Disable bidi bracket matching unless enabled via ASS_Feature

Bracket matching is incompatible with VSFilter (even on modern
Windows), so disable it by default. But as it's generally
a good thing (and 100% more compliant with current Unicode),
keep it available as an ASS_Feature.

It can be toggled individually or enabled as part of the
catch-all ASS_FEATURE_INCOMPATIBLE_EXTENSIONS feature.

If libass is compiled against FriBidi older than 1.0,
bracket matching is impossible. Signal this at runtime
by failing to recognize the ASS_FEATURE_BIDI_BRACKETS
feature. This way, clients who want to use bracket matching
can set the feature without any compile-time checks for
FriBidi and can be freely linked against libass that is itself
compiled against any version of FriBidi; and yet they can
detect at runtime whether the feature is actually enabled.

Fixes https://github.com/libass/libass/issues/374.

1 files changed, 14 insertions, 1 deletions

diff --git a/libass/ass.h b/libass/ass.h
index 35aeeb7..db67ed2 100644
--- a/libass/ass.h
+++ b/libass/ass.h
@@ -24,7 +24,7 @@
 #include <stdarg.h>
 #include "ass_types.h"
 
-#define LIBASS_VERSION 0x01400001
+#define LIBASS_VERSION 0x01400002
 
 #ifdef __cplusplus
 extern "C" {
@@ -209,6 +209,19 @@ typedef enum {
      */
     ASS_FEATURE_INCOMPATIBLE_EXTENSIONS,
 
+    /**
+     * Match bracket pairs in bidirectional text according to the revised
+     * Unicode Bidirectional Algorithm introduced in Unicode 6.3.
+     * This is incompatible with VSFilter and disabled by default.
+     *
+     * (Directional isolates, also introduced in Unicode 6.3,
+     * are unconditionally processed when FriBidi is new enough.)
+     *
+     * This feature may be unavailable at runtime (ass_track_set_feature
+     * may return -1) if libass was compiled against old FriBidi.
+     */
+    ASS_FEATURE_BIDI_BRACKETS,
+
     // New enum values can be added here in new ABI-compatible library releases.
 } ASS_Feature;