tidy_bytes

# File activesupport/lib/active_support/multibyte/unicode.rb, line 227
      def tidy_bytes(string, force = false)
        if force
          return string.unpack("C*").map do |b|
            tidy_byte(b)
          end.flatten.compact.pack("C*").unpack("U*").pack("U*")
        end

        bytes = string.unpack("C*")
        conts_expected = 0
        last_lead = 0

        bytes.each_index do |i|

          byte          = bytes[i]
          is_cont       = byte > 127 && byte < 192
          is_lead       = byte > 191 && byte < 245
          is_unused     = byte > 240
          is_restricted = byte > 244

          # Impossible or highly unlikely byte? Clean it.
          if is_unused || is_restricted
            bytes[i] = tidy_byte(byte)
          elsif is_cont
            # Not expecting contination byte? Clean up. Otherwise, now expect one less.
            conts_expected == 0 ? bytes[i] = tidy_byte(byte) : conts_expected -= 1
          else
            if conts_expected > 0
              # Expected continuation, but got ASCII or leading? Clean backwards up to
              # the leading byte.
              (1..(i - last_lead)).each {|j| bytes[i - j] = tidy_byte(bytes[i - j])}
              conts_expected = 0
            end
            if is_lead
              # Final byte is leading? Clean it.
              if i == bytes.length - 1
                bytes[i] = tidy_byte(bytes.last)
              else
                # Valid leading byte? Expect continuations determined by position of
                # first zero bit, with max of 3.
                conts_expected = byte < 224 ? 1 : byte < 240 ? 2 : 3
                last_lead = i
              end
            end
          end
        end
        bytes.empty? ? "" : bytes.flatten.compact.pack("C*").unpack("U*").pack("U*")
      end