Improve ligature-component handling

We use three bits for lig_id these days, so we finally got a report of
two separate ligatures with the same lig_id happening adjacent to each
other, and then the component-handling code was breaking things.
Protect against that by ignoring same-lig-id but lig-comp=0 glyphs after
a new ligature.

Fixes https://github.com/behdad/harfbuzz/issues/198

src/hb-ot-layout-gsubgpos-private.hh

@@ -845,9 +845,12 @@ static inline bool ligate_input (hb_apply_context_t *c,
     while (buffer->idx < match_positions[i] && !buffer->in_error)
     {
       if (!is_mark_ligature) {
+        unsigned int this_comp = _hb_glyph_info_get_lig_comp (&buffer->cur());
+	if (this_comp == 0)
+	  this_comp = 1;
 	unsigned int new_lig_comp = components_so_far - last_num_components +
-				    MIN (MAX (_hb_glyph_info_get_lig_comp (&buffer->cur()), 1u), last_num_components);
-	_hb_glyph_info_set_lig_props_for_mark (&buffer->cur(), lig_id, new_lig_comp);
+				    MIN (this_comp, last_num_components);
+	  _hb_glyph_info_set_lig_props_for_mark (&buffer->cur(), lig_id, new_lig_comp);
       }
       buffer->next_glyph ();
     }
@@ -864,8 +867,11 @@ static inline bool ligate_input (hb_apply_context_t *c,
     /* Re-adjust components for any marks following. */
     for (unsigned int i = buffer->idx; i < buffer->len; i++) {
       if (last_lig_id == _hb_glyph_info_get_lig_id (&buffer->info[i])) {
+        unsigned int this_comp = _hb_glyph_info_get_lig_comp (&buffer->info[i]);
+	if (!this_comp)
+	  break;
 	unsigned int new_lig_comp = components_so_far - last_num_components +
-				    MIN (MAX (_hb_glyph_info_get_lig_comp (&buffer->info[i]), 1u), last_num_components);
+				    MIN (this_comp, last_num_components);
 	_hb_glyph_info_set_lig_props_for_mark (&buffer->info[i], lig_id, new_lig_comp);
       } else
 	break;

test/shaping/Makefile.am

@@ -52,6 +52,7 @@ TESTS = \
 	tests/indic-joiner-candrabindu.tests \
 	tests/indic-old-spec.tests \
 	tests/indic-pref-blocking.tests \
+	tests/ligature-id.tests \
 	tests/mongolian-variation-selector.tests \
 	tests/spaces.tests \
 	tests/simple.tests \

test/shaping/fonts/sha1sum/MANIFEST

@@ -30,6 +30,7 @@ b9e2aaa0d75fcef6971ec3a96d806ba4a6b31fe2.ttf
 bb29ce50df2bdba2d10726427c6b7609bf460e04.ttf
 bb9473d2403488714043bcfb946c9f78b86ad627.ttf
 c4e48b0886ef460f532fb49f00047ec92c432ec0.ttf
+c78d1f25ea1df1210414f2fe2e0e1e53082bba76.ttf
 d629e7fedc0b350222d7987345fe61613fa3929a.ttf
 df768b9c257e0c9c35786c47cae15c46571d56be.ttf
 e207635780b42f898d58654b65098763e340f5c7.ttf

test/shaping/tests/MANIFEST

@@ -10,6 +10,7 @@ hyphens.tests
 indic-joiner-candrabindu.tests
 indic-old-spec.tests
 indic-pref-blocking.tests
+ligature-id.tests
 mongolian-variation-selector.tests
 simple.tests
 spaces.tests