Making WebSocket unmasking fast in ruby

unmasking_benchmark.rb

module EventMachine
  module WebSocket
    class MaskedString < String
      def read_mask
        raise "Too short" if bytesize < 4 # TODO - change
        @masking_key = String.new(self[0..3])
      end

      def slice_mask
        slice!(0, 4)
      end

      def getbyte(index)
        masked_char = super(index + 4)
        masked_char ? masked_char ^ @masking_key.getbyte(index % 4) : nil
      end

      def getbytes(start_index, count)
        data = ''
        count.times do |i|
          data << getbyte(start_index + i)
        end
        data
      end
    end
  end
end

require 'benchmark'

n = 1000

# Use a 4 byte mask and a 1K string
string = rand.to_s[0..3] + 'a' * 1024

Benchmark.bm do |x|
  x.report("MaskedString:") {
    n.times {
      string = EventMachine::WebSocket::MaskedString.new(string)
      string.read_mask
      string.getbytes(0, 1024)
    }
  }
end

Discussion: https://github.com/igrigorik/em-websocket/issues#issue/40

Check dan's gists, he's getting some wins, but another thread that might be useful: http://markmail.org/message/owwxbh7v67zjo4k3#query:ruby%20string%20and%20xor+page:1+mid:4vibjcxf4r5c2prs+state:results

Or even better: http://blog.50projects.com/2010/12/ruby-string-xor-optimizations.html