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

From: Eli Zaretskii <[email-masked]>
Date: Thu, 11 Jun 2015 19:04:24 +0300

> From: Wolfgang Jenkner <[email-masked]>
> Cc: [email-masked]
> Date: Thu, 11 Jun 2015 17:24:42 +0200
> 
> 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));
>  }

Works for me, thanks.  But please add a comment there about
BYTE_TO_CHAR expecting byte positions that are on a character
boundary, so that the reason for the loop is clear.