[subset] Truncate empty gids at the end in retain-gids mode.

src/hb-subset-plan.cc

@@ -183,9 +183,11 @@ _create_old_gid_to_new_gid_map (const hb_face_t *face,
     | hb_sink (reverse_glyph_map)
     ;
 
-    // TODO(grieger): Should we discard glyphs past the max glyph to keep?
-    // *num_glyphs = + hb_iter (all_gids_to_retain) | hb_reduce (hb_max, 0);
-    *num_glyphs = face->get_num_glyphs ();
+    unsigned max_glyph =
+    + hb_iter (all_gids_to_retain)
+    | hb_reduce (hb_max, 0)
+    ;
+    *num_glyphs = max_glyph + 1;
   }
 
   + reverse_glyph_map->iter ()

test/api/test-subset-glyf.c

@@ -305,6 +305,30 @@ test_subset_glyf_retain_gids (void)
   hb_face_destroy (face_ac);
 }
 
+static void
+test_subset_glyf_retain_gids_truncates (void)
+{
+  hb_face_t *face_abc = hb_test_open_font_file ("fonts/Roboto-Regular.abc.ttf");
+  hb_face_t *face_a = hb_test_open_font_file ("fonts/Roboto-Regular.a.retaingids.ttf");
+
+  hb_set_t *codepoints = hb_set_create();
+  hb_face_t *face_abc_subset;
+  hb_set_add (codepoints, 97);
+
+  hb_subset_input_t *input = hb_subset_test_create_input (codepoints);
+  hb_subset_input_set_retain_gids (input, true);
+  face_abc_subset = hb_subset_test_create_subset (face_abc, input);
+  hb_set_destroy (codepoints);
+
+  hb_subset_test_check (face_a, face_abc_subset, HB_TAG ('g','l','y','f'));
+  hb_subset_test_check (face_a, face_abc_subset, HB_TAG ('l','o','c', 'a'));
+  check_maxp_num_glyphs(face_abc_subset, 2, true);
+
+  hb_face_destroy (face_abc_subset);
+  hb_face_destroy (face_abc);
+  hb_face_destroy (face_a);
+}
+
 // TODO(grieger): test for long loca generation.
 
 int
@@ -322,6 +346,7 @@ main (int argc, char **argv)
   hb_test_add (test_subset_glyf_with_gsub);
   hb_test_add (test_subset_glyf_without_gsub);
   hb_test_add (test_subset_glyf_retain_gids);
+  hb_test_add (test_subset_glyf_retain_gids_truncates);
 
   return hb_test_run();
 }