class ApiVersioning
  class Middleware
    def initialize(app)
      @app = app
    end

    def call(env)
      if version_number = extract_version_number(env)
        ApiVersioning.current_version_number = version_number.to_i
        Rails.application.eager_load! unless Rails.application.config.cache_classes
        ClassDescendantsBuilder.build ApiBeta::ApplicationController,
                                      ApiVersioning.max_version_number
      end
      @app.call env
    end

    private

    def extract_version_number(env)
      env['HTTP_ACCEPT'] && env['HTTP_ACCEPT'][/v([0-9\.]+)\+json/, 1]
    end
  end

  class_attribute :current_version_number

  def self.instance
    @instance || raise('call `.setup` first')
  end

  def self.setup(context, &block)
    @instance = new(context, &block)
  end

  def self.min_version_number
    instance.min_version_number
  end

  def self.max_version_number
    instance.max_version_number
  end

  def initialize(context, &block)
    @context, @definitions = context, {}
    instance_eval &block
  end

  def version(number, &block)
    @definitions[number] = block || Proc.new {}
    @context.scope module: "v#{number}", constraints: ->(*) { ApiVersioning.current_version_number == number } do
      number.downto(min_version_number) do |i|
        @context.instance_eval &@definitions[i]
      end
    end
  end

  def min_version_number
    @definitions.keys.min
  end

  def max_version_number
    @definitions.keys.max
  end
end