Allow requesting a specific glyph for 'rand'

Randomization only happens by default. If the user specifies a value for
'rand', that value is respected.

src/hb-ot-map.cc

@@ -254,6 +254,7 @@ hb_ot_map_builder_t::compile (hb_ot_map_t  &m,
     map->stage[1] = info->stage[1];
     map->auto_zwnj = !(info->flags & F_MANUAL_ZWNJ);
     map->auto_zwj = !(info->flags & F_MANUAL_ZWJ);
+    map->random = !!(info->flags & F_RANDOM);
     if ((info->flags & F_GLOBAL) && info->max_value == 1) {
       /* Uses the global bit */
       map->shift = global_bit_shift;
@@ -304,7 +305,7 @@ hb_ot_map_builder_t::compile (hb_ot_map_t  &m,
 		       m.features[i].mask,
 		       m.features[i].auto_zwnj,
 		       m.features[i].auto_zwj,
-		       m.features[i].tag == HB_TAG ('r','a','n','d'));
+		       m.features[i].random);
 
       /* Sort lookups and merge duplicates */
       if (last_num_lookups < m.lookups[table_index].len)

src/hb-ot-map.hh

@@ -52,6 +52,7 @@ struct hb_ot_map_t
     unsigned int needs_fallback : 1;
     unsigned int auto_zwnj : 1;
     unsigned int auto_zwj : 1;
+    unsigned int random : 1;
 
     inline int cmp (const hb_tag_t *tag_) const
     { return *tag_ < tag ? -1 : *tag_ > tag ? 1 : 0; }
@@ -168,7 +169,8 @@ enum hb_ot_map_feature_flags_t {
   F_HAS_FALLBACK	= 0x0002u, /* Has fallback implementation, so include mask bit even if feature not found. */
   F_MANUAL_ZWNJ		= 0x0004u, /* Don't skip over ZWNJ when matching **context**. */
   F_MANUAL_ZWJ		= 0x0008u, /* Don't skip over ZWJ when matching **input**. */
-  F_GLOBAL_SEARCH	= 0x0010u  /* If feature not found in LangSys, look for it in global feature list and pick one. */
+  F_GLOBAL_SEARCH	= 0x0010u, /* If feature not found in LangSys, look for it in global feature list and pick one. */
+  F_RANDOM		= 0x0020u  /* Randomly select a glyph from an AlternateSubstFormat1 subtable. */
 };
 HB_MARK_AS_FLAG_T (hb_ot_map_feature_flags_t);
 /* Macro version for where const is desired. */

src/hb-ot-shape.cc

@@ -49,7 +49,6 @@ static hb_tag_t common_features[] = {
   HB_TAG('m','a','r','k'),
   HB_TAG('m','k','m','k'),
   HB_TAG('r','l','i','g'),
-  HB_TAG('r','a','n','d'),
 };
 
 
@@ -71,6 +70,7 @@ hb_ot_shape_collect_features (hb_ot_shape_planner_t          *planner,
 			      unsigned int                    num_user_features)
 {
   hb_ot_map_builder_t *map = &planner->map;
+  bool default_rand = true;
 
   map->add_global_bool_feature (HB_TAG('r','v','r','n'));
   map->add_gsub_pause (nullptr);
@@ -123,7 +123,12 @@ hb_ot_shape_collect_features (hb_ot_shape_planner_t          *planner,
     map->add_feature (feature->tag, feature->value,
 		      (feature->start == 0 && feature->end == (unsigned int) -1) ?
 		       F_GLOBAL : F_NONE);
+    if (feature->tag == HB_TAG ('r','a','n','d'))
+      default_rand = false;
   }
+
+  if (default_rand)
+    map->add_feature (HB_TAG ('r','a','n','d'), 1, F_GLOBAL | F_RANDOM);
 }