Assign default values on Group Stage creation

default values are assigned to instant_finalists_amount and
intermediate_round_participants_amount depending on playoff_teams_amount and group amount

app/interactors/add_group_stage_to_tournament.rb

@@ -10,6 +10,9 @@ class AddGroupStageToTournament
     begin
       group_stage = GroupStageService.generate_group_stage(groups)
       tournament.stages = [group_stage]
+      tournament.instant_finalists_amount, tournament.intermediate_round_participants_amount =
+        TournamentService.calculate_default_amount_of_teams_advancing(tournament.playoff_teams_amount,
+                                                                      group_stage.groups.size)
       context.object_to_save = tournament
     rescue StandardError
       context.fail!

app/services/tournament_service.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class TournamentService
+  class << self
+    def calculate_default_amount_of_teams_advancing(playoff_teams_amount, amount_of_groups)
+      # the amount of whole places that advance in a group (e. g. all 1rst places of every group instantly go through)
+      instant_finalists_amount = (playoff_teams_amount.floor / amount_of_groups.floor) * amount_of_groups.floor
+      # the amount of teams that still need to play an intermediate round before advancing to playoffs
+      intermediate_round_participants_amount = (playoff_teams_amount - instant_finalists_amount) * 2
+
+      [instant_finalists_amount, intermediate_round_participants_amount]
+    end
+  end
+end

spec/interactors/add_group_stage_to_tournament_interactor_spec.rb

@@ -14,6 +14,7 @@ RSpec.describe AddGroupStageToTournament, type: :interactor do
     @group_stage_tournament = create(:group_stage_tournament, stage_count: 0, group_count: 0)
     @group_stage = create(:group_stage)
     @groups = Hash[1 => create_list(:team, 4), 2 => create_list(:team, 4)].values
+    @tournament_service_defaults = [78_345, 2_387]
   end
 
   context 'GroupStageService mocked' do
@@ -24,13 +25,28 @@ RSpec.describe AddGroupStageToTournament, type: :interactor do
     end
 
     context 'empty tournament' do
+      before do
+        allow(class_double('TournamentService').as_stubbed_const(transfer_nested_constants: true))
+          .to receive(:calculate_default_amount_of_teams_advancing)
+          .with(@empty_tournament.playoff_teams_amount, @group_stage.groups.size)
+          .and_return(@tournament_service_defaults)
+      end
+
       it 'succeeds' do
         expect(empty_tournament_context).to be_a_success
       end
 
       it 'adds group stage to the tournament' do
-        test = empty_tournament_context.tournament.stages.first
-        expect(test).to eq(@group_stage)
+        expect(empty_tournament_context.tournament.stages.first).to eq(@group_stage)
+      end
+
+      it 'sets default for instant_finalists_amount' do
+        expect(empty_tournament_context.tournament.instant_finalists_amount).to eq(@tournament_service_defaults.first)
+      end
+
+      it 'sets default for intermediate_round_participants_amount' do
+        expect(empty_tournament_context.tournament.intermediate_round_participants_amount)
+          .to eq(@tournament_service_defaults.second)
       end
     end
   end