module ActionDispatch
  module Http
    class Headers
      include Enumerable

      @@env_cache = Hash.new { |h,k| h[k] = "HTTP_#{k.upcase.gsub(/-/, '_')}" }

      def initialize(*args)
        @headers = args.first || {}
      end

      def [](header_name)
        @headers[env_name(header_name)]
      end

      def []=(k,v); @headers[k] = v; end
      def key?(k); @headers.key? k; end
      alias :include? :key?

      def fetch(header_name, *args, &block)
        @headers.fetch env_name(header_name), *args, &block
      end

      def each(&block)
        @headers.each(&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)
          @headers.include?(header_name) ? header_name : @@env_cache[header_name]
        end
    end
  end
end