u16tou8

# File lib/net/imap.rb, line 1225
    def self.u16tou8(s)
      len = s.length
      if len < 2
        return ""
      end
      buf = ""
      i = 0
      while i < len
        c = s[i] << 8 | s[i + 1]
        i += 2
        if c == 0xfeff
          next
        elsif c < 0x0080
          buf.concat(c)
        elsif c < 0x0800
          b2 = c & 0x003f
          b1 = c >> 6
          buf.concat(b1 | 0xc0)
          buf.concat(b2 | 0x80)
        elsif c >= 0xdc00 && c < 0xe000
          raise DataFormatError, "invalid surrogate detected"
        elsif c >= 0xd800 && c < 0xdc00
          if i + 2 > len
            raise DataFormatError, "invalid surrogate detected"
          end
          low = s[i] << 8 | s[i + 1]
          i += 2
          if low < 0xdc00 || low > 0xdfff
            raise DataFormatError, "invalid surrogate detected"
          end
          c = (((c & 0x03ff)) << 10 | (low & 0x03ff)) + 0x10000
          b4 = c & 0x003f
          b3 = (c >> 6) & 0x003f
          b2 = (c >> 12) & 0x003f
          b1 = c >> 18;
          buf.concat(b1 | 0xf0)
          buf.concat(b2 | 0x80)
          buf.concat(b3 | 0x80)
          buf.concat(b4 | 0x80)
        else # 0x0800-0xffff
          b3 = c & 0x003f
          b2 = (c >> 6) & 0x003f
          b1 = c >> 12
          buf.concat(b1 | 0xe0)
          buf.concat(b2 | 0x80)
          buf.concat(b3 | 0x80)
        end
      end
      return buf
    end