[gtk/ngl-glyph-cache: 3/3] ngl: Improve the glyph front cache

[Matthias Clasen <matthiasc src gnome org>]

commit b253aca883e886a5b5b8eaa770594ad1b77e9afe
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Mar 19 14:30:19 2021 -0400

    ngl: Improve the glyph front cache
    
    The effectiveness of the front cache is limited by
    subpixel positioning making it very likely that we
    will meet the same glyph in  different x phases inside
    a single line of text.
    
    Factoring the xphase into the front cache key makes things
    better. For the string eeeeeeeeeeeeeeeeeee
    
    before: 0% front cache hits
    after: >90% front cache hits

 gsk/ngl/gsknglglyphlibraryprivate.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gsk/ngl/gsknglglyphlibraryprivate.h b/gsk/ngl/gsknglglyphlibraryprivate.h
index f48ae6e585..c44068dd50 100644
--- a/gsk/ngl/gsknglglyphlibraryprivate.h
+++ b/gsk/ngl/gsknglglyphlibraryprivate.h
@@ -91,7 +91,7 @@ gsk_ngl_glyph_library_lookup_or_add (GskNglGlyphLibrary      *self,
                                      const GskNglGlyphValue **out_value)
 {
   GskNglTextureAtlasEntry *entry;
-  guint front_index = key->glyph & 0xFF;
+  guint front_index = ((key->glyph << 2) | key->xshift) & 0xFF;
 
   if (memcmp (key, &self->front[front_index], sizeof *key) == 0)
     {