[ot-face] Load num-glyphs from `loca` table before `maxp`

Implements [boring-expansion] [maxp] Relax
https://github.com/be-fonts/boring-expansion-spec/issues/6

src/hb-ot-hmtx-table.hh

@@ -185,7 +185,7 @@ struct hmtxvmtx
 
       table = hb_sanitize_context_t ().reference_table<hmtxvmtx> (face, T::tableTag);
 
-      /* Cap num_metrics() and num_advances() based on table length. */
+      /* Cap num_metrics and num_advances based on table length. */
       unsigned int len = table.get_length ();
       if (unlikely (num_advances * 4 > len))
 	num_advances = len / 4;

src/hb-static.cc

@@ -33,6 +33,7 @@
 #include "hb-aat-layout-feat-table.hh"
 #include "hb-ot-layout-common.hh"
 #include "hb-ot-cmap-table.hh"
+#include "hb-ot-glyf-table.hh"
 #include "hb-ot-head-table.hh"
 #include "hb-ot-maxp-table.hh"
 
@@ -55,17 +56,58 @@ const unsigned char _hb_Null_AAT_Lookup[2] = {0xFF, 0xFF};
 
 /* hb_face_t */
 
+static inline unsigned
+load_num_glyphs_from_loca (const hb_face_t *face)
+{
+  unsigned ret = 0;
+  hb_sanitize_context_t c = hb_sanitize_context_t ();
+  c.set_num_glyphs (0); /* So we don't recurse ad infinitum. */
+
+  /* We cannot use table.head because that would use sanitizer,
+   * which would try accessing face.num_glyphs, which would
+   * recurse here again... */
+  hb_blob_t *head_blob = c.reference_table<OT::head> (face);
+  const OT::head *head_table = head_blob->as<OT::head> ();
+  unsigned indexToLocFormat = head_table->indexToLocFormat;
+  hb_blob_destroy (head_blob);
+
+  if (indexToLocFormat <= 1)
+  {
+    bool short_offset = 0 == indexToLocFormat;
+    hb_blob_t *loca_blob = c.reference_table<OT::loca> (face);
+    ret = hb_max (1u, loca_blob->length / (short_offset ? 2 : 4)) - 1;
+    hb_blob_destroy (loca_blob);
+  }
+
+  return ret;
+}
+
+static inline unsigned
+load_num_glyphs_from_maxp (const hb_face_t *face)
+{
+  unsigned ret = 0;
+  hb_sanitize_context_t c = hb_sanitize_context_t ();
+  c.set_num_glyphs (0); /* So we don't recurse ad infinitum. */
+
+  hb_blob_t *maxp_blob = c.reference_table<OT::maxp> (face);
+  const OT::maxp *maxp_table = maxp_blob->as<OT::maxp> ();
+  ret = maxp_table->get_num_glyphs ();
+  hb_blob_destroy (maxp_blob);
+
+  return ret;
+}
+
 unsigned int
 hb_face_t::load_num_glyphs () const
 {
-  hb_sanitize_context_t c = hb_sanitize_context_t ();
+  unsigned ret = 0;
-  c.set_num_glyphs (0); /* So we don't recurse ad infinitum. */
+
-  hb_blob_t *maxp_blob = c.reference_table<OT::maxp> (this);
+  ret = load_num_glyphs_from_loca (this);
-  const OT::maxp *maxp_table = maxp_blob->as<OT::maxp> ();
+
+  if (!ret)
+    ret = load_num_glyphs_from_maxp (this);
 
-  unsigned int ret = maxp_table->get_num_glyphs ();
   num_glyphs.set_relaxed (ret);
-  hb_blob_destroy (maxp_blob);
   return ret;
 }
 

test/api/Makefile.am

@@ -30,6 +30,7 @@ noinst_PROGRAMS = $(TEST_PROGS)
 TEST_PROGS = \
 	test-aat-layout \
 	test-baseline \
+	test-be-num-glyphs \
 	test-blob \
 	test-buffer \
 	test-c \

test/api/hb-test.h

@@ -52,6 +52,13 @@ HB_BEGIN_DECLS
 				  ((const char *) s)[2], \
 				  ((const char *) s)[3]))
 
+#define HB_FACE_ADD_TABLE(face, tag, data) \
+	hb_face_builder_add_table ((face), \
+				   HB_TAG_CHAR4(tag), \
+				   hb_blob_create_or_fail ((data), \
+							    sizeof (data), \
+							    HB_MEMORY_MODE_READONLY, \
+							    NULL, NULL))
 
 static inline const char *
 srcdir (void)

test/api/meson.build

@@ -6,6 +6,7 @@ endif
 tests = [
   'test-aat-layout.c',
   'test-baseline.c',
+  'test-be-num-glyphs.c',
   'test-blob.c',
   'test-buffer.c',
   'test-c.c',

test/api/test-be-num-glyphs.c

@@ -0,0 +1,69 @@
+/*
+ * Copyright © 2022  Behdad Esfahbod
+ *
+ *  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.
+ */
+
+#include "hb-test.h"
+
+#include <hb.h>
+
+static void
+test_maxp_and_loca (void)
+{
+  hb_face_t *face;
+
+  const char maxp_data[] = "\x00\x00\x50\x00" // version
+			   "\x00\x05" // numGlyphs
+			   ;
+  const char loca_data[18] = "";
+
+  face = hb_face_builder_create ();
+  g_assert_cmpuint (hb_face_get_glyph_count (face), ==, 0);
+  hb_face_destroy (face);
+
+  face = hb_face_builder_create ();
+  HB_FACE_ADD_TABLE (face, "maxp", maxp_data);
+  g_assert_cmpuint (hb_face_get_glyph_count (face), ==, 5);
+  hb_face_destroy (face);
+
+  face = hb_face_builder_create ();
+  HB_FACE_ADD_TABLE (face, "maxp", maxp_data);
+  HB_FACE_ADD_TABLE (face, "loca", loca_data);
+  g_assert_cmpuint (hb_face_get_glyph_count (face), ==, 8);
+  hb_face_destroy (face);
+
+  face = hb_face_builder_create ();
+  HB_FACE_ADD_TABLE (face, "loca", loca_data);
+  g_assert_cmpuint (hb_face_get_glyph_count (face), ==, 8);
+  hb_face_destroy (face);
+}
+
+
+int
+main (int argc, char **argv)
+{
+  hb_test_init (&argc, &argv);
+
+  hb_test_add (test_maxp_and_loca);
+
+  return hb_test_run();
+}