[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.

[behdad]

Would be nice to match HB on this.

[behdad]

Would be nice to match HB on this.

HB has two APIs. One low-level, that can return false if the font has no glyph names. The other will fall back to gidDDD where DDD is the glyph index in decimal.

[simoncozens]

Hah, I didn't realise that but fontspector does the same:

    /// Get a glyph's name by Glyph ID, if present in the font
    pub fn glyph_name_for_id(&self, gid: impl Into<GlyphId>) -> Option<String> {
        self.glyph_name_for_id_impl(gid, false)
    }

    /// Get a glyph's name by Glyph ID, synthesizing a name if not present in the font
    ///
    /// For example GID 1024 will be named "gid1024".
    pub fn glyph_name_for_id_synthesise(&self, gid: impl Into<GlyphId>) -> String {
        #[allow(clippy::unwrap_used)]
        self.glyph_name_for_id_impl(gid, true).unwrap()
    }