module RSpec
  module Core
    module Let

      module ExampleGroupMethods
        # Generates a method whose return value is memoized after the first
        # call. Useful for reducing duplication between examples that assign
        # values to the same local variable.
        #
        # @note `let` _can_ enhance readability when used sparingly (1,2, or
        #   maybe 3 declarations) in any given example group, but that can
        #   quickly degrade with overuse. YMMV.
        #
        # @note `let` uses an `||=` conditional that has the potential to
        #   behave in surprising ways in examples that spawn separate threads,
        #   though we have yet to see this in practice. You've been warned.
        #
        # @example
        #
        #   describe Thing do
        #     let(:thing) { Thing.new }
        #
        #     it "does something" do
        #       # first invocation, executes block, memoizes and returns result
        #       thing.do_something
        #
        #       # second invocation, returns the memoized value
        #       thing.should be_something
        #     end
        #   end
        def let(name, &block)
          define_method(name) do
            __memoized.fetch(name) {|k| __memoized[k] = instance_eval(&block) }
          end
        end

        # Just like `let`, except the block is invoked by an implicit `before`
        # hook. This serves a dual purpose of setting up state and providing a
        # memoized reference to that state.
        #
        # @example
        #
        #   class Thing
        #     def self.count
        #       @count ||= 0
        #     end
        #
        #     def self.count=(val)
        #       @count += val
        #     end
        #
        #     def self.reset_count
        #       @count = 0
        #     end
        #
        #     def initialize
        #       self.class.count += 1
        #     end
        #   end
        #
        #   describe Thing do
        #     after(:each) { Thing.reset_count }
        #
        #     context "using let" do
        #       let(:thing) { Thing.new }
        #
        #       it "is not invoked implicitly" do
        #         Thing.count.should eq(0)
        #       end
        #
        #       it "can be invoked explicitly" do
        #         thing
        #         Thing.count.should eq(1)
        #       end
        #     end
        #
        #     context "using let!" do
        #       let!(:thing) { Thing.new }
        #
        #       it "is invoked implicitly" do
        #         Thing.count.should eq(1)
        #       end
        #
        #       it "returns memoized version on first invocation" do
        #         thing
        #         Thing.count.should eq(1)
        #       end
        #     end
        #   end
        def let!(name, &block)
          let(name, &block)
          before { __send__(name) }
        end
      end

      # @private
      module ExampleMethods
        # @private
        def __memoized
          @__memoized ||= {}
        end
      end

      def self.included(mod)
        mod.extend ExampleGroupMethods
        mod.__send__ :include, ExampleMethods
      end
    end
  end
end