fix VarData serialize to remove unused data sets

add api test case for that

src/hb-ot-layout-common.hh

@@ -1883,7 +1883,11 @@ struct VariationStore
   		  const hb_array_t <hb_bimap_t> &inner_remaps)
   {
     TRACE_SUBSET (this);
-    unsigned int size = min_size + HBUINT32::static_size * inner_remaps.length;
+    unsigned int set_count = 0;
+    for (unsigned int i = 0; i < inner_remaps.length; i++)
+      if (inner_remaps[i].get_count () > 0) set_count++;
+
+    unsigned int size = min_size + HBUINT32::static_size * set_count;
     if (unlikely (!c->allocate_size<HBUINT32> (size))) return_trace (false);
     format = 1;
     if (unlikely (!regions.serialize (c, this)
@@ -1892,10 +1896,12 @@ struct VariationStore
     /* TODO: The following code could be simplified when
      * OffsetListOf::subset () can take a custom param to be passed to VarData::serialize ()
      */
-    dataSets.len = inner_remaps.length;
+    dataSets.len = set_count;
+    unsigned int set_index = 0;
     for (unsigned int i = 0; i < inner_remaps.length; i++)
     {
-      if (unlikely (!dataSets[i].serialize (c, this)
+      if (inner_remaps[i].get_count () == 0) continue;
+      if (unlikely (!dataSets[set_index++].serialize (c, this)
 		      .serialize (c, &(src+src->dataSets[i]), inner_remaps[i])))
 	return_trace (false);
     }

test/api/test-subset-hvar.c

@@ -90,6 +90,26 @@ test_subset_map_HVAR_retaingids (void)
   hb_face_destroy (face_ac);
 }
 
+static void
+test_subset_map_modHVAR (void)
+{
+  hb_face_t *face_abc = hb_test_open_font_file ("fonts/SourceSansVariable-Roman-modHVAR.abc.ttf");
+  hb_face_t *face_ac = hb_test_open_font_file ("fonts/SourceSansVariable-Roman-modHVAR.ac.ttf");
+
+  hb_set_t *codepoints = hb_set_create ();
+  hb_face_t *face_abc_subset;
+  hb_set_add (codepoints, 'a');
+  hb_set_add (codepoints, 'c');
+  face_abc_subset = hb_subset_test_create_subset (face_abc, hb_subset_test_create_input (codepoints));
+  hb_set_destroy (codepoints);
+
+  hb_subset_test_check (face_ac, face_abc_subset, HB_TAG ('H','V','A','R'));
+
+  hb_face_destroy (face_abc_subset);
+  hb_face_destroy (face_abc);
+  hb_face_destroy (face_ac);
+}
+
 static void
 test_subset_identity_HVAR_noop (void)
 {
@@ -160,6 +180,7 @@ main (int argc, char **argv)
   hb_test_add (test_subset_map_HVAR_noop);
   hb_test_add (test_subset_map_HVAR);
   hb_test_add (test_subset_map_HVAR_retaingids);
+  hb_test_add (test_subset_map_modHVAR);
   hb_test_add (test_subset_identity_HVAR_noop);
   hb_test_add (test_subset_identity_HVAR);
   hb_test_add (test_subset_identity_HVAR_retaingids);