UTF-8-aware backwards string searching in Guile, or: fixing centered lyrics ignoring punctuation

[Alexander Kobel]

Dear all,

I'm happily using center-lyrics-ignoring-punctuation.ily in the version 
from 
https://lists.gnu.org/archive/html/lilypond-user/2016-12/msg00382.html 
for a while now. (It's attached as well.)

Now I stumbled across the following unexpected warning:

\version "2.19.82"
\include "center-lyrics-ignoring-punctuation.ily"
{ d4 }
\addlyrics { à }

> GNU LilyPond 2.19.82
> Processing `test.ly'
> Parsing...
> Interpreting music...
> Preprocessing graphical objects...
> (process:2528): Pango-WARNING **: 17:49:34.874: Invalid UTF-8 string passed to 
> pango_layout_set_text()
>
> warning: no glyph for character U+FFFD in font 
> `/usr/share/fonts/tex-gyre/texgyreschola-regular.otf'

Observations:
1. The warning vanishes as soon as I replace the lyrics by àa, but not 
by aà.
2. There is no U+FFFD (REPLACEMENT CHARACTER) anywhere in my input.

This makes me wonder whether the problem is in the backwards 
string-search in string-skip-right or in the substring routine used in 
the make-center-on-word-callback; the only reason I can imagine why this 
pops up is that some blind-to-Unicode slicing cuts some string in the 
middle of a multi-byte Unicode character.

The warning is merely annoying, but harmless; but I suspect that there 
are unfortunate combinations where trailing "to-be-ignored" characters 
are missed, or "not-to-be-ignored" characters are split into an 
ill-defined character and a false "to-be-ignored".

Does anyone have a hint how to approach this one? (Or is the answer 
just: be patient and hope for Guile v2?)


Thanks,
Alex

center-lyrics-ignoring-punctuation.ily
Description: Text document

test.ly
Description: Text Data

smime.p7s
Description: S/MIME Cryptographic Signature