kc3-lang/freetype

2025-04-20T06:27:52

[autofit] Speed up creation of the adjustment database's reverse map. (1/3) As it turns out, the original implementation using `hb_ot_shape_glyphs_closure` is extremely slow if a font has a rich set of OpenType features. For example, this function was called 66954 times while loading font `arial.ttf` version 7.00, increasing FreeType's startup time by a factor of 10, which is unacceptable. The new algorithm uses a completely different, more low-level approach, no longer working with OpenType features but with OpenType lookups. It relies on function `hb_ot_layout_lookup_get_glyph_alternates`, also replacing recursion with a simple loop. In total, this brings the additional startup time back to an acceptable range of a few percent. A side effect of the new approach is that it catches more alternate forms: the old code didn't properly handle script-specific features. To make the change more readable, this commit only adds new code.