[skrifa] Please add a glyph name function

[simoncozens]

I find myself carrying this bit of code everywhere I go:

    fn glyph_name_for_id_impl(&self, gid: impl Into<GlyphId>, synthesize: bool) -> Option<String> {
        let gid: GlyphId = gid.into();
        if self._glyphnames.borrow().is_empty() {
            if let Ok(post) = self.font().post() {
                match post.version() {
                    Version16Dot16::VERSION_1_0 => {
                        let names = DEFAULT_GLYPH_NAMES.into_iter().map(|x| Some(x.to_string()));
                        self._glyphnames.borrow_mut().extend(names);
                    }
                    Version16Dot16::VERSION_2_0 => {
                        let strings: Vec<Option<read_fonts::tables::post::PString>> =
                            post.string_data()?.iter().map(|x| x.ok()).collect();
                        if let Some(index) = post.glyph_name_index() {
                            let names = (0..self.glyph_count).map(|gid| {
                                let idx = index.get(gid)?.get() as usize;
                                if idx < 258 {
                                    Some(DEFAULT_GLYPH_NAMES[idx].to_string())
                                } else {
                                    let entry = strings.get(idx - 258)?;
                                    entry.map(|x| x.to_string())
                                }
                            });
                            self._glyphnames.borrow_mut().extend(names);
                        }
                    }
                    _ => {}
                }
            }
        }
        if let Some(Some(n)) = self._glyphnames.borrow().get(gid.to_u32() as usize) {
            Some(n.to_string())
        } else if synthesize {
            Some(format!("gid{:}", gid))
        } else {
            None
        }
    }

(Note that this gets all the glyph names at once and then uses a _glyphnames cache because random access on the post table is unpleasantly slow #1106; I guess in a library function you may have to bite the bullet and do random access, and end users can cache it if they want to.)

[dfrg]

Agreed that this is a hole in the API and we’ll also need it for HarfRuzz soon. I’ve mostly avoided doing it so far because we don’t currently have support for parsing glyph names from the cff table. I’ll add it to my list.