module ActionDispatch
  module Http
    class Headers < ::Hash
      @@env_cache = Hash.new { |h,k| h[k] = "HTTP_#{k.upcase.gsub(/-/, '_')}" }

      def initialize(*args)

        if args.size == 1 && args[0].is_a?(Hash)
          super()
          update(args[0])
        else
          super
        end
      end

      def [](header_name)
        super env_name(header_name)
      end

      def fetch(header_name, default=nil, &block)
        super env_name(header_name), default, &block
      end

      private
        # Converts a HTTP header name to an environment variable name if it is
        # not contained within the headers hash.
        def env_name(header_name)
          include?(header_name) ? header_name : @@env_cache[header_name]
        end
    end
  end
end