[arabic] Make ZWJ prevent ligatures instead of facilitating it
Unicode 6.2.0 Section 16.2 / Figure 16.3 says:
"For backward compatibility, between Arabic characters a ZWJ acts just
like the sequence <ZWJ, ZWNJ, ZWJ>, preventing a ligature from forming
instead of requesting the use of a ligature that would not normally be
used. As a result, there is no plain text mechanism for requesting the
use of a ligature in Arabic text."
As such, we flip internal zwj to zwnj flags for GSUB matching, which
means it will block ligation in all features, unless the font
explicitly matches U+200D glyph. This doesn't affect joining behavior.
diff --git a/src/hb-ot-shape-complex-arabic.cc b/src/hb-ot-shape-complex-arabic.cc
index a57e81a..d549945 100644
--- a/src/hb-ot-shape-complex-arabic.cc
+++ b/src/hb-ot-shape-complex-arabic.cc
@@ -157,6 +157,11 @@
static void
+nuke_joiners (const hb_ot_shape_plan_t *plan,
+ hb_font_t *font,
+ hb_buffer_t *buffer);
+
+static void
arabic_fallback_shape (const hb_ot_shape_plan_t *plan,
hb_font_t *font,
hb_buffer_t *buffer);
@@ -176,6 +181,8 @@
* TODO: Add test cases for these two.
*/
+ map->add_gsub_pause (nuke_joiners);
+
map->add_global_bool_feature (HB_TAG('c','c','m','p'));
map->add_global_bool_feature (HB_TAG('l','o','c','l'));
@@ -314,6 +321,17 @@
static void
+nuke_joiners (const hb_ot_shape_plan_t *plan HB_UNUSED,
+ hb_font_t *font HB_UNUSED,
+ hb_buffer_t *buffer)
+{
+ unsigned int count = buffer->len;
+ for (unsigned int i = 0; i < count; i++)
+ if (_hb_glyph_info_is_zwj (&buffer->info[i]))
+ _hb_glyph_info_flip_joiners (&buffer->info[i]);
+}
+
+static void
arabic_fallback_shape (const hb_ot_shape_plan_t *plan,
hb_font_t *font,
hb_buffer_t *buffer)
