[ft] Cache advances

I decided to always use the cache, instead of my previous sketch direction
that was to only allocate and use cache if fast advances are not available.
The cache is a mere 1kb, so just use it...

TODO: Invalidate cache on font size change.

Fixes https://github.com/harfbuzz/harfbuzz/issues/651
Fixes https://github.com/harfbuzz/harfbuzz/pull/1082

src/hb-cache.hh

@@ -36,7 +36,7 @@ template <unsigned int key_bits, unsigned int value_bits, unsigned int cache_bit
 struct hb_cache_t
 {
   static_assert ((key_bits >= cache_bits), "");
-  static_assert ((key_bits + value_bits - cache_bits < 8 * sizeof (unsigned int)), "");
+  static_assert ((key_bits + value_bits - cache_bits <= 8 * sizeof (unsigned int)), "");
 
   inline void init (void) { clear (); }
   inline void fini (void) {}

src/hb-ft.cc

@@ -69,6 +69,7 @@ struct hb_ft_font_t
   int load_flags;
   bool symbol; /* Whether selected cmap is symbol cmap. */
   bool unref; /* Whether to destroy ft_face when done. */
+  mutable hb_advance_cache_t advance_cache;
 };
 
 static hb_ft_font_t *
@@ -85,6 +86,8 @@ _hb_ft_font_create (FT_Face ft_face, bool symbol, bool unref)
 
   ft_font->load_flags = FT_LOAD_DEFAULT | FT_LOAD_NO_HINTING;
 
+  ft_font->advance_cache.init ();
+
   return ft_font;
 }
 
@@ -99,6 +102,8 @@ _hb_ft_font_destroy (void *data)
 {
   hb_ft_font_t *ft_font = (hb_ft_font_t *) data;
 
+  ft_font->advance_cache.fini ();
+
   if (ft_font->unref)
     _hb_ft_face_destroy (ft_font->ft_face);
 
@@ -239,31 +244,23 @@ hb_ft_get_glyph_h_advances (hb_font_t* font, void* font_data,
 {
   const hb_ft_font_t *ft_font = (const hb_ft_font_t *) font_data;
   FT_Face ft_face = ft_font->ft_face;
-  int load_flags;
-
+  int load_flags = ft_font->load_flags;
   int mult = font->x_scale < 0 ? -1 : +1;
 
-  unsigned int i = 0;
-
-  load_flags = ft_font->load_flags | FT_ADVANCE_FLAG_FAST_ONLY;
-  for (; i < count; i++)
+  for (unsigned int i = 0; i < count; i++)
   {
     FT_Fixed v = 0;
-    if (unlikely (FT_Get_Advance (ft_face, *first_glyph, load_flags, &v)))
-      goto slow;
-    *first_advance = (v * mult + (1<<9)) >> 10;
-    first_glyph = &StructAtOffset<hb_codepoint_t> (first_glyph, glyph_stride);
-    first_advance = &StructAtOffset<hb_position_t> (first_advance, advance_stride);
-  }
-  return;
+    hb_codepoint_t glyph = *first_glyph;
+
+    unsigned int cv;
+    if (ft_font->advance_cache.get (glyph, &cv))
+      v = cv;
+    else
+    {
+      FT_Get_Advance (ft_face, glyph, load_flags, &v);
+      ft_font->advance_cache.set (glyph, v);
+    }
 
-slow:
-  /* TODO Prepare and use cache. */
-  load_flags = ft_font->load_flags;// & ~FT_ADVANCE_FLAG_FAST_ONLY;
-  for (; i < count; i++)
-  {
-    FT_Fixed v = 0;
-    FT_Get_Advance (ft_face, *first_glyph, load_flags, &v);
     *first_advance = (v * mult + (1<<9)) >> 10;
     first_glyph = &StructAtOffset<hb_codepoint_t> (first_glyph, glyph_stride);
     first_advance = &StructAtOffset<hb_position_t> (first_advance, advance_stride);