remove empty lookup subtables

Added a variant of subset_offset_array which takes an extra arg passed to serialize_subset for this impl.
Added a new api test "test-subset-gpos" for this.

src/hb-ot-layout-common.hh

@@ -155,6 +155,43 @@ struct subset_offset_array_t
   const void *_dest_base;
 };
 
+
+template<typename OutputArray, typename Arg>
+struct subset_offset_array_arg_t
+{
+  subset_offset_array_arg_t
+  (hb_subset_context_t *subset_context,
+   OutputArray& out,
+   const void *src_base,
+   const void *dest_base,
+   Arg &&arg)
+      : _subset_context(subset_context), _out (out), _src_base (src_base), _dest_base (dest_base), _arg (arg) {}
+
+  template <typename T>
+  bool
+  operator ()
+  (T&& offset)
+  {
+    auto *o = _out.serialize_append (_subset_context->serializer);
+    if (unlikely (!o)) return false;
+    auto snap = _subset_context->serializer->snapshot ();
+    bool ret = o->serialize_subset (_subset_context, offset, _src_base, _dest_base, _arg);
+    if (!ret)
+    {
+      _out.pop ();
+      _subset_context->serializer->revert (snap);
+    }
+    return ret;
+  }
+
+  private:
+  hb_subset_context_t *_subset_context;
+  OutputArray &_out;
+  const void *_src_base;
+  const void *_dest_base;
+  Arg &&_arg;
+};
+
 /*
  * Helper to subset an array of offsets. Subsets the thing pointed to by each offset
  * and discards the offset in the array if the subset operation results in an empty
@@ -172,6 +209,19 @@ struct
   {
     return subset_offset_array_t<OutputArray> (subset_context, out, src_base, dest_base);
   }
+
+  /* Variant with one extra argument passed to serialize_subset */
+  template<typename OutputArray, typename Arg>
+  subset_offset_array_arg_t<OutputArray, Arg>
+  operator ()
+  (hb_subset_context_t *subset_context,
+   OutputArray& out,
+   const void *src_base,
+   const void *dest_base,
+   Arg &&arg) const
+  {
+    return subset_offset_array_arg_t<OutputArray, Arg> (subset_context, out, src_base, dest_base, arg);
+  }
 }
 HB_FUNCOBJ (subset_offset_array);
 
@@ -1006,17 +1056,17 @@ struct Lookup
   bool subset (hb_subset_context_t *c) const
   {
     TRACE_SUBSET (this);
-    auto *out = c->serializer->embed (*this);
-    if (unlikely (!out)) return_trace (false);
+    auto *out = c->serializer->start_embed (*this);
+    if (unlikely (!out || !c->serializer->extend_min (out))) return_trace (false);
+    out->lookupType = lookupType;
+    out->lookupFlag = lookupFlag;
 
-    /* Subset the actual subtables. */
-    /* TODO Drop empty ones, either by calling intersects() beforehand,
-     * or just dropping null offsets after. */
-    const OffsetArrayOf<TSubTable>& subtables = get_subtables<TSubTable> ();
-    OffsetArrayOf<TSubTable>& out_subtables = out->get_subtables<TSubTable> ();
-    unsigned int count = subTable.len;
-    for (unsigned int i = 0; i < count; i++)
-      out_subtables[i].serialize_subset (c, subtables[i], this, out, get_type ());
+    const hb_set_t *glyphset = c->plan->glyphset ();
+    unsigned int lookup_type = get_type ();
+    + hb_iter (get_subtables <TSubTable> ())
+    | hb_filter ([=] (const OffsetTo<TSubTable> &_) { return (this+_).intersects (glyphset, lookup_type); })
+    | hb_apply (subset_offset_array (c, out->get_subtables<TSubTable> (), this, out, lookup_type))
+    ;
 
     return_trace (true);
   }

src/hb-ot-layout-gpos-table.hh

@@ -1931,6 +1931,12 @@ struct PosLookupSubTable
     }
   }
 
+  bool intersects (const hb_set_t *glyphs, unsigned int lookup_type) const
+  {
+    hb_intersects_context_t c (glyphs);
+    return dispatch (&c, lookup_type);
+  }
+
   protected:
   union {
   SinglePos		single;

src/hb-ot-layout-gsub-table.hh

@@ -1324,6 +1324,12 @@ struct SubstLookupSubTable
     }
   }
 
+  bool intersects (const hb_set_t *glyphs, unsigned int lookup_type) const
+  {
+    hb_intersects_context_t c (glyphs);
+    return dispatch (&c, lookup_type);
+  }
+
   protected:
   union {
   SingleSubst			single;

test/api/Makefile.am

@@ -56,6 +56,7 @@ TEST_PROGS = \
 	test-subset-hvar \
 	test-subset-vvar \
 	test-subset-sbix \
+	test-subset-gpos \
 	test-unicode \
 	test-version \
 	test-subset-nameids \
@@ -77,6 +78,7 @@ test_subset_hvar_LDADD = $(LDADD) $(top_builddir)/src/libharfbuzz-subset.la
 test_subset_vvar_LDADD = $(LDADD) $(top_builddir)/src/libharfbuzz-subset.la
 test_subset_sbix_LDADD = $(LDADD) $(top_builddir)/src/libharfbuzz-subset.la
 test_subset_nameids_LDADD = $(LDADD) $(top_builddir)/src/libharfbuzz-subset.la
+test_subset_gpos_LDADD = $(LDADD) $(top_builddir)/src/libharfbuzz-subset.la
 
 test_unicode_CPPFLAGS = \
 	$(AM_CPPFLAGS) \

test/api/test-subset-gpos.c

@@ -0,0 +1,65 @@
+/*
+ * Copyright © 2020 Adobe Inc.
+ *
+ *  This is part of HarfBuzz, a text shaping library.
+ *
+ * Permission is hereby granted, without written agreement and without
+ * license or royalty fees, to use, copy, modify, and distribute this
+ * software and its documentation for any purpose, provided that the
+ * above copyright notice and the following two paragraphs appear in
+ * all copies of this software.
+ *
+ * IN NO EVENT SHALL THE COPYRIGHT HOLDER BE LIABLE TO ANY PARTY FOR
+ * DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES
+ * ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN
+ * IF THE COPYRIGHT HOLDER HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
+ * DAMAGE.
+ *
+ * THE COPYRIGHT HOLDER SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING,
+ * BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+ * FITNESS FOR A PARTICULAR PURPOSE.  THE SOFTWARE PROVIDED HEREUNDER IS
+ * ON AN "AS IS" BASIS, AND THE COPYRIGHT HOLDER HAS NO OBLIGATION TO
+ * PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
+ *
+ * Adobe Author(s): Michiharu Ariza
+ */
+
+#include "hb-test.h"
+#include "hb-subset-test.h"
+
+/* Unit tests for GPOS subsetting */
+
+static void
+test_subset_gpos_lookup_subtable (void)
+{
+  hb_face_t *face_pwa = hb_test_open_font_file ("fonts/Roboto-Regular-gpos-.aw.ttf");
+  hb_face_t *face_wa = hb_test_open_font_file ("fonts/Roboto-Regular-gpos-aw.ttf");
+
+  hb_set_t *codepoints = hb_set_create ();
+  hb_face_t *face_pwa_subset;
+  hb_set_add (codepoints, 'a');
+  hb_set_add (codepoints, 'w');
+
+  hb_subset_input_t *input = hb_subset_test_create_input (codepoints);
+
+  hb_set_del (hb_subset_input_drop_tables_set (input), HB_TAG ('G', 'P', 'O', 'S'));
+
+  face_pwa_subset = hb_subset_test_create_subset (face_pwa, input);
+  hb_set_destroy (codepoints);
+
+  hb_subset_test_check (face_wa, face_pwa_subset, HB_TAG ('G','P','O','S'));
+
+  hb_face_destroy (face_pwa_subset);
+  hb_face_destroy (face_pwa);
+  hb_face_destroy (face_wa);
+}
+
+int
+main (int argc, char **argv)
+{
+  hb_test_init (&argc, &argv);
+
+  hb_test_add (test_subset_gpos_lookup_subtable);
+
+  return hb_test_run ();
+}