module ActionController
  module ConditionalGet
    extend ActiveSupport::Concern

    include RackDelegation
    include Head

    included do
      class_attribute :etaggers
      self.etaggers = []
    end

    module ClassMethods
      # Allows you to consider additional controller-wide information when generating an etag.
      # For example, if you serve pages tailored depending on who's logged in at the moment, you
      # may want to add the current user id to be part of the etag to prevent authorized displaying
      # of cached pages.
      #
      #   class InvoicesController < ApplicationController
      #     etag { current_user.try :id }
      #
      #     def show
      #       # Etag will differ even for the same invoice when it's viewed by a different current_user
      #       @invoice = Invoice.find(params[:id])
      #       fresh_when(@invoice)
      #     end
      #   end
      def etag(&etagger)
        self.etaggers += [etagger]
      end
    end

    # Sets the etag, +last_modified+, or both on the response and renders a
    # <tt>304 Not Modified</tt> response if the request is already fresh.
    #
    # === Parameters:
    #
    # * <tt>:etag</tt>.
    # * <tt>:last_modified</tt>.
    # * <tt>:public</tt> By default the Cache-Control header is private, set this to
    #   +true+ if you want your application to be cachable by other devices (proxy caches).
    #
    # === Example:
    #
    #   def show
    #     @article = Article.find(params[:id])
    #     fresh_when(etag: @article, last_modified: @article.created_at, public: true)
    #   end
    #
    # This will render the show template if the request isn't sending a matching etag or
    # If-Modified-Since header and just a <tt>304 Not Modified</tt> response if there's a match.
    #
    # You can also just pass a record where +last_modified+ will be set by calling
    # +updated_at+ and the etag by passing the object itself.
    #
    #   def show
    #     @article = Article.find(params[:id])
    #     fresh_when(@article)
    #   end
    #
    # When passing a record, you can still set whether the public header:
    #
    #   def show
    #     @article = Article.find(params[:id])
    #     fresh_when(@article, public: true)
    #   end
    def fresh_when(record_or_options, additional_options = {})
      if record_or_options.is_a? Hash
        options = record_or_options
        options.assert_valid_keys(:etag, :last_modified, :public)
      else
        record  = record_or_options
        options = { etag: record, last_modified: record.try(:updated_at) }.merge!(additional_options)
      end

      response.etag          = combine_etags(options[:etag]) if options[:etag]
      response.last_modified = options[:last_modified]       if options[:last_modified]
      response.cache_control[:public] = true                 if options[:public]

      head :not_modified if request.fresh?(response)
    end

    # Sets the +etag+ and/or +last_modified+ on the response and checks it against
    # the client request. If the request doesn't match the options provided, the
    # request is considered stale and should be generated from scratch. Otherwise,
    # it's fresh and we don't need to generate anything and a reply of <tt>304 Not Modified</tt> is sent.
    #
    # === Parameters:
    #
    # * <tt>:etag</tt>.
    # * <tt>:last_modified</tt>.
    # * <tt>:public</tt> By default the Cache-Control header is private, set this to
    #   +true+ if you want your application to be cachable by other devices (proxy caches).
    #
    # === Example:
    #
    #   def show
    #     @article = Article.find(params[:id])
    #
    #     if stale?(etag: @article, last_modified: @article.created_at)
    #       @statistics = @article.really_expensive_call
    #       respond_to do |format|
    #         # all the supported formats
    #       end
    #     end
    #   end
    #
    # You can also just pass a record where +last_modified+ will be set by calling
    # updated_at and the etag by passing the object itself.
    #
    #   def show
    #     @article = Article.find(params[:id])
    #
    #     if stale?(@article)
    #       @statistics = @article.really_expensive_call
    #       respond_to do |format|
    #         # all the supported formats
    #       end
    #     end
    #   end
    #
    # When passing a record, you can still set whether the public header:
    #
    #   def show
    #     @article = Article.find(params[:id])
    #
    #     if stale?(@article, public: true)
    #       @statistics = @article.really_expensive_call
    #       respond_to do |format|
    #         # all the supported formats
    #       end
    #     end
    #   end
    def stale?(record_or_options, additional_options = {})
      fresh_when(record_or_options, additional_options)
      !request.fresh?(response)
    end

    # Sets a HTTP 1.1 Cache-Control header. Defaults to issuing a +private+
    # instruction, so that intermediate caches must not cache the response.
    #
    #   expires_in 20.minutes
    #   expires_in 3.hours, public: true
    #   expires_in 3.hours, public: true, must_revalidate: true
    #
    # This method will overwrite an existing Cache-Control header.
    # See http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html for more possibilities.
    #
    # The method will also ensure a HTTP Date header for client compatibility.
    def expires_in(seconds, options = {})
      response.cache_control.merge!(
        :max_age         => seconds,
        :public          => options.delete(:public),
        :must_revalidate => options.delete(:must_revalidate)
      )
      options.delete(:private)

      response.cache_control[:extras] = options.map {|k,v| "#{k}=#{v}"}
      response.date = Time.now unless response.date?
    end

    # Sets a HTTP 1.1 Cache-Control header of <tt>no-cache</tt> so no caching should
    # occur by the browser or intermediate caches (like caching proxy servers).
    def expires_now
      response.cache_control.replace(:no_cache => true)
    end

    private
      def combine_etags(etag)
        [ etag, *etaggers.map { |etagger| instance_exec(&etagger) }.compact ]
      end
  end
end