[layout] Use a cache for main input ClassDef of (Chain)ContextLookups

src/hb-ot-layout-common.hh

@@ -2001,6 +2001,8 @@ struct ClassDefFormat1
     return_trace (c->check_struct (this) && classValue.sanitize (c));
   }
 
+  unsigned cost () const { return 1; }
+
   template <typename set_t>
   bool collect_coverage (set_t *glyphs) const
   {
@@ -2237,6 +2239,8 @@ struct ClassDefFormat2
     return_trace (rangeRecord.sanitize (c));
   }
 
+  unsigned cost () const { return hb_bit_storage ((unsigned) rangeRecord.len); /* bsearch cost */ }
+
   template <typename set_t>
   bool collect_coverage (set_t *glyphs) const
   {
@@ -2477,6 +2481,15 @@ struct ClassDef
     }
   }
 
+  unsigned cost () const
+  {
+    switch (u.format) {
+    case 1: return u.format1.cost ();
+    case 2: return u.format2.cost ();
+    default:return 0u;
+    }
+  }
+
   /* Might return false if array looks unsorted.
    * Used for faster rejection of corrupt data. */
   template <typename set_t>

src/hb-ot-layout-gsubgpos.hh

@@ -394,7 +394,6 @@ struct hb_collect_coverage_context_t :
   set_t *set;
 };
 
-
 struct hb_ot_apply_context_t :
        hb_dispatch_context_t<hb_ot_apply_context_t, bool, HB_DEBUG_APPLY>
 {
@@ -410,7 +409,7 @@ struct hb_ot_apply_context_t :
 	     match_func (nullptr),
 	     match_data (nullptr) {}
 
-    typedef bool (*match_func_t) (hb_codepoint_t glyph_id, const HBUINT16 &value, const void *data);
+    typedef bool (*match_func_t) (hb_glyph_info_t &info, const HBUINT16 &value, const void *data);
 
     void set_ignore_zwnj (bool ignore_zwnj_) { ignore_zwnj = ignore_zwnj_; }
     void set_ignore_zwj (bool ignore_zwj_) { ignore_zwj = ignore_zwj_; }
@@ -428,7 +427,7 @@ struct hb_ot_apply_context_t :
       MATCH_MAYBE
     };
 
-    may_match_t may_match (const hb_glyph_info_t &info,
+    may_match_t may_match (hb_glyph_info_t &info,
 			   const HBUINT16        *glyph_data) const
     {
       if (!(info.mask & mask) ||
@@ -436,7 +435,7 @@ struct hb_ot_apply_context_t :
 	return MATCH_NO;
 
       if (match_func)
-	return match_func (info.codepoint, *glyph_data, match_data) ? MATCH_YES : MATCH_NO;
+	return match_func (info, *glyph_data, match_data) ? MATCH_YES : MATCH_NO;
 
       return MATCH_MAYBE;
     }
@@ -524,7 +523,7 @@ struct hb_ot_apply_context_t :
       while (idx + num_items < end)
       {
 	idx++;
-	const hb_glyph_info_t &info = c->buffer->info[idx];
+	hb_glyph_info_t &info = c->buffer->info[idx];
 
 	matcher_t::may_skip_t skip = matcher.may_skip (c, info);
 	if (unlikely (skip == matcher_t::SKIP_YES))
@@ -557,7 +556,7 @@ struct hb_ot_apply_context_t :
       while (idx > num_items - 1)
       {
 	idx--;
-	const hb_glyph_info_t &info = c->buffer->out_info[idx];
+	hb_glyph_info_t &info = c->buffer->out_info[idx];
 
 	matcher_t::may_skip_t skip = matcher.may_skip (c, info);
 	if (unlikely (skip == matcher_t::SKIP_YES))
@@ -639,6 +638,7 @@ struct hb_ot_apply_context_t :
   bool per_syllable = false;
   bool random = false;
   uint32_t random_state = 1;
+  unsigned new_syllables = (unsigned) -1;
 
   hb_ot_apply_context_t (unsigned int table_index_,
 			 hb_font_t *font_,
@@ -736,6 +736,9 @@ struct hb_ot_apply_context_t :
 			  bool ligature = false,
 			  bool component = false) const
   {
+    if (new_syllables != (unsigned) -1)
+      buffer->cur().syllable() = new_syllables;
+
     unsigned int props = _hb_glyph_info_get_glyph_props (&buffer->cur());
     props |= HB_OT_LAYOUT_GLYPH_PROPS_SUBSTITUTED;
     if (ligature)
@@ -800,15 +803,60 @@ struct hb_accelerate_subtables_context_t :
     return typed_obj->apply (c);
   }
 
+  template <typename T>
+  static inline auto apply_cached_ (const T *obj, OT::hb_ot_apply_context_t *c, hb_priority<1>) HB_RETURN (bool, obj->apply_cached (c) )
+  template <typename T>
+  static inline auto apply_cached_ (const T *obj, OT::hb_ot_apply_context_t *c, hb_priority<0>) HB_RETURN (bool, obj->apply (c) )
+  template <typename Type>
+  static inline bool apply_cached_to (const void *obj, OT::hb_ot_apply_context_t *c)
+  {
+    const Type *typed_obj = (const Type *) obj;
+    return apply_cached_ (typed_obj, c, hb_prioritize);
+  }
+
+  template <typename T>
+  static inline auto cache_enter_ (const T *obj, OT::hb_ot_apply_context_t *c, hb_priority<1>) HB_RETURN (void, obj->cache_enter (c) )
+  template <typename T>
+  static inline void cache_enter_ (const T *obj, OT::hb_ot_apply_context_t *c, hb_priority<0>) {}
+  template <typename Type>
+  static inline void cache_enter_to (const void *obj, OT::hb_ot_apply_context_t *c)
+  {
+    const Type *typed_obj = (const Type *) obj;
+    return cache_enter_ (typed_obj, c, hb_prioritize);
+  }
+
+  template <typename T>
+  static inline auto cache_leave_ (const T *obj, OT::hb_ot_apply_context_t *c, hb_priority<1>) HB_RETURN (void, obj->cache_leave (c) )
+  template <typename T>
+  static inline void cache_leave_ (const T *obj, OT::hb_ot_apply_context_t *c, hb_priority<0>) {}
+  template <typename Type>
+  static inline void cache_leave_to (const void *obj, OT::hb_ot_apply_context_t *c)
+  {
+    const Type *typed_obj = (const Type *) obj;
+    return cache_leave_ (typed_obj, c, hb_prioritize);
+  }
+
   typedef bool (*hb_apply_func_t) (const void *obj, OT::hb_ot_apply_context_t *c);
+  typedef void (*hb_cache_enter_func_t) (const void *obj, OT::hb_ot_apply_context_t *c);
+  typedef void (*hb_cache_leave_func_t) (const void *obj, OT::hb_ot_apply_context_t *c);
 
   struct hb_applicable_t
   {
+    friend struct hb_accelerate_subtables_context_t;
+    friend struct hb_ot_layout_lookup_accelerator_t;
+
     template <typename T>
-    void init (const T &obj_, hb_apply_func_t apply_func_)
+    void init (const T &obj_,
+	       hb_apply_func_t apply_func_,
+	       hb_apply_func_t apply_cached_func_,
+	       hb_cache_enter_func_t cache_enter_func_,
+	       hb_cache_leave_func_t cache_leave_func_)
     {
       obj = &obj_;
       apply_func = apply_func_;
+      apply_cached_func = apply_cached_func_;
+      cache_enter_func = cache_enter_func_;
+      cache_leave_func = cache_leave_func_;
       digest.init ();
       obj_.get_coverage ().collect_coverage (&digest);
     }
@@ -817,21 +865,59 @@ struct hb_accelerate_subtables_context_t :
     {
       return digest.may_have (c->buffer->cur().codepoint) && apply_func (obj, c);
     }
+    bool apply_cached (OT::hb_ot_apply_context_t *c) const
+    {
+      return digest.may_have (c->buffer->cur().codepoint) &&  apply_cached_func (obj, c);
+    }
+
+    void cache_enter (OT::hb_ot_apply_context_t *c) const
+    {
+      cache_enter_func (obj, c);
+    }
+    void cache_leave (OT::hb_ot_apply_context_t *c) const
+    {
+      cache_leave_func (obj, c);
+    }
 
     private:
     const void *obj;
     hb_apply_func_t apply_func;
+    hb_apply_func_t apply_cached_func;
+    hb_cache_enter_func_t cache_enter_func;
+    hb_cache_leave_func_t cache_leave_func;
     hb_set_digest_t digest;
   };
 
   typedef hb_vector_t<hb_applicable_t> array_t;
 
+  template <typename T>
+  auto cache_cost (const T &obj, hb_priority<1>) HB_AUTO_RETURN ( obj.cache_cost () )
+
+  template <typename T>
+  auto cache_cost (const T &obj, hb_priority<0>) HB_AUTO_RETURN ( 0u )
+
   /* Dispatch interface. */
   template <typename T>
   return_t dispatch (const T &obj)
   {
-    hb_applicable_t *entry = array.push();
+    hb_applicable_t entry;
-    entry->init (obj, apply_to<T>);
+
+    entry.init (obj,
+		apply_to<T>,
+		apply_cached_to<T>,
+		cache_enter_to<T>,
+		cache_leave_to<T>);
+
+    array.push (entry);
+
+    // Cache handling
+    unsigned cost = cache_cost (obj, hb_prioritize);
+    if (cost > cache_user_cost && !array.in_error ())
+    {
+      cache_user_idx = array.length - 1;
+      cache_user_cost = cost;
+    }
+
     return hb_empty_t ();
   }
   static return_t default_return_value () { return hb_empty_t (); }
@@ -840,15 +926,15 @@ struct hb_accelerate_subtables_context_t :
 				     array (array_) {}
 
   array_t &array;
+  unsigned cache_user_idx = (unsigned) -1;
+  unsigned cache_user_cost = 0;
 };
 
 
-
-
 typedef bool (*intersects_func_t) (const hb_set_t *glyphs, const HBUINT16 &value, const void *data);
 typedef void (*intersected_glyphs_func_t) (const hb_set_t *glyphs, const void *data, unsigned value, hb_set_t *intersected_glyphs);
 typedef void (*collect_glyphs_func_t) (hb_set_t *glyphs, const HBUINT16 &value, const void *data);
-typedef bool (*match_func_t) (hb_codepoint_t glyph_id, const HBUINT16 &value, const void *data);
+typedef bool (*match_func_t) (hb_glyph_info_t &info, const HBUINT16 &value, const void *data);
 
 struct ContextClosureFuncs
 {
@@ -863,6 +949,10 @@ struct ContextApplyFuncs
 {
   match_func_t match;
 };
+struct ChainContextApplyFuncs
+{
+  match_func_t match[3];
+};
 
 
 static inline bool intersects_glyph (const hb_set_t *glyphs, const HBUINT16 &value, const void *data HB_UNUSED)
@@ -939,19 +1029,30 @@ static inline void collect_array (hb_collect_glyphs_context_t *c HB_UNUSED,
 }
 
 
-static inline bool match_glyph (hb_codepoint_t glyph_id, const HBUINT16 &value, const void *data HB_UNUSED)
+static inline bool match_glyph (hb_glyph_info_t &info, const HBUINT16 &value, const void *data HB_UNUSED)
 {
-  return glyph_id == value;
+  return info.codepoint == value;
 }
-static inline bool match_class (hb_codepoint_t glyph_id, const HBUINT16 &value, const void *data)
+static inline bool match_class (hb_glyph_info_t &info, const HBUINT16 &value, const void *data)
 {
   const ClassDef &class_def = *reinterpret_cast<const ClassDef *>(data);
-  return class_def.get_class (glyph_id) == value;
+  return class_def.get_class (info.codepoint) == value;
 }
-static inline bool match_coverage (hb_codepoint_t glyph_id, const HBUINT16 &value, const void *data)
+static inline bool match_class_cached (hb_glyph_info_t &info, const HBUINT16 &value, const void *data)
+{
+  unsigned klass = info.syllable();
+  if (klass < 255)
+    return klass == value;
+  const ClassDef &class_def = *reinterpret_cast<const ClassDef *>(data);
+  klass = class_def.get_class (info.codepoint);
+  if (likely (klass < 255))
+    info.syllable() = klass;
+  return klass == value;
+}
+static inline bool match_coverage (hb_glyph_info_t &info, const HBUINT16 &value, const void *data)
 {
   const Offset16To<Coverage> &coverage = (const Offset16To<Coverage>&)value;
-  return (data+coverage).get_coverage (glyph_id) != NOT_COVERED;
+  return (data+coverage).get_coverage (info.codepoint) != NOT_COVERED;
 }
 
 static inline bool would_match_input (hb_would_apply_context_t *c,
@@ -964,8 +1065,12 @@ static inline bool would_match_input (hb_would_apply_context_t *c,
     return false;
 
   for (unsigned int i = 1; i < count; i++)
-    if (likely (!match_func (c->glyphs[i], input[i - 1], match_data)))
+  {
+    hb_glyph_info_t info;
+    info.codepoint = c->glyphs[i];
+    if (likely (!match_func (info, input[i - 1], match_data)))
       return false;
+  }
 
   return true;
 }
@@ -2125,19 +2230,47 @@ struct ContextFormat2
 
   const Coverage &get_coverage () const { return this+coverage; }
 
-  bool apply (hb_ot_apply_context_t *c) const
+  unsigned cache_cost () const
+  {
+    unsigned c = (this+classDef).cost () * ruleSet.len;
+    return c >= 4 ? c : 0;
+  }
+  void cache_enter (hb_ot_apply_context_t *c) const
+  {
+    auto &info = c->buffer->info;
+    unsigned count = c->buffer->len;
+    for (unsigned i = 0; i < count; i++)
+      info[i].syllable() = 255;
+    c->new_syllables = 255;
+  }
+  void cache_leave (hb_ot_apply_context_t *c) const
+  {
+    c->new_syllables = (unsigned) -1;
+  }
+  bool apply_cached (hb_ot_apply_context_t *c) const { return apply (c, true); }
+
+  bool apply (hb_ot_apply_context_t *c, bool cached = false) const
   {
     TRACE_APPLY (this);
     unsigned int index = (this+coverage).get_coverage (c->buffer->cur().codepoint);
     if (likely (index == NOT_COVERED)) return_trace (false);
 
     const ClassDef &class_def = this+classDef;
-    index = class_def.get_class (c->buffer->cur().codepoint);
+
-    const RuleSet &rule_set = this+ruleSet[index];
     struct ContextApplyLookupContext lookup_context = {
-      {match_class},
+      {cached ? match_class_cached : match_class},
       &class_def
     };
+
+    if (cached && c->buffer->cur().syllable() < 255)
+      index = c->buffer->cur().syllable ();
+    else
+    {
+      index = class_def.get_class (c->buffer->cur().codepoint);
+      if (cached && index < 255)
+	c->buffer->cur().syllable() = index;
+    }
+    const RuleSet &rule_set = this+ruleSet[index];
     return_trace (rule_set.apply (c, lookup_context));
   }
 
@@ -2411,7 +2544,7 @@ struct ChainContextCollectGlyphsLookupContext
 
 struct ChainContextApplyLookupContext
 {
-  ContextApplyFuncs funcs;
+  ChainContextApplyFuncs funcs;
   const void *match_data[3];
 };
 
@@ -2499,7 +2632,7 @@ static inline bool chain_context_would_apply_lookup (hb_would_apply_context_t *c
   return (c->zero_context ? !backtrackCount && !lookaheadCount : true)
       && would_match_input (c,
 			    inputCount, input,
-			    lookup_context.funcs.match, lookup_context.match_data[1]);
+			    lookup_context.funcs.match[1], lookup_context.match_data[1]);
 }
 
 static inline bool chain_context_apply_lookup (hb_ot_apply_context_t *c,
@@ -2518,11 +2651,11 @@ static inline bool chain_context_apply_lookup (hb_ot_apply_context_t *c,
   unsigned match_positions[HB_MAX_CONTEXT_LENGTH];
   if (!(match_input (c,
 		     inputCount, input,
-		     lookup_context.funcs.match, lookup_context.match_data[1],
+		     lookup_context.funcs.match[1], lookup_context.match_data[1],
 		     &match_end, match_positions) && (end_index = match_end)
        && match_lookahead (c,
 			   lookaheadCount, lookahead,
-			   lookup_context.funcs.match, lookup_context.match_data[2],
+			   lookup_context.funcs.match[2], lookup_context.match_data[2],
 			   match_end, &end_index)))
   {
     c->buffer->unsafe_to_concat (c->buffer->idx, end_index);
@@ -2532,7 +2665,7 @@ static inline bool chain_context_apply_lookup (hb_ot_apply_context_t *c,
   unsigned start_index = c->buffer->out_len;
   if (!match_backtrack (c,
 			backtrackCount, backtrack,
-			lookup_context.funcs.match, lookup_context.match_data[0],
+			lookup_context.funcs.match[0], lookup_context.match_data[0],
 			&start_index))
   {
     c->buffer->unsafe_to_concat_from_outbuffer (start_index, end_index);
@@ -2934,7 +3067,7 @@ struct ChainContextFormat1
   {
     const ChainRuleSet &rule_set = this+ruleSet[(this+coverage).get_coverage (c->glyphs[0])];
     struct ChainContextApplyLookupContext lookup_context = {
-      {match_glyph},
+      {{match_glyph, match_glyph, match_glyph}},
       {nullptr, nullptr, nullptr}
     };
     return rule_set.would_apply (c, lookup_context);
@@ -2950,7 +3083,7 @@ struct ChainContextFormat1
 
     const ChainRuleSet &rule_set = this+ruleSet[index];
     struct ChainContextApplyLookupContext lookup_context = {
-      {match_glyph},
+      {{match_glyph, match_glyph, match_glyph}},
       {nullptr, nullptr, nullptr}
     };
     return_trace (rule_set.apply (c, lookup_context));
@@ -3134,7 +3267,7 @@ struct ChainContextFormat2
     unsigned int index = input_class_def.get_class (c->glyphs[0]);
     const ChainRuleSet &rule_set = this+ruleSet[index];
     struct ChainContextApplyLookupContext lookup_context = {
-      {match_class},
+      {{match_class, match_class, match_class}},
       {&backtrack_class_def,
        &input_class_def,
        &lookahead_class_def}
@@ -3144,7 +3277,26 @@ struct ChainContextFormat2
 
   const Coverage &get_coverage () const { return this+coverage; }
 
-  bool apply (hb_ot_apply_context_t *c) const
+  unsigned cache_cost () const
+  {
+    unsigned c = (this+inputClassDef).cost () * ruleSet.len;
+    return c >= 4 ? c : 0;
+  }
+  void cache_enter (hb_ot_apply_context_t *c) const
+  {
+    auto &info = c->buffer->info;
+    unsigned count = c->buffer->len;
+    for (unsigned i = 0; i < count; i++)
+      info[i].syllable() = 255;
+    c->new_syllables = 255;
+  }
+  void cache_leave (hb_ot_apply_context_t *c) const
+  {
+    c->new_syllables = (unsigned) -1;
+  }
+  bool apply_cached (hb_ot_apply_context_t *c) const { return apply (c, true); }
+
+  bool apply (hb_ot_apply_context_t *c, bool cached = false) const
   {
     TRACE_APPLY (this);
     unsigned int index = (this+coverage).get_coverage (c->buffer->cur().codepoint);
@@ -3154,14 +3306,24 @@ struct ChainContextFormat2
     const ClassDef &input_class_def = this+inputClassDef;
     const ClassDef &lookahead_class_def = this+lookaheadClassDef;
 
-    index = input_class_def.get_class (c->buffer->cur().codepoint);
-    const ChainRuleSet &rule_set = this+ruleSet[index];
     struct ChainContextApplyLookupContext lookup_context = {
-      {match_class},
+      {{cached && &backtrack_class_def == &input_class_def ? match_class_cached : match_class,
+        cached ? match_class_cached : match_class,
+        cached && &lookahead_class_def == &input_class_def ? match_class_cached : match_class}},
       {&backtrack_class_def,
        &input_class_def,
        &lookahead_class_def}
     };
+
+    if (cached && c->buffer->cur().syllable() < 255)
+      index = c->buffer->cur().syllable ();
+    else
+    {
+      index = input_class_def.get_class (c->buffer->cur().codepoint);
+      if (cached && index < 255)
+	c->buffer->cur().syllable() = index;
+    }
+    const ChainRuleSet &rule_set = this+ruleSet[index];
     return_trace (rule_set.apply (c, lookup_context));
   }
 
@@ -3359,7 +3521,7 @@ struct ChainContextFormat3
     const Array16OfOffset16To<Coverage> &lookahead = StructAfter<Array16OfOffset16To<Coverage>> (input);
     const Array16Of<LookupRecord> &lookup = StructAfter<Array16Of<LookupRecord>> (lookahead);
     struct ChainContextApplyLookupContext lookup_context = {
-      {match_coverage},
+      {{match_coverage, match_coverage, match_coverage}},
       {this, this, this}
     };
     return chain_context_would_apply_lookup (c,
@@ -3386,7 +3548,7 @@ struct ChainContextFormat3
     const Array16OfOffset16To<Coverage> &lookahead = StructAfter<Array16OfOffset16To<Coverage>> (input);
     const Array16Of<LookupRecord> &lookup = StructAfter<Array16Of<LookupRecord>> (lookahead);
     struct ChainContextApplyLookupContext lookup_context = {
-      {match_coverage},
+      {{match_coverage, match_coverage, match_coverage}},
       {this, this, this}
     };
     return_trace (chain_context_apply_lookup (c,
@@ -3625,23 +3787,54 @@ struct hb_ot_layout_lookup_accelerator_t
     subtables.init ();
     OT::hb_accelerate_subtables_context_t c_accelerate_subtables (subtables);
     lookup.dispatch (&c_accelerate_subtables);
+    cache_user_idx = c_accelerate_subtables.cache_user_idx;
+    for (unsigned i = 0; i < subtables.length; i++)
+      if (i != cache_user_idx)
+	subtables[i].apply_cached_func = subtables[i].apply_func;
   }
   void fini () { subtables.fini (); }
 
   bool may_have (hb_codepoint_t g) const
   { return digest.may_have (g); }
 
-  bool apply (hb_ot_apply_context_t *c) const
+  bool apply (hb_ot_apply_context_t *c, bool use_cache) const
+  {
+    if (use_cache)
+    {
+      for (unsigned int i = 0; i < subtables.length; i++)
+        if (subtables[i].apply_cached (c))
+	  return true;
+    }
+    else
     {
       for (unsigned int i = 0; i < subtables.length; i++)
         if (subtables[i].apply (c))
 	  return true;
+    }
     return false;
   }
 
+  bool cache_enter (OT::hb_ot_apply_context_t *c) const
+  {
+    if (cache_user_idx == (unsigned) -1 ||
+	!HB_BUFFER_TRY_ALLOCATE_VAR (c->buffer, syllable))
+      return false;
+
+    subtables[cache_user_idx].cache_enter (c);
+
+    return true;
+  }
+  void cache_leave (OT::hb_ot_apply_context_t *c) const
+  {
+    subtables[cache_user_idx].cache_leave (c);
+    HB_BUFFER_DEALLOCATE_VAR (c->buffer, syllable);
+  }
+
+
   private:
   hb_set_digest_t digest;
   hb_accelerate_subtables_context_t::array_t subtables;
+  unsigned cache_user_idx = (unsigned) -1;
 };
 
 struct GSUBGPOS

src/hb-ot-layout.cc

@@ -1827,6 +1827,8 @@ static inline bool
 apply_forward (OT::hb_ot_apply_context_t *c,
 	       const OT::hb_ot_layout_lookup_accelerator_t &accel)
 {
+  bool use_cache = accel.cache_enter (c);
+
   bool ret = false;
   hb_buffer_t *buffer = c->buffer;
   while (buffer->idx < buffer->len && buffer->successful)
@@ -1836,7 +1838,7 @@ apply_forward (OT::hb_ot_apply_context_t *c,
 	(buffer->cur().mask & c->lookup_mask) &&
 	c->check_glyph_property (&buffer->cur(), c->lookup_props))
      {
-       applied = accel.apply (c);
+       applied = accel.apply (c, use_cache);
      }
 
     if (applied)
@@ -1844,6 +1846,10 @@ apply_forward (OT::hb_ot_apply_context_t *c,
     else
       (void) buffer->next_glyph ();
   }
+
+  if (use_cache)
+    accel.cache_leave (c);
+
   return ret;
 }
 
@@ -1858,7 +1864,7 @@ apply_backward (OT::hb_ot_apply_context_t *c,
     if (accel.may_have (buffer->cur().codepoint) &&
 	(buffer->cur().mask & c->lookup_mask) &&
 	c->check_glyph_property (&buffer->cur(), c->lookup_props))
-     ret |= accel.apply (c);
+     ret |= accel.apply (c, false);
 
     /* The reverse lookup doesn't "advance" cursor (for good reason). */
     buffer->idx--;