Add rindex to ActiveSupport::Multibyte::Chars.

Signed-off-by: Jeremy Kemper <jeremy@bitsweat.net>

activesupport/lib/active_support/multibyte/chars.rb

@@ -210,6 +210,19 @@ module ActiveSupport #:nodoc:
         index ? (self.class.u_unpack(@wrapped_string.slice(0...index)).size) : nil
       end
 
+      # Returns the position _needle_ in the string, counting in
+      # codepoints, searching backward from _offset_ or the end of the
+      # string. Returns +nil+ if _needle_ isn't found.
+      #
+      # Example:
+      #   'Café périferôl'.mb_chars.rindex('é') #=> 5
+      #   'Café périferôl'.mb_chars.rindex(/\w/u) #=> 13
+      def rindex(needle, offset=nil)
+        offset ||= length
+        index = @wrapped_string.rindex(needle, offset)
+        index ? (self.class.u_unpack(@wrapped_string.slice(0...index)).size) : nil
+      end
+
       # Like <tt>String#[]=</tt>, except instead of byte offsets you specify character offsets.
       #
       # Example:

activesupport/test/multibyte_chars_test.rb

@@ -234,6 +234,13 @@ class MultibyteCharsUTF8BehaviourTest < Test::Unit::TestCase
     assert_equal 3, @chars.index('わ')
   end
 
+  def test_rindex_should_return_character_offset
+    assert_nil @chars.rindex('u')
+    assert_equal 1, @chars.rindex('に')
+    assert_equal 6, 'Café périferôl'.mb_chars.rindex('é')
+    assert_equal 12, 'Café périferôl'.mb_chars.rindex(/\w/u)
+  end
+
   def test_indexed_insert_should_take_character_offsets
     @chars[2] = 'a'
     assert_equal 'こにaわ', @chars