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[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.
This commit is contained in:
Behdad Esfahbod 2013-10-16 13:42:38 +02:00
parent 1a31f9f820
commit c52ddab72e
2 changed files with 24 additions and 0 deletions
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6
src/hb-ot-layout-private.hh
View File

@ -107,6 +107,12 @@ _hb_glyph_info_is_zwj (const hb_glyph_info_t *info)
return !!(info->unicode_props0() & MASK0_ZWJ);
}
inline void
_hb_glyph_info_flip_joiners (hb_glyph_info_t *info)
{
info->unicode_props0() ^= MASK0_ZWNJ | MASK0_ZWJ;
}
#define hb_ot_layout_from_face(face) ((hb_ot_layout_t *) face->shaper_data.ot)

18
src/hb-ot-shape-complex-arabic.cc
View File

@ -156,6 +156,11 @@ static const struct arabic_state_table_entry {
};
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,
@ -176,6 +181,8 @@ collect_features_arabic (hb_ot_shape_planner_t *plan)
* 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'));
@ -313,6 +320,17 @@ setup_masks_arabic (const hb_ot_shape_plan_t *plan,
}
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,