From 718bf5aab397290ab5214c6a6bff3e248c4ab094 Mon Sep 17 00:00:00 2001
From: Garret Rieger <grieger@google.com>
Date: Tue, 29 Sep 2020 13:16:01 -0700
Subject: [PATCH] [subset] only keep features reachable from script in the
 final subset. Matches fontTools behaviour.

---
 src/hb-ot-layout-gsubgpos.hh | 31 +++++++++++++++++++++----------
 src/hb-subset-plan.cc        | 21 +++++++++++++++++----
 2 files changed, 38 insertions(+), 14 deletions(-)

diff --git a/src/hb-ot-layout-gsubgpos.hh b/src/hb-ot-layout-gsubgpos.hh
index afa00d20c..b3028e655 100644
--- a/src/hb-ot-layout-gsubgpos.hh
+++ b/src/hb-ot-layout-gsubgpos.hh
@@ -3380,20 +3380,31 @@ struct GSUBGPOS
     return_trace (true);
   }
 
-  void closure_features (const hb_map_t *lookup_indexes, /* IN */
-			 hb_set_t       *feature_indexes /* OUT */) const
+  void prune_features (const hb_map_t *lookup_indices, /* IN */
+                       hb_set_t       *feature_indices /* IN/OUT */) const
   {
-    unsigned int feature_count = hb_min (get_feature_count (), (unsigned) HB_MAX_FEATURES);
-    for (unsigned i = 0; i < feature_count; i++)
+#ifndef HB_NO_VAR
+    // This is the set of feature indices which have alternate versions defined
+    // if the FeatureVariation's table and the alternate version(s) intersect the
+    // set of lookup indices.
+    hb_set_t alternate_feature_indices;
+    if (version.to_int () >= 0x00010001u)
+      (this+featureVars).closure_features (lookup_indices, &alternate_feature_indices);
+    if (alternate_feature_indices.in_error()) {
+      feature_indices->successful = false;
+      return;
+    }
+#endif
+
+    for (unsigned i : feature_indices->iter())
     {
       const Feature& f = get_feature (i);
-      if ((!f.featureParams.is_null ()) || f.intersects_lookup_indexes (lookup_indexes))
-	feature_indexes->add (i);
+
+      if (f.featureParams.is_null ()
+          && !f.intersects_lookup_indexes (lookup_indices)
+          && !alternate_feature_indices.has (i))
+        feature_indices->del (i);
     }
-#ifndef HB_NO_VAR
-    if (version.to_int () >= 0x00010001u)
-      (this+featureVars).closure_features (lookup_indexes, feature_indexes);
-#endif
   }
 
   unsigned int get_size () const
diff --git a/src/hb-subset-plan.cc b/src/hb-subset-plan.cc
index 24beada3e..b58c27eeb 100644
--- a/src/hb-subset-plan.cc
+++ b/src/hb-subset-plan.cc
@@ -88,10 +88,17 @@ _gsub_closure_glyphs_lookups_features (hb_face_t *face,
 			 &lookup_indices);
   _remap_indexes (&lookup_indices, gsub_lookups);
 
-  //closure features
+  // Collect and prune features
   hb_set_t feature_indices;
-  gsub->closure_features (gsub_lookups, &feature_indices);
+  hb_ot_layout_collect_features (face,
+                                 HB_OT_TAG_GSUB,
+                                 nullptr,
+                                 nullptr,
+                                 nullptr,
+                                 &feature_indices);
+  gsub->prune_features (gsub_lookups, &feature_indices);
   _remap_indexes (&feature_indices, gsub_features);
+
   gsub.destroy ();
 }
 
@@ -114,9 +121,15 @@ _gpos_closure_lookups_features (hb_face_t      *face,
 			 &lookup_indices);
   _remap_indexes (&lookup_indices, gpos_lookups);
 
-  //closure features
+  // Collect and prune features
   hb_set_t feature_indices;
-  gpos->closure_features (gpos_lookups, &feature_indices);
+  hb_ot_layout_collect_features (face,
+                                 HB_OT_TAG_GPOS,
+                                 nullptr,
+                                 nullptr,
+                                 nullptr,
+                                 &feature_indices);
+  gpos->prune_features (gpos_lookups, &feature_indices);
   _remap_indexes (&feature_indices, gpos_features);
   gpos.destroy ();
 }