P (*chp))
+	{
+	  pos_byte--;
+	  /* There's no buffer gap in the middle of a character.  */
+	  chp--;
+	}
+    }
+  return make_number (BYTE_TO_CHAR (pos_byte));
 }
 
 DEFUN ("following-char", Ffollowing_char, Sfollowing_char, 0, 0, 0,
-- 
2.4.2