Only allow positive powers of two for playoff_teams_amount

app/models/tournament.rb

@@ -10,6 +10,8 @@ class Tournament < ApplicationRecord
   validates :name, presence: true
   validates :code, presence: true, uniqueness: true
 
+  validate :playoff_teams_amount_is_positive_power_of_two
+
   alias_attribute :owner, :user
 
   after_initialize :generate_code
@@ -24,4 +26,11 @@ class Tournament < ApplicationRecord
       break if errors['code'].blank?
     end
   end
+
+  def playoff_teams_amount_is_positive_power_of_two
+    return if (Utils.po2?(playoff_teams_amount) && playoff_teams_amount.positive?) || playoff_teams_amount.zero?
+
+    errors.add(:playoff_teams_amount_isnt_positive_power_of_two,
+               'playoff_teams_amount needs to be a positive power of two')
+  end
 end

db/migrate/0000_create_schema.rb

@@ -51,7 +51,7 @@ class CreateSchema < ActiveRecord::Migration[5.2]
       t.string :code, null: false, index: { unique: true }
       t.string :description
       t.boolean :public, default: true
-      t.integer :playoff_teams_amount
+      t.integer :playoff_teams_amount, default: 0
       t.integer :instant_finalists_amount, default: 0
       t.integer :intermediate_round_participants_amount, default: 0
 

spec/controllers/tournaments_controller_spec.rb

@@ -194,6 +194,29 @@ RSpec.describe TournamentsController, type: :controller do
             expect(@group_stage_tournament.playoff_teams_amount)
               .to eq(create_group_tournament_data[:playoff_teams_amount])
           end
+
+          context 'playoff_teams_amount unacceptable' do
+            it 'is not a power of two' do
+              post :create, params: create_group_tournament_data.merge(playoff_teams_amount: 18)
+              expect(response).to have_http_status(:unprocessable_entity)
+              expect(deserialize_response(response).values.first.first)
+                .to eq('playoff_teams_amount needs to be a positive power of two')
+            end
+
+            it 'isn\'t positive' do
+              post :create, params: create_group_tournament_data.merge(playoff_teams_amount: -16)
+              expect(response).to have_http_status(:unprocessable_entity)
+              expect(deserialize_response(response).values.first.first)
+                .to eq('playoff_teams_amount needs to be a positive power of two')
+            end
+
+            it 'isn\'t positive nor a power of two' do
+              post :create, params: create_group_tournament_data.merge(playoff_teams_amount: -42)
+              expect(response).to have_http_status(:unprocessable_entity)
+              expect(deserialize_response(response).values.first.first)
+                .to eq('playoff_teams_amount needs to be a positive power of two')
+            end
+          end
         end
 
         it 'renders a JSON response with the new tournament' do
@@ -234,12 +257,19 @@ RSpec.describe TournamentsController, type: :controller do
       end
 
       context 'with invalid parameters' do
+        shared_examples_for 'renders unprocessable entity response' do
           it 'renders an unprocessable entity response' do
-          put :create, params: { teams: [1, 2, 3] }
             expect(response).to have_http_status(:unprocessable_entity)
           end
         end
 
+        before do
+          put :create, params: context
+        end
+
+        context ''
+      end
+
       context 'with empty team objects' do
         it 'renders an unprocessable entity response' do
           data = create_group_tournament_data