[subset] Make cmap4 packing more optimal.

The current CMAP4 implementation uses whatever the current codepoint ranges are and then encodes them as indivudal glyph ids or as a delta if possible. However, it's often possible to save bytes by splitting up existing ranges and encoding parts of them using deltas where the cost of splitting the range is less than encoding each glyph individual.

src/hb-ot-cmap-table.hh

@@ -93,120 +93,168 @@ struct CmapSubtableFormat0
 struct CmapSubtableFormat4
 {
 
+
   template<typename Iterator,
+      typename Writer,
 	   hb_requires (hb_is_iterator (Iterator))>
-  HBUINT16* serialize_endcode_array (hb_serialize_context_t *c,
-				     Iterator it)
+  void to_ranges (Iterator it, Writer& range_writer)
   {
-    HBUINT16 *endCode = c->start_embed<HBUINT16> ();
-    hb_codepoint_t prev_endcp = 0xFFFF;
+    hb_codepoint_t start_cp, run_start_cp, end_cp, last_gid;
+    int run_length, delta;
 
-    for (const auto& _ : +it)
-    {
-      if (prev_endcp != 0xFFFF && prev_endcp + 1u != _.first)
-      {
-	HBUINT16 end_code;
-	end_code = prev_endcp;
-	c->copy<HBUINT16> (end_code);
-      }
-      prev_endcp = _.first;
+    enum {
+      FIRST_RANGE,
+      FOLLOWING_RANGE,
+    } mode;
+
+    while (it) {
+      // Start a new range
+      start_cp = (*it).first;
+      run_start_cp = (*it).first;
+      end_cp = (*it).first;
+      last_gid = (*it).second;
+      run_length = 1;
+      delta = (*it).second - (*it).first;
+      mode = FIRST_RANGE;
+      it++;
+
+      while (it) {
+        // Process range
+        hb_codepoint_t next_cp = (*it).first;
+        hb_codepoint_t next_gid = (*it).second;
+        if (next_cp != end_cp + 1) {
+          // Current range is over, stop processing.
+          break;
         }
 
-    {
-      // last endCode
-      HBUINT16 endcode;
-      endcode = prev_endcp;
-      if (unlikely (!c->copy<HBUINT16> (endcode))) return nullptr;
-      // There must be a final entry with end_code == 0xFFFF.
-      if (prev_endcp != 0xFFFF)
-      {
-	HBUINT16 finalcode;
-	finalcode = 0xFFFF;
-	if (unlikely (!c->copy<HBUINT16> (finalcode))) return nullptr;
+        if (next_gid == last_gid + 1) {
+          // The current run continues.
+          end_cp = next_cp;
+          run_length++;
+          last_gid = next_gid;
+          it++;
+          continue;
+        }
+
+        // A new run is starting, decide if we want to commit the current run.
+        int split_cost = (mode == FIRST_RANGE) ? 8 : 16;
+        int run_cost = run_length * 2;
+        if (run_cost >= split_cost) {
+          commit_current_range(start_cp, run_start_cp, end_cp, delta, split_cost, range_writer);
+          mode = FOLLOWING_RANGE;
+          start_cp = next_cp;
+        }
+
+        // Start the new run
+        run_start_cp = next_cp;
+        end_cp = next_cp;
+        delta = next_gid - run_start_cp;
+        run_length = 1;
+        last_gid = next_gid;
+        it++;
+      }
+
+      // Finalize range
+      commit_current_range (start_cp, run_start_cp, end_cp, delta, 8, range_writer);
+    }
+
+    if (likely (end_cp != 0xFFFF)) {
+      range_writer (0xFFFF, 0xFFFF, 1);
     }
   }
 
-    return endCode;
+  /*
+   * Writes the current range as either one or two ranges depending on what is most efficient.
+   */
+  template<typename Writer>
+  void commit_current_range (hb_codepoint_t start,
+                             hb_codepoint_t run_start,
+                             hb_codepoint_t end,
+                             int run_delta,
+                             int split_cost,
+                             Writer& range_writer) {
+    bool should_split = false;
+    if (start < run_start && run_start < end) {
+      int run_cost = (end - run_start + 1) * 2;
+      if (run_cost >= split_cost) {
+        should_split = true;
+      }
+    }
+
+    if (should_split) {
+      range_writer (start, run_start - 1, 0);
+      range_writer (run_start, end, run_delta);
+      return;
+    }
+
+
+    if (start == run_start) {
+      // Range is only a run
+      range_writer (start, end, run_delta);
+      return;
+    }
+
+    // Write only a single non-run range.
+    run_delta = (start == end) ? run_delta : 0;
+    range_writer (start, end, run_delta);
   }
 
   template<typename Iterator,
 	   hb_requires (hb_is_iterator (Iterator))>
-  HBUINT16* serialize_startcode_array (hb_serialize_context_t *c,
-				       Iterator it)
-  {
-    HBUINT16 *startCode = c->start_embed<HBUINT16> ();
-    hb_codepoint_t prev_cp = 0xFFFF;
+  unsigned serialize_find_segcount (Iterator it) {
+    struct Counter {
+      unsigned segcount = 0;
 
-    for (const auto& _ : +it)
-    {
-      if (prev_cp == 0xFFFF || prev_cp + 1u != _.first)
-      {
-	HBUINT16 start_code;
-	start_code = _.first;
-	c->copy<HBUINT16> (start_code);
+      void operator() (hb_codepoint_t start,
+                       hb_codepoint_t end,
+                       int delta) {
+        segcount++;
+      }
+    } counter;
+
+    to_ranges (+it, counter);
+    return counter.segcount;
   }
 
-      prev_cp = _.first;
-    }
-
-    // There must be a final entry with end_code == 0xFFFF.
-    if (it.len () == 0 || prev_cp != 0xFFFF)
-    {
-      HBUINT16 finalcode;
-      finalcode = 0xFFFF;
-      if (unlikely (!c->copy<HBUINT16> (finalcode))) return nullptr;
-    }
-
-    return startCode;
-  }
 
   template<typename Iterator,
 	   hb_requires (hb_is_iterator (Iterator))>
-  HBINT16* serialize_idDelta_array (hb_serialize_context_t *c,
+  bool serialize_start_end_delta_arrays (hb_serialize_context_t *c,
                                          Iterator it,
-				    HBUINT16 *endCode,
-				    HBUINT16 *startCode,
-				    unsigned segcount)
+                                         int segcount)
   {
-    unsigned i = 0;
-    hb_codepoint_t last_gid = 0, start_gid = 0, last_cp = 0xFFFF;
-    bool use_delta = true;
+    struct Writer {
+      hb_serialize_context_t *serializer_;
+      HBUINT16* end_code_;
+      HBUINT16* start_code_;
+      HBINT16* id_delta_;
+      int index_;
 
-    HBINT16 *idDelta = c->start_embed<HBINT16> ();
-    if ((char *)idDelta - (char *)startCode != (int) segcount * (int) HBINT16::static_size)
-      return nullptr;
-
-    for (const auto& _ : +it)
-    {
-      if (_.first == startCode[i])
-      {
-	use_delta = true;
-	start_gid = _.second;
+      Writer(hb_serialize_context_t *serializer)
+          : serializer_(serializer),
+            end_code_(nullptr),
+            start_code_(nullptr),
+            id_delta_(nullptr),
+            index_ (0) {}
+      void operator() (hb_codepoint_t start,
+                       hb_codepoint_t end,
+                       int delta) {
+        start_code_[index_] = start;
+        end_code_[index_] = end;
+        id_delta_[index_] = delta;
+        index_++;
       }
-      else if (_.second != last_gid + 1) use_delta = false;
+    } writer(c);
 
-      if (_.first == endCode[i])
-      {
-	HBINT16 delta;
-	if (use_delta) delta = (int)start_gid - (int)startCode[i];
-	else delta = 0;
-	c->copy<HBINT16> (delta);
+    writer.end_code_ = c->allocate_size<HBUINT16> (HBUINT16::static_size * segcount);
+    c->allocate_size<HBUINT16> (2); // padding
+    writer.start_code_ = c->allocate_size<HBUINT16> (HBUINT16::static_size * segcount);
+    writer.id_delta_ = c->allocate_size<HBINT16> (HBINT16::static_size * segcount);
 
-	i++;
-      }
+    if (unlikely (!writer.end_code_ || !writer.start_code_ || !writer.id_delta_)) return false;
 
-      last_gid = _.second;
-      last_cp = _.first;
-    }
-
-    if (it.len () == 0 || last_cp != 0xFFFF)
-    {
-      HBINT16 delta;
-      delta = 1;
-      if (unlikely (!c->copy<HBINT16> (delta))) return nullptr;
-    }
-
-    return idDelta;
+    to_ranges (+it, writer);
+    return true;
   }
 
   template<typename Iterator,
@@ -257,22 +305,14 @@ struct CmapSubtableFormat4
     if (unlikely (!c->extend_min (this))) return;
     this->format = 4;
 
-    //serialize endCode[]
-    HBUINT16 *endCode = serialize_endcode_array (c, format4_iter);
-    if (unlikely (!endCode)) return;
+    //serialize endCode[], startCode[], idDelta[]
+    HBUINT16* endCode = c->start_embed<HBUINT16> ();
+    unsigned segcount = serialize_find_segcount (format4_iter);
+    if (unlikely (!serialize_start_end_delta_arrays (c, format4_iter, segcount)))
+      return;
 
-    unsigned segcount = (c->length () - min_size) / HBUINT16::static_size;
-
-    // 2 bytes of padding.
-    if (unlikely (!c->allocate_size<HBUINT16> (HBUINT16::static_size))) return; // 2 bytes of padding.
-
-   // serialize startCode[]
-    HBUINT16 *startCode = serialize_startcode_array (c, format4_iter);
-    if (unlikely (!startCode)) return;
-
-    //serialize idDelta[]
-    HBINT16 *idDelta = serialize_idDelta_array (c, format4_iter, endCode, startCode, segcount);
-    if (unlikely (!idDelta)) return;
+    HBUINT16 *startCode = endCode + segcount + 1;
+    HBINT16 *idDelta = ((HBINT16*)startCode) + segcount;
 
     HBUINT16 *idRangeOffset = serialize_rangeoffset_glyid (c, format4_iter, endCode, startCode, idDelta, segcount);
     if (unlikely (!c->check_success (idRangeOffset))) return;