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.

6 files changed, 60 insertions, 16 deletions

diff --git a/libass/ass.c b/libass/ass.c
index 13cfea5..811a9a7 100644
--- a/libass/ass.c
+++ b/libass/ass.c
@@ -36,6 +36,7 @@
 #include "ass_utils.h"
 #include "ass_library.h"
 #include "ass_priv.h"
+#include "ass_shaper.h"
 #include "ass_string.h"
 
 #define ass_atof(STR) (ass_strtod((STR),NULL))
@@ -1468,7 +1469,11 @@ int ass_track_set_feature(ASS_Track *track, ASS_Feature feature, int enable)
 {
     switch (feature) {
     case ASS_FEATURE_INCOMPATIBLE_EXTENSIONS:
-        track->parser_priv->enable_extensions = !!enable;
+        //-fallthrough
+#ifdef USE_FRIBIDI_EX_API
+    case ASS_FEATURE_BIDI_BRACKETS:
+        track->parser_priv->bidi_brackets = !!enable;
+#endif
         return 0;
     default:
         return -1;
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;
 
diff --git a/libass/ass_priv.h b/libass/ass_priv.h
index 3ee2b45..98b2711 100644
--- a/libass/ass_priv.h
+++ b/libass/ass_priv.h
@@ -19,8 +19,11 @@
 #ifndef LIBASS_PRIV_H
 #define LIBASS_PRIV_H
 
+#include <stdbool.h>
 #include <stdint.h>
 
+#include "ass_shaper.h"
+
 typedef enum {
     PST_UNKNOWN = 0,
     PST_INFO,
@@ -55,10 +58,12 @@ struct parser_priv {
     int read_order_elems; // size in uint32_t units of read_order_bitmap
     int check_readorder;
 
-    int enable_extensions;
-
     // tracks [Script Info] headers set by the script
     uint32_t header_flags;
+
+#ifdef USE_FRIBIDI_EX_API
+    bool bidi_brackets;
+#endif
 };
 
 #endif /* LIBASS_PRIV_H */
diff --git a/libass/ass_render.c b/libass/ass_render.c
index 7bc9554..28b6461 100644
--- a/libass/ass_render.c
+++ b/libass/ass_render.c
@@ -27,6 +27,7 @@
 #include "ass_outline.h"
 #include "ass_render.h"
 #include "ass_parse.h"
+#include "ass_priv.h"
 #include "ass_shaper.h"
 
 #define MAX_GLYPHS_INITIAL 1024
@@ -2833,6 +2834,10 @@ ass_start_frame(ASS_Renderer *render_priv, ASS_Track *track,
     ass_shaper_set_kerning(render_priv->shaper, track->Kerning);
     ass_shaper_set_language(render_priv->shaper, track->Language);
     ass_shaper_set_level(render_priv->shaper, render_priv->settings.shaper);
+#ifdef USE_FRIBIDI_EX_API
+    ass_shaper_set_bidi_brackets(render_priv->shaper,
+            track->parser_priv->bidi_brackets);
+#endif
 
     // PAR correction
     double par = render_priv->settings.par;
diff --git a/libass/ass_shaper.c b/libass/ass_shaper.c
index ab94f10..fc12974 100644
--- a/libass/ass_shaper.c
+++ b/libass/ass_shaper.c
@@ -39,10 +39,6 @@ enum {
 #define NUM_FEATURES 5
 #endif
 
-#if FRIBIDI_MAJOR_VERSION >= 1
-#define USE_FRIBIDI_EX_API
-#endif
-
 struct ass_shaper {
     ASS_ShapingLevel shaping_level;
 
@@ -50,9 +46,6 @@ struct ass_shaper {
     int n_glyphs;
     FriBidiChar *event_text;
     FriBidiCharType *ctypes;
-#ifdef USE_FRIBIDI_EX_API
-    FriBidiBracketType *btypes;
-#endif
     FriBidiLevel *emblevels;
     FriBidiStrIndex *cmap;
     FriBidiParType base_direction;
@@ -66,6 +59,11 @@ struct ass_shaper {
     // Glyph metrics cache, to speed up shaping
     Cache *metrics_cache;
 #endif
+
+#ifdef USE_FRIBIDI_EX_API
+    FriBidiBracketType *btypes;
+    bool bidi_brackets;
+#endif
 };
 
 #ifdef CONFIG_HARFBUZZ
@@ -105,7 +103,7 @@ static bool check_allocations(ASS_Shaper *shaper, size_t new_size)
         if (!ASS_REALLOC_ARRAY(shaper->event_text, new_size) ||
             !ASS_REALLOC_ARRAY(shaper->ctypes, new_size) ||
 #ifdef USE_FRIBIDI_EX_API
-            !ASS_REALLOC_ARRAY(shaper->btypes, new_size) ||
+            (shaper->bidi_brackets && !ASS_REALLOC_ARRAY(shaper->btypes, new_size)) ||
 #endif
             !ASS_REALLOC_ARRAY(shaper->emblevels, new_size) ||
             !ASS_REALLOC_ARRAY(shaper->cmap, new_size))
@@ -860,6 +858,13 @@ void ass_shaper_set_level(ASS_Shaper *shaper, ASS_ShapingLevel level)
     shaper->shaping_level = level;
 }
 
+#ifdef USE_FRIBIDI_EX_API
+void ass_shaper_set_bidi_brackets(ASS_Shaper *shaper, bool match_brackets)
+{
+    shaper->bidi_brackets = match_brackets;
+}
+#endif
+
 /**
   * \brief Remove all zero-width invisible characters from the text.
   * \param text_info text
@@ -907,11 +912,15 @@ int ass_shaper_shape(ASS_Shaper *shaper, TextInfo *text_info)
             fribidi_get_bidi_types(shaper->event_text + last_break,
                     i - last_break + 1, shaper->ctypes + last_break);
 #ifdef USE_FRIBIDI_EX_API
-            fribidi_get_bracket_types(shaper->event_text + last_break,
-                    i - last_break + 1, shaper->ctypes + last_break,
-                    shaper->btypes + last_break);
+            FriBidiBracketType *btypes = NULL;
+            if (shaper->bidi_brackets) {
+                btypes = shaper->btypes + last_break;
+                fribidi_get_bracket_types(shaper->event_text + last_break,
+                        i - last_break + 1, shaper->ctypes + last_break,
+                        btypes);
+            }
             ret = fribidi_get_par_embedding_levels_ex(
-                    shaper->ctypes + last_break, shaper->btypes + last_break,
+                    shaper->ctypes + last_break, btypes,
                     i - last_break + 1, &dir, shaper->emblevels + last_break);
 #else
             ret = fribidi_get_par_embedding_levels(shaper->ctypes + last_break,
diff --git a/libass/ass_shaper.h b/libass/ass_shaper.h
index d12870f..9ff96e7 100644
--- a/libass/ass_shaper.h
+++ b/libass/ass_shaper.h
@@ -25,6 +25,10 @@ typedef struct ass_shaper ASS_Shaper;
 #include <stdbool.h>
 #include "ass_render.h"
 
+#if FRIBIDI_MAJOR_VERSION >= 1
+#define USE_FRIBIDI_EX_API
+#endif
+
 void ass_shaper_info(ASS_Library *lib);
 ASS_Shaper *ass_shaper_new(void);
 void ass_shaper_free(ASS_Shaper *shaper);
@@ -35,6 +39,9 @@ void ass_shaper_find_runs(ASS_Shaper *shaper, ASS_Renderer *render_priv,
 void ass_shaper_set_base_direction(ASS_Shaper *shaper, FriBidiParType dir);
 void ass_shaper_set_language(ASS_Shaper *shaper, const char *code);
 void ass_shaper_set_level(ASS_Shaper *shaper, ASS_ShapingLevel level);
+#ifdef USE_FRIBIDI_EX_API
+void ass_shaper_set_bidi_brackets(ASS_Shaper *shaper, bool match_brackets);
+#endif
 int ass_shaper_shape(ASS_Shaper *shaper, TextInfo *text_info);
 void ass_shaper_cleanup(ASS_Shaper *shaper, TextInfo *text_info);
 FriBidiStrIndex *ass_shaper_reorder(ASS_Shaper *shaper, TextInfo *text_info);