[kerx] Implement Format4 'ankr'-based mark attachment

Tested with Kannada MN:

$ HB_OPTIONS=aat ./hb-shape Kannada\ MN.ttc -u 0CCD,0C95,0CD6
[kn_ka.vattu=0+230|kn_ai_length_mark=1@326,0+607]

src/hb-aat-layout-common.hh

@@ -531,6 +531,7 @@ struct hb_aat_apply_context_t :
   hb_buffer_t *buffer;
   hb_sanitize_context_t sanitizer;
   const ankr &ankr_table;
+  const char *ankr_end;
 
   /* Unused. For debug tracing only. */
   unsigned int lookup_index;
@@ -540,9 +541,12 @@ struct hb_aat_apply_context_t :
 				 hb_font_t *font_,
 				 hb_buffer_t *buffer_,
 				 hb_blob_t *table,
-				 const ankr &ankr_table_ = Null(ankr)) :
+				 const ankr &ankr_table_ = Null(ankr),
+				 const char *ankr_end_ = nullptr) :
 		plan (plan_), font (font_), face (font->face), buffer (buffer_),
-		sanitizer (), ankr_table (ankr_table_), lookup_index (0), debug_depth (0)
+		sanitizer (),
+		ankr_table (ankr_table_), ankr_end (ankr_end_),
+		lookup_index (0), debug_depth (0)
   {
     sanitizer.init (table);
     sanitizer.set_num_glyphs (face->get_num_glyphs ());

src/hb-aat-layout-kerx-table.hh

@@ -30,6 +30,7 @@
 
 #include "hb-open-type.hh"
 #include "hb-aat-layout-common.hh"
+#include "hb-ot-layout-gpos-table.hh"
 #include "hb-ot-kern-table.hh"
 
 /*
@@ -362,9 +363,22 @@ struct KerxSubTableFormat4
 	    const HBUINT16 *data = &ankrData[entry->data.ankrActionIndex];
 	    if (!c->sanitizer.check_array (data, 2))
 	      return false;
-	    HB_UNUSED unsigned int markAnchorPoint = *data++;
+	    unsigned int markAnchorPoint = *data++;
-	    HB_UNUSED unsigned int currAnchorPoint = *data++;
+	    unsigned int currAnchorPoint = *data++;
-	    /* TODO */
+	    const Anchor markAnchor = c->ankr_table.get_anchor (c->buffer->info[mark].codepoint,
+								markAnchorPoint,
+								c->face->get_num_glyphs (),
+								c->ankr_end);
+	    const Anchor currAnchor = c->ankr_table.get_anchor (c->buffer->cur ().codepoint,
+								currAnchorPoint,
+								c->face->get_num_glyphs (),
+								c->ankr_end);
+	    hb_glyph_position_t &o = buffer->cur_pos();
+	    o.x_offset = c->font->em_scale_x (markAnchor.xCoordinate) - c->font->em_scale_x (currAnchor.xCoordinate);
+	    o.y_offset = c->font->em_scale_y (markAnchor.yCoordinate) - c->font->em_scale_y (currAnchor.yCoordinate);
+	    o.attach_type() = ATTACH_TYPE_MARK;
+	    o.attach_chain() = (int) mark - (int) buffer->idx;
+	    buffer->scratch_flags |= HB_BUFFER_SCRATCH_FLAG_HAS_GPOS_ATTACHMENT;
 	  }
 	  break;
 

src/hb-aat-layout.cc

@@ -69,10 +69,18 @@ _get_kerx (hb_face_t *face, hb_blob_t **blob = nullptr)
   return kerx;
 }
 static inline const AAT::ankr&
-_get_ankr (hb_face_t *face)
+_get_ankr (hb_face_t *face, hb_blob_t **blob = nullptr)
 {
-  if (unlikely (!hb_ot_shaper_face_data_ensure (face))) return Null(AAT::ankr);
+  if (unlikely (!hb_ot_shaper_face_data_ensure (face)))
-  return *(hb_ot_face_data (face)->ankr.get ());
+  {
+    if (blob)
+      *blob = hb_blob_get_empty ();
+    return Null(AAT::ankr);
+  }
+  const AAT::ankr& ankr = *(hb_ot_face_data (face)->ankr.get ());
+  if (blob)
+    *blob = hb_ot_face_data (face)->ankr.get_blob ();
+  return ankr;
 }
 
 
@@ -109,6 +117,10 @@ hb_aat_layout_position (hb_ot_shape_plan_t *plan,
   hb_blob_t *blob;
   const AAT::kerx& kerx = _get_kerx (font->face, &blob);
 
-  AAT::hb_aat_apply_context_t c (plan, font, buffer, blob, _get_ankr (font->face));
+  hb_blob_t *ankr_blob;
+  const AAT::ankr& ankr = _get_ankr (font->face, &ankr_blob);
+
+  AAT::hb_aat_apply_context_t c (plan, font, buffer, blob,
+				 ankr, ankr_blob->data + ankr_blob->length);
   kerx.apply (&c);
 }

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

@@ -1755,10 +1755,6 @@ template <typename context_t>
 struct GPOS_accelerator_t : GPOS::accelerator_t {};
 
 
-#undef attach_chain
-#undef attach_type
-
-
 } /* namespace OT */