require 'digest/md5'

module Rack
  # Automatically sets the ETag header on all String bodies
  class ETag
    def initialize(app)
      @app = app
    end

    def call(env)
      status, headers, body = @app.call(env)

      if !headers.has_key?('ETag')
        parts = []
        body.each { |part| parts << part.to_s }
        headers['ETag'] = %("#{Digest::MD5.hexdigest(parts.join(""))}")
        [status, headers, parts]
      else
        [status, headers, body]
      end
    end
  end
end