[ot-map] Speed up feature finding

New API:
- hb_ot_layout_collect_feature_map()

docs/harfbuzz-sections.txt

@@ -619,6 +619,7 @@ hb_ot_tags_from_script_and_language
 hb_ot_tags_to_script_and_language
 hb_ot_layout_collect_lookups
 hb_ot_layout_collect_features
+hb_ot_layout_collect_features_map
 hb_ot_layout_feature_get_characters
 hb_ot_layout_feature_get_lookups
 hb_ot_layout_feature_get_name_ids

src/hb-ot-layout.cc

@@ -1241,7 +1241,7 @@ script_collect_features (hb_collect_features_context_t *c,
  *   terminated by %HB_TAG_NONE
  * @features: (nullable) (array zero-terminated=1): The array of features to collect,
  *   terminated by %HB_TAG_NONE
- * @feature_indexes: (out): The array of feature indexes found for the query
+ * @feature_indexes: (out): The set of feature indexes found for the query
  *
  * Fetches a list of all feature indexes in the specified face's GSUB table
  * or GPOS table, underneath the specified scripts, languages, and features.
@@ -1282,6 +1282,43 @@ hb_ot_layout_collect_features (hb_face_t      *face,
   }
 }
 
+/**
+ * hb_ot_layout_collect_features_map:
+ * @face: #hb_face_t to work upon
+ * @table_tag: #HB_OT_TAG_GSUB or #HB_OT_TAG_GPOS
+ * @script_index: The index of the requested script tag
+ * @language_index: The index of the requested language tag
+ * @feature_map: (out): The map of feature tag to feature index.
+ *
+ * Fetches the mapping from feature tags to feature indexes for
+ * the specified script and language.
+ *
+ * XSince: REPLACEME
+ **/
+void
+hb_ot_layout_collect_features_map (hb_face_t      *face,
+				   hb_tag_t        table_tag,
+				   unsigned        script_index,
+				   unsigned        language_index,
+				   hb_map_t       *feature_map /* OUT */)
+{
+  const OT::GSUBGPOS &g = get_gsubgpos_table (face, table_tag);
+  const OT::LangSys &l = g.get_script (script_index).get_lang_sys (language_index);
+
+  unsigned int count = l.get_feature_indexes (0, nullptr, nullptr);
+
+  for (unsigned int i = 0; i < count; i++)
+  {
+    unsigned feature_index = 0;
+    unsigned feature_count = 1;
+    l.get_feature_indexes (i, &feature_count, &feature_index);
+    if (!feature_count)
+      break;
+    hb_tag_t feature_tag = g.get_feature_tag (feature_index);
+    feature_map->set (feature_tag, feature_index);
+  }
+}
+
 
 /**
  * hb_ot_layout_collect_lookups:

src/hb-ot-layout.h

@@ -324,6 +324,13 @@ hb_ot_layout_collect_features (hb_face_t      *face,
 			       const hb_tag_t *features,
 			       hb_set_t       *feature_indexes /* OUT */);
 
+HB_EXTERN void
+hb_ot_layout_collect_features_map (hb_face_t      *face,
+				   hb_tag_t        table_tag,
+				   unsigned        script_index,
+				   unsigned        language_index,
+				   hb_map_t       *feature_map /* OUT */);
+
 HB_EXTERN void
 hb_ot_layout_collect_lookups (hb_face_t      *face,
 			      hb_tag_t        table_tag,

src/hb-ot-map.cc

@@ -239,6 +239,13 @@ hb_ot_map_builder_t::compile (hb_ot_map_t                  &m,
     feature_infos.shrink (j + 1);
   }
 
+  hb_map_t feature_indices[2];
+  for (unsigned int table_index = 0; table_index < 2; table_index++)
+    hb_ot_layout_collect_features_map (face,
+				       table_tags[table_index],
+				       script_index[table_index],
+				       language_index[table_index],
+				       &feature_indices[table_index]);
 
   /* Allocate bits now */
   static_assert ((!(HB_GLYPH_FLAG_DEFINED & (HB_GLYPH_FLAG_DEFINED + 1))), "");
@@ -261,7 +268,6 @@ hb_ot_map_builder_t::compile (hb_ot_map_t                  &m,
     if (!info->max_value || next_bit + bits_needed >= global_bit_shift)
       continue; /* Feature disabled, or not enough bits. */
 
-
     bool found = false;
     unsigned int feature_index[2];
     for (unsigned int table_index = 0; table_index < 2; table_index++)
@@ -269,12 +275,14 @@ hb_ot_map_builder_t::compile (hb_ot_map_t                  &m,
       if (required_feature_tag[table_index] == info->tag)
 	required_feature_stage[table_index] = info->stage[table_index];
 
-      found |= (bool) hb_ot_layout_language_find_feature (face,
-							  table_tags[table_index],
-							  script_index[table_index],
-							  language_index[table_index],
-							  info->tag,
-							  &feature_index[table_index]);
+      hb_codepoint_t *index;
+      if (feature_indices[table_index].has (info->tag, &index))
+      {
+        feature_index[table_index] = *index;
+        found = true;
+      }
+      else
+        feature_index[table_index] = HB_OT_LAYOUT_NO_FEATURE_INDEX;
     }
     if (!found && (info->flags & F_GLOBAL_SEARCH))
     {