instance method strenc

# File activesupport/lib/active_support/json/backends/okjson.rb, line 451
    def strenc(s)
      t = StringIO.new
      t.putc(?")
      r = 0

      # In ruby >= 1.9, s[r] is a codepoint, not a byte.
      rubydoesenc = s.class.method_defined?(:encoding)

      while r < s.length
        case s[r]
        when ?"  then t.print('\\"')
        when ?\\ then t.print('\\\\')
        when ?\b then t.print('\\b')
        when ?\f then t.print('\\f')
        when ?\n then t.print('\\n')
        when ?\r then t.print('\\r')
        when ?\t then t.print('\\t')
        else
          c = s[r]
          case true
          when rubydoesenc
            begin
              c.ord # will raise an error if c is invalid UTF-8
              t.write(c)
            rescue
              t.write(Ustrerr)
            end
          when Spc <= c && c <= ?~
            t.putc(c)
          else
            n = ucharcopy(t, s, r) # ensure valid UTF-8 output
            r += n - 1 # r is incremented below
          end
        end
        r += 1
      end
      t.putc(?")
      t.string
    end