tor-browser

hb-aat-layout-kerx-table.hh (35501B)

---

/*
* Copyright © 2018  Ebrahim Byagowi
* Copyright © 2018  Google, Inc.
*
*  This is part of HarfBuzz, a text shaping library.
*
* Permission is hereby granted, without written agreement and without
* license or royalty fees, to use, copy, modify, and distribute this
* software and its documentation for any purpose, provided that the
* above copyright notice and the following two paragraphs appear in
* all copies of this software.
*
* IN NO EVENT SHALL THE COPYRIGHT HOLDER BE LIABLE TO ANY PARTY FOR
* DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES
* ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN
* IF THE COPYRIGHT HOLDER HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH
* DAMAGE.
*
* THE COPYRIGHT HOLDER SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING,
* BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
* FITNESS FOR A PARTICULAR PURPOSE.  THE SOFTWARE PROVIDED HEREUNDER IS
* ON AN "AS IS" BASIS, AND THE COPYRIGHT HOLDER HAS NO OBLIGATION TO
* PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.
*
* Google Author(s): Behdad Esfahbod
*/

#ifndef HB_AAT_LAYOUT_KERX_TABLE_HH
#define HB_AAT_LAYOUT_KERX_TABLE_HH

#include "hb-kern.hh"
#include "hb-aat-layout-ankr-table.hh"
#include "hb-set-digest.hh"

/*
* kerx -- Extended Kerning
* https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6kerx.html
*/
#define HB_AAT_TAG_kerx HB_TAG('k','e','r','x')


namespace AAT {

using namespace OT;


static inline int
kerxTupleKern (int value,
       unsigned int tupleCount,
       const void *base,
       hb_aat_apply_context_t *c)
{
 if (likely (!tupleCount || !c)) return value;

 unsigned int offset = value;
 const FWORD *pv = &StructAtOffset<FWORD> (base, offset);
 if (unlikely (!c->sanitizer.check_array (pv, tupleCount))) return 0;
 hb_barrier ();
 return *pv;
}


struct hb_glyph_pair_t
{
 hb_codepoint_t left;
 hb_codepoint_t right;
};

struct KernPair
{
 int get_kerning () const { return value; }

 int cmp (const hb_glyph_pair_t &o) const
 {
   int ret = left.cmp (o.left);
   if (ret) return ret;
   return right.cmp (o.right);
 }

 bool sanitize (hb_sanitize_context_t *c) const
 {
   TRACE_SANITIZE (this);
   return_trace (c->check_struct (this));
 }

 public:
 HBGlyphID16	left;
 HBGlyphID16	right;
 FWORD		value;
 public:
 DEFINE_SIZE_STATIC (6);
};

template <typename KernSubTableHeader>
struct KerxSubTableFormat0
{
 int get_kerning (hb_codepoint_t left, hb_codepoint_t right,
	   hb_aat_apply_context_t *c = nullptr) const
 {
   hb_glyph_pair_t pair = {left, right};
   int v = pairs.bsearch (pair).get_kerning ();
   return kerxTupleKern (v, header.tuple_count (), this, c);
 }

 bool apply (hb_aat_apply_context_t *c) const
 {
   TRACE_APPLY (this);

   if (!c->plan->requested_kerning)
     return_trace (false);

   if (header.coverage & header.Backwards)
     return_trace (false);

   accelerator_t accel (*this, c);
   hb_kern_machine_t<accelerator_t> machine (accel, header.coverage & header.CrossStream);
   machine.kern (c->font, c->buffer, c->plan->kern_mask);

   return_trace (true);
 }

 template <typename set_t>
 void collect_glyphs (set_t &first_set, set_t &second_set, unsigned num_glyphs) const
 {
   for (const KernPair& pair : pairs)
   {
     first_set.add (pair.left);
     second_set.add (pair.right);
   }
 }

 struct accelerator_t
 {
   const KerxSubTableFormat0 &table;
   hb_aat_apply_context_t *c;

   accelerator_t (const KerxSubTableFormat0 &table_,
	   hb_aat_apply_context_t *c_) :
	     table (table_), c (c_) {}

   int get_kerning (hb_codepoint_t left, hb_codepoint_t right) const
   {
     if (!(*c->first_set)[left] || !(*c->second_set)[right]) return 0;
     return table.get_kerning (left, right, c);
   }
 };


 bool sanitize (hb_sanitize_context_t *c) const
 {
   TRACE_SANITIZE (this);
   return_trace (likely (pairs.sanitize (c)));
 }

 protected:
 KernSubTableHeader	header;
 BinSearchArrayOf<KernPair, typename KernSubTableHeader::Types::HBUINT>
		pairs;	/* Sorted kern records. */
 public:
 DEFINE_SIZE_ARRAY (KernSubTableHeader::static_size + 16, pairs);
};


template <bool extended>
struct Format1Entry;

template <>
struct Format1Entry<true>
{
 enum Flags
 {
   Push		= 0x8000,	/* If set, push this glyph on the kerning stack. */
   DontAdvance		= 0x4000,	/* If set, don't advance to the next glyph
				 * before going to the new state. */
   Reset		= 0x2000,	/* If set, reset the kerning data (clear the stack) */
   Reserved		= 0x1FFF,	/* Not used; set to 0. */
 };

 struct EntryData
 {
   HBUINT16	kernActionIndex;/* Index into the kerning value array. If
			 * this index is 0xFFFF, then no kerning
			 * is to be performed. */
   public:
   DEFINE_SIZE_STATIC (2);
 };

 static bool initiateAction (const Entry<EntryData> &entry)
 { return entry.flags & Push; }

 static bool performAction (const Entry<EntryData> &entry)
 { return entry.data.kernActionIndex != 0xFFFF; }

 static unsigned int kernActionIndex (const Entry<EntryData> &entry)
 { return entry.data.kernActionIndex; }
};
template <>
struct Format1Entry<false>
{
 enum Flags
 {
   Push		= 0x8000,	/* If set, push this glyph on the kerning stack. */
   DontAdvance		= 0x4000,	/* If set, don't advance to the next glyph
				 * before going to the new state. */
   Offset		= 0x3FFF,	/* Byte offset from beginning of subtable to the
				 * value table for the glyphs on the kerning stack. */

   Reset		= 0x0000,	/* Not supported? */
 };

 typedef void EntryData;

 static bool initiateAction (const Entry<EntryData> &entry)
 { return entry.flags & Push; }

 static bool performAction (const Entry<EntryData> &entry)
 { return entry.flags & Offset; }

 static unsigned int kernActionIndex (const Entry<EntryData> &entry)
 { return entry.flags & Offset; }
};

template <typename KernSubTableHeader>
struct KerxSubTableFormat1
{
 typedef typename KernSubTableHeader::Types Types;
 typedef typename Types::HBUINT HBUINT;

 typedef Format1Entry<Types::extended> Format1EntryT;
 typedef typename Format1EntryT::EntryData EntryData;

 enum Flags
 {
   DontAdvance	= Format1EntryT::DontAdvance,
 };

 bool is_action_initiable (const Entry<EntryData> &entry) const
 {
   return Format1EntryT::initiateAction (entry);
 }
 bool is_actionable (const Entry<EntryData> &entry) const
 {
   return Format1EntryT::performAction (entry);
 }

 struct driver_context_t
 {
   static constexpr bool in_place = true;

   driver_context_t (const KerxSubTableFormat1 *table_,
	      hb_aat_apply_context_t *c_) :
c (c_),
table (table_),
/* Apparently the offset kernAction is from the beginning of the state-machine,
 * similar to offsets in morx table, NOT from beginning of this table, like
 * other subtables in kerx.  Discovered via testing. */
kernAction (&table->machine + table->kernAction),
depth (0),
crossStream (table->header.coverage & table->header.CrossStream) {}

   void transition (hb_buffer_t *buffer,
	     StateTableDriver<Types, EntryData, Flags> *driver,
	     const Entry<EntryData> &entry)
   {
     unsigned int flags = entry.flags;

     if (flags & Format1EntryT::Reset)
depth = 0;

     if (flags & Format1EntryT::Push)
     {
if (likely (depth < ARRAY_LENGTH (stack)))
  stack[depth++] = buffer->idx;
else
  depth = 0; /* Probably not what CoreText does, but better? */
     }

     if (Format1EntryT::performAction (entry) && depth)
     {
unsigned int tuple_count = hb_max (1u, table->header.tuple_count ());

unsigned int kern_idx = Format1EntryT::kernActionIndex (entry);
kern_idx = Types::byteOffsetToIndex (kern_idx, &table->machine, kernAction.arrayZ);
const FWORD *actions = &kernAction[kern_idx];
if (!c->sanitizer.check_array (actions, depth, tuple_count))
{
  depth = 0;
  return;
}
hb_barrier ();

hb_mask_t kern_mask = c->plan->kern_mask;

/* From Apple 'kern' spec:
 * "Each pops one glyph from the kerning stack and applies the kerning value to it.
 * The end of the list is marked by an odd value... */
bool last = false;
while (!last && depth)
{
  unsigned int idx = stack[--depth];
  int v = *actions;
  actions += tuple_count;
  if (idx >= buffer->len) continue;

  /* "The end of the list is marked by an odd value..." */
  last = v & 1;
  v &= ~1;

  hb_glyph_position_t &o = buffer->pos[idx];

  if (HB_DIRECTION_IS_HORIZONTAL (buffer->props.direction))
  {
    if (crossStream)
    {
      /* The following flag is undocumented in the spec, but described
       * in the 'kern' table example. */
      if (v == -0x8000)
      {
	o.attach_type() = OT::Layout::GPOS_impl::ATTACH_TYPE_NONE;
	o.attach_chain() = 0;
	o.y_offset = 0;
      }
      else if (o.attach_type())
      {
	o.y_offset += c->font->em_scale_y (v);
	buffer->scratch_flags |= HB_BUFFER_SCRATCH_FLAG_HAS_GPOS_ATTACHMENT;
      }
    }
    else if (buffer->info[idx].mask & kern_mask)
    {
      auto scaled = c->font->em_scale_x (v);
      o.x_advance += scaled;
      o.x_offset += scaled;
    }
  }
  else
  {
    if (crossStream)
    {
      /* CoreText doesn't do crossStream kerning in vertical.  We do. */
      if (v == -0x8000)
      {
	o.attach_type() = OT::Layout::GPOS_impl::ATTACH_TYPE_NONE;
	o.attach_chain() = 0;
	o.x_offset = 0;
      }
      else if (o.attach_type())
      {
	o.x_offset += c->font->em_scale_x (v);
	buffer->scratch_flags |= HB_BUFFER_SCRATCH_FLAG_HAS_GPOS_ATTACHMENT;
      }
    }
    else if (buffer->info[idx].mask & kern_mask)
    {
      o.y_advance += c->font->em_scale_y (v);
      o.y_offset += c->font->em_scale_y (v);
    }
  }
}
     }
   }

   public:
   hb_aat_apply_context_t *c;
   const KerxSubTableFormat1 *table;
   private:
   const UnsizedArrayOf<FWORD> &kernAction;
   unsigned int stack[8];
   unsigned int depth;
   bool crossStream;
 };

 bool apply (hb_aat_apply_context_t *c) const
 {
   TRACE_APPLY (this);

   if (!c->plan->requested_kerning &&
!(header.coverage & header.CrossStream))
     return false;

   driver_context_t dc (this, c);

   StateTableDriver<Types, EntryData, Flags> driver (machine, c->font->face);

   driver.drive (&dc, c);

   return_trace (true);
 }

 bool sanitize (hb_sanitize_context_t *c) const
 {
   TRACE_SANITIZE (this);
   /* The rest of array sanitizations are done at run-time. */
   return_trace (likely (c->check_struct (this) &&
		  machine.sanitize (c)));
 }

 template <typename set_t>
 void collect_glyphs (set_t &first_set, set_t &second_set, unsigned num_glyphs) const
 {
   machine.collect_initial_glyphs (first_set, num_glyphs, *this);
   //machine.collect_glyphs (second_set, num_glyphs); // second_set is unused for machine kerning
 }

 protected:
 KernSubTableHeader				header;
 StateTable<Types, EntryData>			machine;
 NNOffsetTo<UnsizedArrayOf<FWORD>, HBUINT>	kernAction;
 public:
 DEFINE_SIZE_STATIC (KernSubTableHeader::static_size + (StateTable<Types, EntryData>::static_size + HBUINT::static_size));
};

template <typename KernSubTableHeader>
struct KerxSubTableFormat2
{
 typedef typename KernSubTableHeader::Types Types;
 typedef typename Types::HBUINT HBUINT;

 int get_kerning (hb_codepoint_t left, hb_codepoint_t right,
	   hb_aat_apply_context_t *c) const
 {
   unsigned int num_glyphs = c->sanitizer.get_num_glyphs ();
   unsigned int l = (this+leftClassTable).get_class (left, num_glyphs, 0);
   unsigned int r = (this+rightClassTable).get_class (right, num_glyphs, 0);

   const UnsizedArrayOf<FWORD> &arrayZ = this+array;
   unsigned int kern_idx = l + r;
   kern_idx = Types::offsetToIndex (kern_idx, this, arrayZ.arrayZ);
   const FWORD *v = &arrayZ[kern_idx];
   if (unlikely (!v->sanitize (&c->sanitizer))) return 0;
   hb_barrier ();

   return kerxTupleKern (*v, header.tuple_count (), this, c);
 }

 bool apply (hb_aat_apply_context_t *c) const
 {
   TRACE_APPLY (this);

   if (!c->plan->requested_kerning)
     return_trace (false);

   if (header.coverage & header.Backwards)
     return_trace (false);

   accelerator_t accel (*this, c);
   hb_kern_machine_t<accelerator_t> machine (accel, header.coverage & header.CrossStream);
   machine.kern (c->font, c->buffer, c->plan->kern_mask);

   return_trace (true);
 }

 template <typename set_t>
 void collect_glyphs (set_t &first_set, set_t &second_set, unsigned num_glyphs) const
 {
   (this+leftClassTable).collect_glyphs (first_set, num_glyphs);
   (this+rightClassTable).collect_glyphs (second_set, num_glyphs);
 }

 struct accelerator_t
 {
   const KerxSubTableFormat2 &table;
   hb_aat_apply_context_t *c;

   accelerator_t (const KerxSubTableFormat2 &table_,
	   hb_aat_apply_context_t *c_) :
	     table (table_), c (c_) {}

   int get_kerning (hb_codepoint_t left, hb_codepoint_t right) const
   {
     if (!(*c->first_set)[left] || !(*c->second_set)[right]) return 0;
     return table.get_kerning (left, right, c);
   }
 };

 bool sanitize (hb_sanitize_context_t *c) const
 {
   TRACE_SANITIZE (this);
   return_trace (likely (c->check_struct (this) &&
		  leftClassTable.sanitize (c, this) &&
		  rightClassTable.sanitize (c, this) &&
		  hb_barrier () &&
		  c->check_range (this, array)));
 }

 protected:
 KernSubTableHeader	header;
 HBUINT		rowWidth;	/* The width, in bytes, of a row in the table. */
 NNOffsetTo<typename Types::ClassTypeWide, HBUINT>
		leftClassTable;	/* Offset from beginning of this subtable to
				 * left-hand class table. */
 NNOffsetTo<typename Types::ClassTypeWide, HBUINT>
		rightClassTable;/* Offset from beginning of this subtable to
				 * right-hand class table. */
 NNOffsetTo<UnsizedArrayOf<FWORD>, HBUINT>
		 array;		/* Offset from beginning of this subtable to
				 * the start of the kerning array. */
 public:
 DEFINE_SIZE_STATIC (KernSubTableHeader::static_size + 4 * sizeof (HBUINT));
};

template <typename KernSubTableHeader>
struct KerxSubTableFormat4
{
 typedef ExtendedTypes Types;

 struct EntryData
 {
   HBUINT16	ankrActionIndex;/* Either 0xFFFF (for no action) or the index of
			 * the action to perform. */
   public:
   DEFINE_SIZE_STATIC (2);
 };

 enum Flags
 {
   Mark		= 0x8000,	/* If set, remember this glyph as the marked glyph. */
   DontAdvance		= 0x4000,	/* If set, don't advance to the next glyph before
				 * going to the new state. */
   Reserved		= 0x3FFF,	/* Not used; set to 0. */
 };

 bool is_action_initiable (const Entry<EntryData> &entry) const
 {
   return (entry.flags & Mark);
 }
 bool is_actionable (const Entry<EntryData> &entry) const
 {
   return entry.data.ankrActionIndex != 0xFFFF;
 }

 struct driver_context_t
 {
   static constexpr bool in_place = true;
   enum SubTableFlags
   {
     ActionType	= 0xC0000000,	/* A two-bit field containing the action type. */
     Unused		= 0x3F000000,	/* Unused - must be zero. */
     Offset		= 0x00FFFFFF,	/* Masks the offset in bytes from the beginning
				 * of the subtable to the beginning of the control
				 * point table. */
   };

   driver_context_t (const KerxSubTableFormat4 *table_,
	      hb_aat_apply_context_t *c_) :
c (c_),
table (table_),
action_type ((table->flags & ActionType) >> 30),
ankrData ((HBUINT16 *) ((const char *) &table->machine + (table->flags & Offset))),
mark_set (false),
mark (0) {}

   void transition (hb_buffer_t *buffer,
	     StateTableDriver<Types, EntryData, Flags> *driver,
	     const Entry<EntryData> &entry)
   {
     if (mark_set && entry.data.ankrActionIndex != 0xFFFF && buffer->idx < buffer->len)
     {
hb_glyph_position_t &o = buffer->cur_pos();
switch (action_type)
{
  case 0: /* Control Point Actions.*/
  {
    /* Indexed into glyph outline. */
    /* Each action (record in ankrData) contains two 16-bit fields, so we must
       double the ankrActionIndex to get the correct offset here. */
    const HBUINT16 *data = &ankrData[entry.data.ankrActionIndex * 2];
    if (!c->sanitizer.check_array (data, 2)) return;
    hb_barrier ();
    unsigned int markControlPoint = *data++;
    unsigned int currControlPoint = *data++;
    hb_position_t markX = 0;
    hb_position_t markY = 0;
    hb_position_t currX = 0;
    hb_position_t currY = 0;
    if (!c->font->get_glyph_contour_point_for_origin (c->buffer->info[mark].codepoint,
						      markControlPoint,
						      HB_DIRECTION_LTR /*XXX*/,
						      &markX, &markY) ||
	!c->font->get_glyph_contour_point_for_origin (c->buffer->cur ().codepoint,
						      currControlPoint,
						      HB_DIRECTION_LTR /*XXX*/,
						      &currX, &currY))
      return;

    o.x_offset = markX - currX;
    o.y_offset = markY - currY;
  }
  break;

  case 1: /* Anchor Point Actions. */
  {
    /* Indexed into 'ankr' table. */
    /* Each action (record in ankrData) contains two 16-bit fields, so we must
       double the ankrActionIndex to get the correct offset here. */
    const HBUINT16 *data = &ankrData[entry.data.ankrActionIndex * 2];
    if (!c->sanitizer.check_array (data, 2)) return;
    hb_barrier ();
    unsigned int markAnchorPoint = *data++;
    unsigned int currAnchorPoint = *data++;
    const Anchor &markAnchor = c->ankr_table->get_anchor (c->buffer->info[mark].codepoint,
							  markAnchorPoint,
							  c->sanitizer.get_num_glyphs ());
    const Anchor &currAnchor = c->ankr_table->get_anchor (c->buffer->cur ().codepoint,
							  currAnchorPoint,
							  c->sanitizer.get_num_glyphs ());

    o.x_offset = c->font->em_scale_x (markAnchor.xCoordinate) - c->font->em_scale_x (currAnchor.xCoordinate);
    o.y_offset = c->font->em_scale_y (markAnchor.yCoordinate) - c->font->em_scale_y (currAnchor.yCoordinate);
  }
  break;

  case 2: /* Control Point Coordinate Actions. */
  {
    /* Each action contains four 16-bit fields, so we multiply the ankrActionIndex
       by 4 to get the correct offset for the given action. */
    const FWORD *data = (const FWORD *) &ankrData[entry.data.ankrActionIndex * 4];
    if (!c->sanitizer.check_array (data, 4)) return;
    hb_barrier ();
    int markX = *data++;
    int markY = *data++;
    int currX = *data++;
    int currY = *data++;

    o.x_offset = c->font->em_scale_x (markX) - c->font->em_scale_x (currX);
    o.y_offset = c->font->em_scale_y (markY) - c->font->em_scale_y (currY);
  }
  break;
}
o.attach_type() = OT::Layout::GPOS_impl::ATTACH_TYPE_MARK;
o.attach_chain() = (int) mark - (int) buffer->idx;
if (c->buffer_is_reversed)
  o.attach_chain() = -o.attach_chain();
buffer->scratch_flags |= HB_BUFFER_SCRATCH_FLAG_HAS_GPOS_ATTACHMENT;
     }

     if (entry.flags & Mark)
     {
mark_set = true;
mark = buffer->idx;
     }
   }

   public:
   hb_aat_apply_context_t *c;
   const KerxSubTableFormat4 *table;
   private:
   unsigned int action_type;
   const HBUINT16 *ankrData;
   bool mark_set;
   unsigned int mark;
 };

 bool apply (hb_aat_apply_context_t *c) const
 {
   TRACE_APPLY (this);

   driver_context_t dc (this, c);

   StateTableDriver<Types, EntryData, Flags> driver (machine, c->font->face);

   driver.drive (&dc, c);

   return_trace (true);
 }

 bool sanitize (hb_sanitize_context_t *c) const
 {
   TRACE_SANITIZE (this);
   /* The rest of array sanitizations are done at run-time. */
   return_trace (likely (c->check_struct (this) &&
		  machine.sanitize (c)));
 }

 template <typename set_t>
 void collect_glyphs (set_t &first_set, set_t &second_set, unsigned num_glyphs) const
 {
   machine.collect_initial_glyphs (first_set, num_glyphs, *this);
   //machine.collect_glyphs (second_set, num_glyphs); // second_set is unused for machine kerning
 }

 protected:
 KernSubTableHeader		header;
 StateTable<Types, EntryData>	machine;
 HBUINT32			flags;
 public:
 DEFINE_SIZE_STATIC (KernSubTableHeader::static_size + (StateTable<Types, EntryData>::static_size + HBUINT32::static_size));
};

template <typename KernSubTableHeader>
struct KerxSubTableFormat6
{
 enum Flags
 {
   ValuesAreLong	= 0x00000001,
 };

 bool is_long () const { return flags & ValuesAreLong; }

 int get_kerning (hb_codepoint_t left, hb_codepoint_t right,
	   hb_aat_apply_context_t *c) const
 {
   unsigned int num_glyphs = c->sanitizer.get_num_glyphs ();
   if (is_long ())
   {
     const auto &t = u.l;
     unsigned int l = (this+t.rowIndexTable).get_value_or_null (left, num_glyphs);
     unsigned int r = (this+t.columnIndexTable).get_value_or_null (right, num_glyphs);
     unsigned int offset = l + r;
     if (unlikely (offset < l)) return 0; /* Addition overflow. */
     if (unlikely (hb_unsigned_mul_overflows (offset, sizeof (FWORD32)))) return 0;
     const FWORD32 *v = &StructAtOffset<FWORD32> (&(this+t.array), offset * sizeof (FWORD32));
     if (unlikely (!v->sanitize (&c->sanitizer))) return 0;
     hb_barrier ();
     return kerxTupleKern (*v, header.tuple_count (), &(this+vector), c);
   }
   else
   {
     const auto &t = u.s;
     unsigned int l = (this+t.rowIndexTable).get_value_or_null (left, num_glyphs);
     unsigned int r = (this+t.columnIndexTable).get_value_or_null (right, num_glyphs);
     unsigned int offset = l + r;
     const FWORD *v = &StructAtOffset<FWORD> (&(this+t.array), offset * sizeof (FWORD));
     if (unlikely (!v->sanitize (&c->sanitizer))) return 0;
     hb_barrier ();
     return kerxTupleKern (*v, header.tuple_count (), &(this+vector), c);
   }
 }

 bool apply (hb_aat_apply_context_t *c) const
 {
   TRACE_APPLY (this);

   if (!c->plan->requested_kerning)
     return_trace (false);

   if (header.coverage & header.Backwards)
     return_trace (false);

   accelerator_t accel (*this, c);
   hb_kern_machine_t<accelerator_t> machine (accel, header.coverage & header.CrossStream);
   machine.kern (c->font, c->buffer, c->plan->kern_mask);

   return_trace (true);
 }

 bool sanitize (hb_sanitize_context_t *c) const
 {
   TRACE_SANITIZE (this);
   return_trace (likely (c->check_struct (this) &&
		  hb_barrier () &&
		  (is_long () ?
		   (
		     u.l.rowIndexTable.sanitize (c, this) &&
		     u.l.columnIndexTable.sanitize (c, this) &&
		     c->check_range (this, u.l.array)
		   ) : (
		     u.s.rowIndexTable.sanitize (c, this) &&
		     u.s.columnIndexTable.sanitize (c, this) &&
		     c->check_range (this, u.s.array)
		   )) &&
		  (header.tuple_count () == 0 ||
		   c->check_range (this, vector))));
 }

 template <typename set_t>
 void collect_glyphs (set_t &first_set, set_t &second_set, unsigned num_glyphs) const
 {
   if (is_long ())
   {
     const auto &t = u.l;
     (this+t.rowIndexTable).collect_glyphs (first_set, num_glyphs);
     (this+t.columnIndexTable).collect_glyphs (second_set, num_glyphs);
   }
   else
   {
     const auto &t = u.s;
     (this+t.rowIndexTable).collect_glyphs (first_set, num_glyphs);
     (this+t.columnIndexTable).collect_glyphs (second_set, num_glyphs);
   }
 }

 struct accelerator_t
 {
   const KerxSubTableFormat6 &table;
   hb_aat_apply_context_t *c;

   accelerator_t (const KerxSubTableFormat6 &table_,
	   hb_aat_apply_context_t *c_) :
	     table (table_), c (c_) {}

   int get_kerning (hb_codepoint_t left, hb_codepoint_t right) const
   {
     if (!(*c->first_set)[left] || !(*c->second_set)[right]) return 0;
     return table.get_kerning (left, right, c);
   }
 };

 protected:
 KernSubTableHeader		header;
 HBUINT32			flags;
 HBUINT16			rowCount;
 HBUINT16			columnCount;
 union U
 {
   struct Long
   {
     NNOffset32To<Lookup<HBUINT32>>		rowIndexTable;
     NNOffset32To<Lookup<HBUINT32>>		columnIndexTable;
     NNOffset32To<UnsizedArrayOf<FWORD32>>	array;
   } l;
   struct Short
   {
     NNOffset32To<Lookup<HBUINT16>>		rowIndexTable;
     NNOffset32To<Lookup<HBUINT16>>		columnIndexTable;
     NNOffset32To<UnsizedArrayOf<FWORD>>	array;
   } s;
 } u;
 NNOffset32To<UnsizedArrayOf<FWORD>>	vector;
 public:
 DEFINE_SIZE_STATIC (KernSubTableHeader::static_size + 24);
};


struct KerxSubTableHeader
{
 typedef ExtendedTypes Types;

 unsigned   tuple_count () const { return tupleCount; }
 bool     is_horizontal () const { return !(coverage & Vertical); }

 enum Coverage
 {
   Vertical	= 0x80000000u,	/* Set if table has vertical kerning values. */
   CrossStream	= 0x40000000u,	/* Set if table has cross-stream kerning values. */
   Variation	= 0x20000000u,	/* Set if table has variation kerning values. */
   Backwards	= 0x10000000u,	/* If clear, process the glyphs forwards, that
			 * is, from first to last in the glyph stream.
			 * If we, process them from last to first.
			 * This flag only applies to state-table based
			 * 'kerx' subtables (types 1 and 4). */
   Reserved	= 0x0FFFFF00u,	/* Reserved, set to zero. */
   SubtableType= 0x000000FFu,	/* Subtable type. */
 };

 bool sanitize (hb_sanitize_context_t *c) const
 {
   TRACE_SANITIZE (this);
   return_trace (c->check_struct (this));
 }

 public:
 HBUINT32	length;
 HBUINT32	coverage;
 HBUINT32	tupleCount;
 public:
 DEFINE_SIZE_STATIC (12);
};

struct KerxSubTable
{
 friend struct kerx;

 unsigned int get_size () const { return u.header.length; }
 unsigned int get_type () const { return u.header.coverage & u.header.SubtableType; }

 template <typename context_t, typename ...Ts>
 typename context_t::return_t dispatch (context_t *c, Ts&&... ds) const
 {
   unsigned int subtable_type = get_type ();
   TRACE_DISPATCH (this, subtable_type);
   switch (subtable_type) {
   case 0:	return_trace (c->dispatch (u.format0, std::forward<Ts> (ds)...));
   case 1:	return_trace (c->dispatch (u.format1, std::forward<Ts> (ds)...));
   case 2:	return_trace (c->dispatch (u.format2, std::forward<Ts> (ds)...));
   case 4:	return_trace (c->dispatch (u.format4, std::forward<Ts> (ds)...));
   case 6:	return_trace (c->dispatch (u.format6, std::forward<Ts> (ds)...));
   default:	return_trace (c->default_return_value ());
   }
 }

 template <typename set_t>
 void collect_glyphs (set_t &first_set, set_t &second_set, unsigned num_glyphs) const
 {
   unsigned int subtable_type = get_type ();
   switch (subtable_type) {
   case 0:	u.format0.collect_glyphs (first_set, second_set, num_glyphs); return;
   case 1:	u.format1.collect_glyphs (first_set, second_set, num_glyphs); return;
   case 2:	u.format2.collect_glyphs (first_set, second_set, num_glyphs); return;
   case 4:	u.format4.collect_glyphs (first_set, second_set, num_glyphs); return;
   case 6:	u.format6.collect_glyphs (first_set, second_set, num_glyphs); return;
   default:	return;
   }
 }

 bool sanitize (hb_sanitize_context_t *c) const
 {
   TRACE_SANITIZE (this);
   if (!(u.header.sanitize (c) &&
  hb_barrier () &&
  u.header.length >= u.header.static_size &&
  c->check_range (this, u.header.length)))
     return_trace (false);

   return_trace (dispatch (c));
 }

 public:
 union {
 KerxSubTableHeader				header;
 KerxSubTableFormat0<KerxSubTableHeader>	format0;
 KerxSubTableFormat1<KerxSubTableHeader>	format1;
 KerxSubTableFormat2<KerxSubTableHeader>	format2;
 KerxSubTableFormat4<KerxSubTableHeader>	format4;
 KerxSubTableFormat6<KerxSubTableHeader>	format6;
 } u;
 public:
 DEFINE_SIZE_MIN (12);
};


/*
* The 'kerx' Table
*/

struct kern_subtable_accelerator_data_t
{
 hb_bit_set_t first_set;
 hb_bit_set_t second_set;
 mutable hb_aat_class_cache_t class_cache;
};

struct kern_accelerator_data_t
{
 hb_vector_t<kern_subtable_accelerator_data_t> subtable_accels;
 hb_aat_scratch_t scratch;
};

template <typename T>
struct KerxTable
{
 /* https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern */
 const T* thiz () const { return static_cast<const T *> (this); }

 bool has_state_machine () const
 {
   typedef typename T::SubTable SubTable;

   const SubTable *st = &thiz()->firstSubTable;
   unsigned int count = thiz()->tableCount;
   for (unsigned int i = 0; i < count; i++)
   {
     if (st->get_type () == 1)
return true;

     // TODO: What about format 4? What's this API used for anyway?

     st = &StructAfter<SubTable> (*st);
   }
   return false;
 }

 bool has_cross_stream () const
 {
   typedef typename T::SubTable SubTable;

   const SubTable *st = &thiz()->firstSubTable;
   unsigned int count = thiz()->tableCount;
   for (unsigned int i = 0; i < count; i++)
   {
     if (st->u.header.coverage & st->u.header.CrossStream)
return true;
     st = &StructAfter<SubTable> (*st);
   }
   return false;
 }

 int get_h_kerning (hb_codepoint_t left, hb_codepoint_t right) const
 {
   typedef typename T::SubTable SubTable;

   int v = 0;
   const SubTable *st = &thiz()->firstSubTable;
   unsigned int count = thiz()->tableCount;
   for (unsigned int i = 0; i < count; i++)
   {
     if ((st->u.header.coverage & (st->u.header.Variation | st->u.header.CrossStream)) ||
  !st->u.header.is_horizontal ())
continue;
     v += st->get_kerning (left, right);
     st = &StructAfter<SubTable> (*st);
   }
   return v;
 }

 bool apply (AAT::hb_aat_apply_context_t *c,
      const kern_accelerator_data_t &accel_data) const
 {
   c->buffer->unsafe_to_concat ();

   c->setup_buffer_glyph_set ();

   typedef typename T::SubTable SubTable;

   bool ret = false;
   bool seenCrossStream = false;
   c->set_lookup_index (0);
   const SubTable *st = &thiz()->firstSubTable;
   unsigned int count = thiz()->tableCount;
   for (unsigned int i = 0; i < count; i++)
   {
     bool reverse;

     auto &subtable_accel = accel_data.subtable_accels[i];

     if (!T::Types::extended && (st->u.header.coverage & st->u.header.Variation))
goto skip;

     if (HB_DIRECTION_IS_HORIZONTAL (c->buffer->props.direction) != st->u.header.is_horizontal ())
goto skip;

     c->first_set = &subtable_accel.first_set;
     c->second_set = &subtable_accel.second_set;
     c->machine_class_cache = &subtable_accel.class_cache;

     if (!c->buffer_intersects_machine ())
     {
(void) c->buffer->message (c->font, "skipped subtable %u because no glyph matches", c->lookup_index);
goto skip;
     }

     reverse = bool (st->u.header.coverage & st->u.header.Backwards) !=
	HB_DIRECTION_IS_BACKWARD (c->buffer->props.direction);

     if (!c->buffer->message (c->font, "start subtable %u", c->lookup_index))
goto skip;

     if (!seenCrossStream &&
  (st->u.header.coverage & st->u.header.CrossStream))
     {
/* Attach all glyphs into a chain. */
seenCrossStream = true;
hb_glyph_position_t *pos = c->buffer->pos;
unsigned int count = c->buffer->len;
for (unsigned int i = 0; i < count; i++)
{
  pos[i].attach_type() = OT::Layout::GPOS_impl::ATTACH_TYPE_CURSIVE;
  pos[i].attach_chain() = HB_DIRECTION_IS_FORWARD (c->buffer->props.direction) ? -1 : +1;
  /* We intentionally don't set HB_BUFFER_SCRATCH_FLAG_HAS_GPOS_ATTACHMENT,
   * since there needs to be a non-zero attachment for post-positioning to
   * be needed. */
}
     }

     if (reverse != c->buffer_is_reversed)
       c->reverse_buffer ();

     {
/* See comment in sanitize() for conditional here. */
hb_sanitize_with_object_t with (&c->sanitizer, i < count - 1 ? st : (const SubTable *) nullptr);
ret |= st->dispatch (c);
     }

     (void) c->buffer->message (c->font, "end subtable %u", c->lookup_index);

   skip:
     st = &StructAfter<SubTable> (*st);
     c->set_lookup_index (c->lookup_index + 1);
   }
   if (c->buffer_is_reversed)
     c->reverse_buffer ();

   return ret;
 }

 bool sanitize (hb_sanitize_context_t *c) const
 {
   TRACE_SANITIZE (this);
   if (unlikely (!(thiz()->version.sanitize (c) &&
	    hb_barrier () &&
	    (unsigned) thiz()->version >= (unsigned) T::minVersion &&
	    thiz()->tableCount.sanitize (c))))
     return_trace (false);

   typedef typename T::SubTable SubTable;

   const SubTable *st = &thiz()->firstSubTable;
   unsigned int count = thiz()->tableCount;
   for (unsigned int i = 0; i < count; i++)
   {
     if (unlikely (!st->u.header.sanitize (c)))
return_trace (false);
     hb_barrier ();
     /* OpenType kern table has 2-byte subtable lengths.  That's limiting.
      * MS implementation also only supports one subtable, of format 0,
      * anyway.  Certain versions of some fonts, like Calibry, contain
      * kern subtable that exceeds 64kb.  Looks like, the subtable length
      * is simply ignored.  Which makes sense.  It's only needed if you
      * have multiple subtables.  To handle such fonts, we just ignore
      * the length for the last subtable. */
     hb_sanitize_with_object_t with (c, i < count - 1 ? st : (const SubTable *) nullptr);

     if (unlikely (!st->sanitize (c)))
return_trace (false);

     st = &StructAfter<SubTable> (*st);
   }

   unsigned majorVersion = thiz()->version;
   if (sizeof (thiz()->version) == 4)
     majorVersion = majorVersion >> 16;
   if (majorVersion >= 3)
   {
     const SubtableGlyphCoverage *coverage = (const SubtableGlyphCoverage *) st;
     if (!coverage->sanitize (c, count))
       return_trace (false);
   }

   return_trace (true);
 }

 kern_accelerator_data_t create_accelerator_data (unsigned num_glyphs) const
 {
   kern_accelerator_data_t accel_data;

   typedef typename T::SubTable SubTable;

   const SubTable *st = &thiz()->firstSubTable;
   unsigned int count = thiz()->tableCount;
   for (unsigned int i = 0; i < count; i++)
   {
     auto &subtable_accel = *accel_data.subtable_accels.push ();
     if (unlikely (accel_data.subtable_accels.in_error ()))
  return accel_data;

     st->collect_glyphs (subtable_accel.first_set, subtable_accel.second_set, num_glyphs);
     subtable_accel.class_cache.clear ();

     st = &StructAfter<SubTable> (*st);
   }

   return accel_data;
 }

 struct accelerator_t
 {
   accelerator_t (hb_face_t *face)
   {
     hb_sanitize_context_t sc;
     this->table = sc.reference_table<T> (face);
     this->accel_data = this->table->create_accelerator_data (face->get_num_glyphs ());
   }
   ~accelerator_t ()
   {
     this->table.destroy ();
   }

   hb_blob_t *get_blob () const { return table.get_blob (); }

   bool apply (AAT::hb_aat_apply_context_t *c) const
   {
     return table->apply (c, accel_data);
   }

   hb_blob_ptr_t<T> table;
   kern_accelerator_data_t accel_data;
   hb_aat_scratch_t scratch;
 };
};

struct kerx : KerxTable<kerx>
{
 friend struct KerxTable<kerx>;

 static constexpr hb_tag_t tableTag = HB_AAT_TAG_kerx;
 static constexpr unsigned minVersion = 2u;

 typedef KerxSubTableHeader SubTableHeader;
 typedef SubTableHeader::Types Types;
 typedef KerxSubTable SubTable;

 bool has_data () const { return version; }

 protected:
 HBUINT16	version;	/* The version number of the extended kerning table
			 * (currently 2, 3, or 4). */
 HBUINT16	unused;		/* Set to 0. */
 HBUINT32	tableCount;	/* The number of subtables included in the extended kerning
			 * table. */
 SubTable	firstSubTable;	/* Subtables. */
/*subtableGlyphCoverageArray*/	/* Only if version >= 3. We don't use. */

 public:
 DEFINE_SIZE_MIN (8);
};

struct kerx_accelerator_t : kerx::accelerator_t {
 kerx_accelerator_t (hb_face_t *face) : kerx::accelerator_t (face) {}
};

} /* namespace AAT */

#endif /* HB_AAT_LAYOUT_KERX_TABLE_HH */