Add two new parameters to tournament

instant_finalists_amount is the amount of teams that instantly are in
the finals after the group stage ends.

intermediate_round_participants_amount is the amount of teams that have
a chance to advance after group stage but must play a relegation game to
actually do so.

Both of these values need to match, so that
instant_finalists_amount + (intermediate_round_participants_amount / 2)
is a power of 2. Or to be more precise, the power of two that is saved
in playoff_teams_amount of the tournament.

app/serializers/tournament_serializer.rb

@@ -1,7 +1,8 @@
 # frozen_string_literal: true
 
 class TournamentSerializer < SimpleTournamentSerializer
-  attributes :description
+  attributes :description, :playoff_teams_amount,
+             :instant_finalists_amount, :intermediate_round_participants_amount
   has_many :stages
   has_many :teams
 

app/services/tournament_service.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class TournamentService
+  class << self
+    def calculate_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

db/migrate/0000_create_schema.rb

@@ -52,6 +52,8 @@ class CreateSchema < ActiveRecord::Migration[5.2]
       t.string :description
       t.boolean :public, default: true
       t.integer :playoff_teams_amount
+      t.integer :instant_finalists_amount, default: 0
+      t.integer :intermediate_round_participants_amount, default: 0
 
       # relation to owner
       t.belongs_to :user, index: true, null: false, foreign_key: { on_delete: :cascade }