[kern] Implement negative state numbers

Let the fuzzing bots rip this code apart...

src/hb-aat-layout-common.hh

@@ -430,8 +430,8 @@ struct StateTable
     CLASS_END_OF_LINE = 3,
   };
 
-  inline unsigned int new_state (unsigned int newState) const
-  { return Types::extended ? newState : (newState - stateArrayTable) / nClasses; }
+  inline int new_state (unsigned int newState) const
+  { return Types::extended ? newState : ((int) newState - (int) stateArrayTable) / nClasses; }
 
   inline unsigned int get_class (hb_codepoint_t glyph_id, unsigned int num_glyphs) const
   {
@@ -444,7 +444,7 @@ struct StateTable
     return (this+entryTable).arrayZ;
   }
 
-  inline const Entry<Extra> *get_entryZ (unsigned int state, unsigned int klass) const
+  inline const Entry<Extra> *get_entryZ (int state, unsigned int klass) const
   {
     if (unlikely (klass >= nClasses)) return nullptr;
 
@@ -452,7 +452,7 @@ struct StateTable
     const Entry<Extra> *entries = (this+entryTable).arrayZ;
 
     unsigned int entry = states[state * nClasses + klass];
-    DEBUG_MSG (APPLY, nullptr, "e%d", entry);
+    DEBUG_MSG (APPLY, nullptr, "e%u", entry);
 
     return &entries[entry];
   }
@@ -468,28 +468,69 @@ struct StateTable
     const Entry<Extra> *entries = (this+entryTable).arrayZ;
 
     unsigned int num_classes = nClasses;
+    if (unlikely (hb_unsigned_mul_overflows (num_classes, states[0].static_size)))
+      return_trace (false);
+    unsigned int row_stride = num_classes * states[0].static_size;
 
-    unsigned int num_states = 1;
+    /* Apple 'kern' table has this peculiarity:
+     *
+     * "Because the stateTableOffset in the state table header is (strictly
+     * speaking) redundant, some 'kern' tables use it to record an initial
+     * state where that should not be StartOfText. To determine if this is
+     * done, calculate what the stateTableOffset should be. If it's different
+     * from the actual stateTableOffset, use it as the initial state."
+     *
+     * We implement this by calling the initial state zero, but allow *negative*
+     * states if the start state indeed was not the first state.  Since the code
+     * is shared, this will also apply to 'mort' table.  The 'kerx' / 'morx'
+     * tables are not affected since those address states by index, not offset.
+     */
+
+    int min_state = 0;
+    int max_state = 0;
     unsigned int num_entries = 0;
 
-    unsigned int state = 0;
+    int state_pos = 0;
+    int state_neg = 0;
     unsigned int entry = 0;
-    while (state < num_states)
+    while (min_state < state_neg || state_pos <= max_state)
     {
-      if (unlikely (hb_unsigned_mul_overflows (num_classes, states[0].static_size)))
-	return_trace (false);
+      if (min_state < state_neg)
+      {
+	/* Negative states. */
+	if (unlikely (hb_unsigned_mul_overflows (min_state, num_classes)))
+	  return_trace (false);
+	if (unlikely (!c->check_array (&states[min_state * num_classes], -min_state, row_stride)))
+	  return_trace (false);
+	if ((c->max_ops -= state_neg - min_state) < 0)
+	  return_trace (false);
+	{ /* Sweep new states. */
+	  const HBUSHORT *stop = &states[min_state * num_classes];
+	  if (unlikely (stop > states))
+	    return_trace (false);
+	  for (const HBUSHORT *p = states; stop < p; p--)
+	    num_entries = MAX<unsigned int> (num_entries, *(p - 1) + 1);
+	  state_neg = min_state;
+	}
+      }
 
-      if (unlikely (!c->check_array (states,
-				     num_states,
-				     num_classes * states[0].static_size)))
-	return_trace (false);
-      if ((c->max_ops -= num_states - state) < 0)
-	return_trace (false);
-      { /* Sweep new states. */
-	const HBUSHORT *stop = &states[num_states * num_classes];
-	for (const HBUSHORT *p = &states[state * num_classes]; p < stop; p++)
-	  num_entries = MAX<unsigned int> (num_entries, *p + 1);
-	state = num_states;
+      if (state_pos <= max_state)
+      {
+	/* Positive states. */
+	if (unlikely (!c->check_array (states, max_state + 1, row_stride)))
+	  return_trace (false);
+	if ((c->max_ops -= max_state - state_pos + 1) < 0)
+	  return_trace (false);
+	{ /* Sweep new states. */
+	  if (unlikely (hb_unsigned_mul_overflows ((max_state + 1), num_classes)))
+	    return_trace (false);
+	  const HBUSHORT *stop = &states[(max_state + 1) * num_classes];
+	  if (unlikely (stop < states))
+	    return_trace (false);
+	  for (const HBUSHORT *p = &states[state_pos * num_classes]; p < stop; p++)
+	    num_entries = MAX<unsigned int> (num_entries, *p + 1);
+	  state_pos = max_state + 1;
+	}
       }
 
       if (unlikely (!c->check_array (entries, num_entries)))
@@ -500,8 +541,9 @@ struct StateTable
 	const Entry<Extra> *stop = &entries[num_entries];
 	for (const Entry<Extra> *p = &entries[entry]; p < stop; p++)
 	{
-	  unsigned int newState = new_state (p->newState);
-	  num_states = MAX<unsigned int> (num_states, newState + 1);
+	  int newState = new_state (p->newState);
+	  min_state = MIN (min_state, newState);
+	  max_state = MAX (max_state, newState);
 	}
 	entry = num_entries;
       }
@@ -623,14 +665,14 @@ struct StateTableDriver
     if (!c->in_place)
       buffer->clear_output ();
 
-    unsigned int state = StateTable<Types, EntryData>::STATE_START_OF_TEXT;
+    int state = StateTable<Types, EntryData>::STATE_START_OF_TEXT;
     bool last_was_dont_advance = false;
     for (buffer->idx = 0; buffer->successful;)
     {
       unsigned int klass = buffer->idx < buffer->len ?
 			   machine.get_class (buffer->info[buffer->idx].codepoint, num_glyphs) :
 			   (unsigned) StateTable<Types, EntryData>::CLASS_END_OF_TEXT;
-      DEBUG_MSG (APPLY, nullptr, "c%d at %d", klass, buffer->idx);
+      DEBUG_MSG (APPLY, nullptr, "c%u at %u", klass, buffer->idx);
       const Entry<EntryData> *entry = machine.get_entryZ (state, klass);
       if (unlikely (!entry))
 	break;

src/hb-aat-layout-kerx-table.hh

@@ -238,16 +238,6 @@ struct KerxSubTableFormat1
 	depth (0),
 	crossStream (table->header.coverage & table->header.CrossStream) {}
 
-    /* TODO
-     * 'kern' table has this pecularity, we don't currently implement.
-     *
-     * "Because the stateTableOffset in the state table header is (strictly
-     * speaking) redundant, some 'kern' tables use it to record an initial
-     * state where that should not be StartOfText. To determine if this is
-     * done, calculate what the stateTableOffset should be. If it's different
-     * from the actual stateTableOffset, use it as the initial state."
-     */
-
     inline bool is_actionable (StateTableDriver<Types, EntryData> *driver HB_UNUSED,
 			       const Entry<EntryData> *entry)
     {