gistfile1.txt

diff --cc src/hb-ot-layout-gsub-table.hh
index 364ef189,364ef189..4de505aa
--- a/src/hb-ot-layout-gsub-table.hh
+++ b/src/hb-ot-layout-gsub-table.hh
@@@ -899,8 -899,8 +899,31 @@@ struct LigatureSubstFormat
    bool subset (hb_subset_context_t *c) const
    {
      TRACE_SUBSET (this);
--    // TODO(subset)
--    return_trace (false);
++    const hb_set_t &glyphset = *c->plan->glyphset ();
++    const hb_map_t &glyph_map = *c->plan->glyph_map;
++
++    auto *out = c->serializer->start_embed (*this);
++    if (unlikely (!c->extend_min (out))) trace_return (false);
++    out->format = format;
++
++    hb_sorted_vector_t<hb_codepoint_t> new_coverage;
++    + hb_zip (this+coverage, ligatureSet)
++    | hb_filter (glyphset, hb_first)
++    | hb_filter ([&] (const OffsetTo<LigatureSet>& _)
++               {
++                 if (unlikely (!out->ligatureSet.serialize_append (c->serializer))) return false;
++                 bool ret = out.ligatureSet.tail ().serialize_subset (c, this+_, out);
++                 if (!ret) out->ligatureSet.len--; umm.
++                 return ret;
++               },
++               hb_second)
++    | hb_map (hb_first)
++    | hb_map (glyph_map)
++    | hb_sink (new_coverage);
++    ;
++    out->coverage.serialize (c->serializer, out)
++               .serialize (c->serializer, new_coverage.iter ());
++    return_trace (bool (new_coverage));
    }
  
    bool sanitize (hb_sanitize_context_t *c) const