resolves asciidoctor#491 support reversed option on ordered list

- implement pred method for String
- implement pred method for RomanNumeral
- call pred method instead of next if reversed option is set on list
- switch to integer value below first char or roman numeral in sequence

lib/asciidoctor-pdf/converter.rb

@@ -657,7 +657,9 @@ def convert_olist node
     else
       '1'
     end
-    if (skip = (node.attr 'start', 1).to_i - 1) > 0
+    start = (node.attr 'start', nil, false) || ((node.option? 'reversed') ? node.items.size : 1)
+    # TODO support negative start values
+    if (skip = start.to_i - 1) > 0
       skip.times { list_number = list_number.next  }
     end
     @list_numbers << list_number
@@ -732,7 +734,8 @@ def convert_outline_list_item node, complex = false
         end
       end
     when :olist
-      @list_numbers << (index = @list_numbers.pop).next
+      dir = (node.parent.option? 'reversed') ? :pred : :next
+      @list_numbers << ((index = @list_numbers.pop).public_send dir)
       marker = %(#{index}.)
     else
       warn %(asciidoctor: WARNING: unknown list type #{list_type.inspect})

lib/asciidoctor-pdf/core_ext.rb

@@ -1,2 +1,3 @@
 require_relative 'core_ext/array'
 require_relative 'core_ext/numeric'
+require_relative 'core_ext/string'

lib/asciidoctor-pdf/core_ext/string.rb

@@ -0,0 +1,11 @@
+class String
+  def pred
+    begin
+      # integers
+      %(#{(Integer self) - 1})
+    rescue ::ArgumentError
+      # chars (lower alpha, upper alpha, lower greek)
+      ([65, 97, 945].include? ord) ? '0' : ([ord - 1].pack 'U*')
+    end
+  end unless respond_to? :pred
+end

lib/asciidoctor-pdf/roman_numeral.rb

@@ -62,7 +62,10 @@ def to_s
   end
 
   def to_r
-    roman = RomanNumeral.int_to_roman @integer_value
+    if (int = @integer_value) <= 0
+      return int.to_s
+    end
+    roman = RomanNumeral.int_to_roman int
     @letter_case == :lower ? roman.downcase : roman
   end
 
@@ -79,6 +82,10 @@ def next!
     self
   end
 
+  def pred
+    RomanNumeral.new @integer_value - 1, @letter_case
+  end
+
   def self.int_to_roman value
     result = ''
     BaseDigits.keys.reverse.each do |ival|