Add a set-digest for second glyph in PairPosFormat1

Doesn't show any speedup.

src/OT/Layout/GPOS/PairPosFormat1.hh

diff --git a/src/OT/Layout/GPOS/PairPosFormat1.hh b/src/OT/Layout/GPOS/PairPosFormat1.hh
index a8ded1e..8d06b3b 100644
--- a/src/OT/Layout/GPOS/PairPosFormat1.hh
+++ b/src/OT/Layout/GPOS/PairPosFormat1.hh
@@ -106,6 +106,7 @@
   struct external_cache_t
   {
     hb_ot_layout_mapping_cache_t coverage;
+    hb_set_digest_t seconds;
   };
   void *external_cache_create () const
   {
@@ -113,6 +114,12 @@
     if (likely (cache))
     {
       cache->coverage.clear ();
+
+      cache->seconds.init ();
+      + hb_iter (pairSet)
+      | hb_map (hb_add (this))
+      | hb_apply ([cache, this] (const PairSet &_) { _.collect_seconds (cache->seconds, this->valueFormat); })
+      ;
     }
     return cache;
   }
@@ -125,8 +132,10 @@
 
 #ifndef HB_NO_OT_LAYOUT_LOOKUP_CACHE
     external_cache_t *cache = (external_cache_t *) external_cache;
+    const hb_set_digest_t *seconds = cache ? &cache->seconds : nullptr;
     unsigned int index = (this+coverage).get_coverage  (buffer->cur().codepoint, cache ? &cache->coverage : nullptr);
 #else
+    const hb_set_digest_t *seconds = nullptr;
     unsigned int index = (this+coverage).get_coverage  (buffer->cur().codepoint);
 #endif
     if (index == NOT_COVERED) return_trace (false);
@@ -140,6 +149,12 @@
       return_trace (false);
     }
 
+    if (seconds && !seconds->may_have (buffer->info[skippy_iter.idx].codepoint))
+    {
+      buffer->unsafe_to_concat (buffer->idx, skippy_iter.idx + 1);
+      return_trace (false);
+    }
+
     return_trace ((this+pairSet[index]).apply (c, valueFormat, skippy_iter.idx));
   }
 

src/OT/Layout/GPOS/PairSet.hh

diff --git a/src/OT/Layout/GPOS/PairSet.hh b/src/OT/Layout/GPOS/PairSet.hh
index 5560fab..ca54657 100644
--- a/src/OT/Layout/GPOS/PairSet.hh
+++ b/src/OT/Layout/GPOS/PairSet.hh
@@ -84,6 +84,21 @@
     c->input->add_array (&record->secondGlyph, len, record_size);
   }
 
+  template <typename set_t>
+  void collect_seconds (set_t &s,
+			const ValueFormat *valueFormats) const
+  {
+    unsigned record_size = get_size (valueFormats);
+
+    const PairValueRecord *record = &firstPairValueRecord;
+    unsigned count = len;
+    for (unsigned i = 0; i < count; i++)
+    {
+      s.add (record->secondGlyph);
+      record = &StructAtOffset<const PairValueRecord> (record, record_size);
+    }
+  }
+
   void collect_variation_indices (hb_collect_variation_indices_context_t *c,
                                   const ValueFormat *valueFormats) const
   {