translate_offset

# File activesupport/lib/active_support/multibyte/chars.rb, line 626
        def translate_offset(byte_offset) #:nodoc:
          return nil if byte_offset.nil?
          return 0   if @wrapped_string == ''
          chunk = @wrapped_string[0..byte_offset]
          begin
            begin
              chunk.unpack('U*').length - 1
            rescue ArgumentError => e
              chunk = @wrapped_string[0..(byte_offset+=1)]
              # Stop retrying at the end of the string
              raise e unless byte_offset < chunk.length 
              # We damaged a character, retry
              retry
            end
          # Catch the ArgumentError so we can throw our own
          rescue ArgumentError 
            raise EncodingError, 'malformed UTF-8 character'
          end
        end