Implement Cast#auto_pay_mana

lib/magic/actions/cast.rb

@@ -98,6 +98,11 @@ def pay_mana(payment)
         self
       end
 
+      def auto_pay_mana
+        mana_cost.auto_pay(player: player)
+        self
+      end
+
       def pay_kicker(payment)
         kicker_cost.pay(player:, payment:)
       end

lib/magic/costs/mana.rb

@@ -62,11 +62,11 @@ def pay(player:, payment:)
         pay_colors(payment.slice(*Magic::Mana::COLORS))
       end
 
-      def auto_pay(player)
+      def auto_pay(player:)
         raise CannotPay unless can_pay?(player)
 
         pay_colors(color_costs)
-        pay_generic(generic: cost[:generic]) if cost[:generic]
+        auto_pay_generic_costs(player) if cost[:generic]
       end
 
       def finalize!(player)
@@ -132,6 +132,15 @@ def pay_generic(payment)
         @payments[:generic].merge!(payment) { |key, old_value, new_value| old_value + new_value }
       end
 
+
+      def auto_pay_generic_costs(player)
+        available_mana = player.mana_pool.flat_map do |color, amount|
+          [color] * amount
+        end
+
+        pay_generic(available_mana.take(cost[:generic]).tally)
+      end
+
       def pay_colors(color_payments)
         color_payments.each_with_object(balance) do |(color, amount), remaining_balance|
           remaining_balance[color] -= amount

spec/cards/titanic_growth_spec.rb

@@ -7,7 +7,7 @@
   it "gets +4/+4 until end of turn" do
     p1.add_mana(green: 2)
     p1.cast(card: Card("Titanic Growth")) do
-      _1.pay_mana(green: 1, generic: { green: 1 })
+      _1.auto_pay_mana
       _1.targeting(wood_elves)
     end