Add hb_segment_properties_overlay()

New API:
+ hb_segment_properties_overlay()

docs/harfbuzz-sections.txt

@@ -103,6 +103,7 @@ hb_buffer_serialize_format_to_string
 hb_buffer_serialize_list_formats
 hb_segment_properties_equal
 hb_segment_properties_hash
+hb_segment_properties_overlay
 hb_buffer_diff
 hb_buffer_set_message_func
 hb_buffer_t

src/hb-buffer.cc

@@ -86,7 +86,46 @@ hb_segment_properties_hash (const hb_segment_properties_t *p)
 	 (intptr_t) (p->language);
 }
 
+/**
+ * hb_segment_properties_overlay:
+ * @p: #hb_segment_properties_t to fill in.
+ * @src: #hb_segment_properties_t to fill in from.
+ *
+ * Fills in missing fields of @p from @src in a considered manner.
+ *
+ * First, if @p does not have direction set, direction is copied from @src.
+ *
+ * Next, if @p and @src have the same direction (which can be unset), if @p
+ * does not have script set, script is copied from @src.
+ *
+ * Finally, if @p and @src have the same direction and script (which either
+ * can be unset), if @p does not have language set, language is copied from
+ * @src.
+ *
+ * Since: REPLACEME
+ **/
+HB_EXTERN void
+hb_segment_properties_overlay (hb_segment_properties_t *p,
+			       const hb_segment_properties_t *src)
+{
+  if (unlikely (!p || !src))
+    return;
 
+  if (!p->direction)
+    p->direction = src->direction;
+
+  if (p->direction != src->direction)
+    return;
+
+  if (!p->script)
+    p->script = src->script;
+
+  if (p->script != src->script)
+    return;
+
+  if (!p->language)
+    p->language = src->language;
+}
 
 /* Here is how the buffer works internally:
  *

src/hb-buffer.h

@@ -170,6 +170,9 @@ hb_segment_properties_equal (const hb_segment_properties_t *a,
 HB_EXTERN unsigned int
 hb_segment_properties_hash (const hb_segment_properties_t *p);
 
+HB_EXTERN void
+hb_segment_properties_overlay (hb_segment_properties_t *p,
+			       const hb_segment_properties_t *src);
 
 
 /**