Fix maxp table

Original approach was right but there was an error in field sizes. This
lead to shift of the following field. One of the fields is
maxStackElements which defines stack size for font program. FreeFont
(also used in Ghostscript) is very strict about stack size. Incorrect
maxStackElements value lead to stack overflow in FreeFont. Consequently,
Ghostscript couldn't parse the font and completely discarded it.

lib/ttfunk/table/maxp.rb

@@ -5,6 +5,9 @@
 module TTFunk
   class Table
     class Maxp < Table
+      DEFAULT_MAX_COMPONENT_DEPTH = 1
+      MAX_V1_TABLE_LENGTH = 32
+
       attr_reader :version
       attr_reader :num_glyphs
       attr_reader :max_points
@@ -21,21 +24,149 @@ class Maxp < Table
       attr_reader :max_component_elements
       attr_reader :max_component_depth
 
-      def self.encode(maxp, mapping)
-        num_glyphs = mapping.length
-        raw = maxp.raw
-        raw[4, 2] = [num_glyphs].pack('n')
-        raw
+      class << self
+        def encode(maxp, new2old_glyph)
+          ''.b.tap do |table|
+            num_glyphs = new2old_glyph.length
+            table << [maxp.version, num_glyphs].pack('Nn')
+
+            if maxp.version == 0x10000
+              stats = stats_for(
+                maxp, glyphs_from_ids(maxp, new2old_glyph.values)
+              )
+
+              table << [
+                stats[:max_points],
+                stats[:max_contours],
+                stats[:max_component_points],
+                stats[:max_component_contours],
+                # these all come from the fpgm and cvt tables, which
+                # we don't support at the moment
+                maxp.max_zones,
+                maxp.max_twilight_points,
+                maxp.max_storage,
+                maxp.max_function_defs,
+                maxp.max_instruction_defs,
+                maxp.max_stack_elements,
+                stats[:max_size_of_instructions],
+                stats[:max_component_elements],
+                stats[:max_component_depth]
+              ].pack('n*')
+            end
+          end
+        end
+
+        private
+
+        def glyphs_from_ids(maxp, glyph_ids)
+          glyph_ids.each_with_object([]) do |glyph_id, ret|
+            if (glyph = maxp.file.glyph_outlines.for(glyph_id))
+              ret << glyph
+            end
+          end
+        end
+
+        def stats_for(maxp, glyphs)
+          stats_for_simple(maxp, glyphs)
+            .merge(stats_for_compound(maxp, glyphs))
+            .transform_values { |agg| agg.value_or(0) }
+        end
+
+        def stats_for_simple(_maxp, glyphs)
+          max_component_elements = Max.new
+          max_points = Max.new
+          max_contours = Max.new
+          max_size_of_instructions = Max.new
+
+          glyphs.each do |glyph|
+            if glyph.compound?
+              max_component_elements << glyph.glyph_ids.size
+            else
+              max_points << glyph.end_point_of_last_contour
+              max_contours << glyph.number_of_contours
+              max_size_of_instructions << glyph.instruction_length
+            end
+          end
+
+          {
+            max_component_elements: max_component_elements,
+            max_points: max_points,
+            max_contours: max_contours,
+            max_size_of_instructions: max_size_of_instructions
+          }
+        end
+
+        def stats_for_compound(maxp, glyphs)
+          max_component_points = Max.new
+          max_component_depth = Max.new
+          max_component_contours = Max.new
+
+          glyphs.each do |glyph|
+            next unless glyph.compound?
+
+            stats = totals_for_compound(maxp, [glyph], 0)
+            max_component_points << stats[:total_points]
+            max_component_depth << stats[:max_depth]
+            max_component_contours << stats[:total_contours]
+          end
+
+          {
+            max_component_points: max_component_points,
+            max_component_depth: max_component_depth,
+            max_component_contours: max_component_contours
+          }
+        end
+
+        def totals_for_compound(maxp, glyphs, depth)
+          total_points = Sum.new
+          total_contours = Sum.new
+          max_depth = Max.new(depth)
+
+          glyphs.each do |glyph|
+            if glyph.compound?
+              stats = totals_for_compound(
+                maxp, glyphs_from_ids(maxp, glyph.glyph_ids), depth + 1
+              )
+
+              total_points << stats[:total_points]
+              total_contours << stats[:total_contours]
+              max_depth << stats[:max_depth]
+            else
+              stats = stats_for_simple(maxp, [glyph])
+              total_points << stats[:max_points]
+              total_contours << stats[:max_contours]
+            end
+          end
+
+          {
+            total_points: total_points,
+            total_contours: total_contours,
+            max_depth: max_depth
+          }
+        end
       end
 
       private
 
       def parse!
-        @version, @num_glyphs, @max_points, @max_contours,
-          @max_component_points, @max_component_contours, @max_zones,
-          @max_twilight_points, @max_storage, @max_function_defs,
-          @max_instruction_defs, @max_stack_elements, @max_size_of_instructions,
-          @max_component_elements, @max_component_depth = read(length, 'Nn*')
+        @version, @num_glyphs = read(6, 'Nn')
+
+        if @version == 0x10000
+          @max_points, @max_contours, @max_component_points,
+            @max_component_contours, @max_zones, @max_twilight_points,
+            @max_storage, @max_function_defs, @max_instruction_defs,
+            @max_stack_elements, @max_size_of_instructions,
+            @max_component_elements = read(24, 'n*')
+
+          # a number of fonts omit these last two bytes for some reason,
+          # so we have to supply a default here to prevent nils
+          @max_component_depth =
+            if length == MAX_V1_TABLE_LENGTH
+              read(2, 'n').first
+            else
+              DEFAULT_MAX_COMPONENT_DEPTH
+            end
+        end
       end
     end
   end

spec/ttfunk/ttf_encoder_spec.rb

@@ -65,7 +65,7 @@
 
       # verified via the Font-Validator tool at:
       # https://github.com/HinTak/Font-Validator
-      expect(checksum).to eq(0xEE3A9625)
+      expect(checksum).to eq(0xEEAE9DCF)
     end
   end
 end