shaper: don't mingle run ID and bidi embedding level

A libass-chosen shaping run boundary can coincide with a bidi embedding level
boundary such that the differences in shaper_run_id and emblevel cancel out.
For example:

    English עבריתEnglish again (with parentheses)

(with no space after the Hebrew)
has ass_shaper_find_runs split runs after the Hebrew, adding 1 to shape_run_id,
while the RTL embed also ends in the same place, removing 1 from emblevel.

As a result, two shape runs that should be separate merge together.
In this example, this leads to HarfBuzz thinking that "English again (with
parentheses)" is still right-to-left, so it flips the parenthesis glyphs.

To avoid this, don't attempt to add these numbers together. Just require
explicitly that both numbers stay constant, rather than their sum.

1 files changed, 6 insertions, 9 deletions

diff --git a/libass/ass_shaper.c b/libass/ass_shaper.c
index 2de12d5..4e93d2b 100644
--- a/libass/ass_shaper.c
+++ b/libass/ass_shaper.c
@@ -639,18 +639,20 @@ static void shape_harfbuzz(ASS_Shaper *shaper, GlyphInfo *glyphs, size_t len)
     for (i = 0; i < len; i++) {
         int offset = i;
         hb_font_t *font = get_hb_font(shaper, glyphs + offset);
-        int level = glyphs[offset].shape_run_id;
-        int direction = FRIBIDI_LEVEL_IS_RTL(shaper->emblevels[offset]);
+        int run_id = glyphs[offset].shape_run_id;
+        int level = shaper->emblevels[offset];
 
         // advance in text until end of run
-        while (i < (len - 1) && level == glyphs[i+1].shape_run_id)
+        while (i < (len - 1) && run_id == glyphs[i + 1].shape_run_id &&
+                level == shaper->emblevels[i + 1])
             i++;
 
         hb_buffer_pre_allocate(buf, i - offset + 1);
         hb_buffer_add_utf32(buf, shaper->event_text + offset, i - offset + 1,
                 0, i - offset + 1);
 
-        props.direction = direction ? HB_DIRECTION_RTL : HB_DIRECTION_LTR;
+        props.direction = FRIBIDI_LEVEL_IS_RTL(level) ?
+            HB_DIRECTION_RTL : HB_DIRECTION_LTR;
         props.script = glyphs[offset].script;
         props.language  = hb_shaper_get_run_language(shaper, props.script);
         hb_buffer_set_segment_properties(buf, &props);
@@ -910,11 +912,6 @@ int ass_shaper_shape(ASS_Shaper *shaper, TextInfo *text_info)
         }
     }
 
-    // add embedding levels to shape runs for final runs
-    for (i = 0; i < text_info->length; i++) {
-        glyphs[i].shape_run_id += shaper->emblevels[i];
-    }
-
 #ifdef CONFIG_HARFBUZZ
     switch (shaper->shaping_level) {
     case ASS_SHAPING_SIMPLE: