[Arabic] Respect Arabic joining from neighboring context

Now we respect Arabic joining across runs.

src/hb-ot-shape-complex-arabic.cc

@@ -229,10 +229,23 @@ static void
 arabic_joining (hb_buffer_t *buffer)
 {
   unsigned int count = buffer->len;
-  unsigned int prev = 0, state = 0;
+  unsigned int prev = (unsigned int) -1, state = 0;
 
   HB_BUFFER_ALLOCATE_VAR (buffer, arabic_shaping_action);
 
+  /* Check pre-context */
+  for (unsigned int i = 0; i < buffer->context_len[0]; i++)
+  {
+    unsigned int this_type = get_joining_type (buffer->context[0][i], buffer->unicode->general_category (buffer->context[0][i]));
+
+    if (unlikely (this_type == JOINING_TYPE_T))
+      continue;
+
+    const arabic_state_table_entry *entry = &arabic_state_table[state][this_type];
+    state = entry->next_state;
+    break;
+  }
+
   for (unsigned int i = 0; i < count; i++)
   {
     unsigned int this_type = get_joining_type (buffer->info[i].codepoint, _hb_glyph_info_get_general_category (&buffer->info[i]));
@@ -244,7 +257,7 @@ arabic_joining (hb_buffer_t *buffer)
 
     const arabic_state_table_entry *entry = &arabic_state_table[state][this_type];
 
-    if (entry->prev_action != NONE)
+    if (entry->prev_action != NONE && prev != (unsigned int) -1)
       buffer->info[prev].arabic_shaping_action() = entry->prev_action;
 
     buffer->info[i].arabic_shaping_action() = entry->curr_action;
@@ -253,6 +266,20 @@ arabic_joining (hb_buffer_t *buffer)
     state = entry->next_state;
   }
 
+  for (unsigned int i = 0; i < buffer->context_len[1]; i++)
+  {
+    unsigned int this_type = get_joining_type (buffer->context[1][i], buffer->unicode->general_category (buffer->context[0][i]));
+
+    if (unlikely (this_type == JOINING_TYPE_T))
+      continue;
+
+    const arabic_state_table_entry *entry = &arabic_state_table[state][this_type];
+    if (entry->prev_action != NONE && prev != (unsigned int) -1)
+      buffer->info[prev].arabic_shaping_action() = entry->prev_action;
+    break;
+  }
+
+
   HB_BUFFER_DEALLOCATE_VAR (buffer, arabic_shaping_action);
 }