require 'faraday'
require 'forwardable'
require 'digest/sha1'

module FaradayMiddleware
  # Public: Caches GET responses and pulls subsequent ones from the cache.
  class Caching < Faraday::Middleware
    attr_reader :cache
    # Internal: List of status codes that can be cached:
    # * 200 - 'OK'
    # * 203 - 'Non-Authoritative Information'
    # * 300 - 'Multiple Choices'
    # * 301 - 'Moved Permanently'
    # * 302 - 'Found'
    # * 404 - 'Not Found'
    # * 410 - 'Gone'
    CACHEABLE_STATUS_CODES = [200, 203, 300, 301, 302, 404, 410]

    extend Forwardable
    def_delegators :'Faraday::Utils', :parse_query, :build_query

    # Public: initialize the middleware.
    #
    # cache   - An object that responds to read and write (default: nil).
    # options - An options Hash (default: {}):
    #           :ignore_params - String name or Array names of query params
    #                            that should be ignored when forming the cache
    #                            key (default: []).
    #           :write_options - Hash of settings that should be passed as the third
    #                            options parameter to the cache's #write method. If not
    #                            specified, no options parameter will be passed.
    #           :full_key      - Boolean - use full URL (url.host + url.request_uri) as cache key
    #
    # Yields if no cache is given. The block should return a cache object.
    def initialize(app, cache = nil, options = {})
      super(app)
      options, cache = cache, nil if cache.is_a? Hash and block_given?
      @cache = cache || yield
      @options = options
    end

    def call(env)
      if :get == env[:method]
        if env[:parallel_manager]
          # callback mode
          cache_on_complete(env)
        else
          # synchronous mode
          key = cache_key(env)
          unless response = cache.read(key) and response
            response = @app.call(env)
            store_response_in_cache(key, response)
          end
          finalize_response(response, env)
        end
      else
        @app.call(env)
      end
    end

    def cache_key(env)
      url = env[:url].dup
      if url.query && params_to_ignore.any?
        params = parse_query url.query
        params.reject! {|k,| params_to_ignore.include? k }
        url.query = params.any? ? build_query(params) : nil
      end
      url.normalize!
      Digest::SHA1.hexdigest(full_key? ? url.host + url.request_uri : url.request_uri)
    end

    def params_to_ignore
      @params_to_ignore ||= Array(@options[:ignore_params]).map { |p| p.to_s }
    end

    def full_key?
      @full_key ||= @options[:full_key]
    end

    def cache_on_complete(env)
      key = cache_key(env)
      if cached_response = cache.read(key)
        finalize_response(cached_response, env)
      else
        # response.status is nil at this point, any checks need to be done inside on_complete block
        @app.call(env).on_complete do |response_env|
          store_response_in_cache(key, response_env.response)
          response_env
        end
      end
    end

    def store_response_in_cache(key, response)
      return unless CACHEABLE_STATUS_CODES.include?(response.status)

      if @options[:write_options]
        cache.write(key, response, @options[:write_options])
      else
        cache.write(key, response)
      end
    end

    def finalize_response(response, env)
      response = response.dup if response.frozen?
      env[:response] = response
      unless env[:response_headers]
        env.update response.env
        # FIXME: omg hax
        response.instance_variable_set('@env', env)
      end
      response
    end
  end
end