Added server-side validation for game setup form

public/css/main.css

@@ -41,6 +41,13 @@ input#setup-submit {
   width: 80px;
 }
 
+#setup-form p {
+  color: red;
+  font-size: 0.9em;
+  font-weight: 300;
+  margin: 5px;
+}
+
 input#cell-btn {
   border: 1px solid black;
   border-radius: 3px;

tic_tac_toe_controller.rb

@@ -10,11 +10,16 @@ class TicTacToeController < Sinatra::Base
   end
 
   post '/setup' do
+    redirect to('/invalid_setup') if params[:player_mark] == nil || params[:opponent] == nil
     session[:mark] = params[:player_mark]
     session[:opponent] = params[:opponent]
     redirect to('/game')
   end
 
+  get '/invalid_setup' do
+    erb :index_with_validation_messages
+  end
+
   get '/game' do
     @board = TicTacToe::Board.new(cells: session[:moves])
     session[:moves] = @board.to_array

views/index_with_validation_messages.erb

@@ -0,0 +1,29 @@
+<div>
+  <form id="setup-form" action="/setup" method="POST">
+
+    <div class="form-section">
+      Player 1, select your mark:
+      <input type="radio" name="player_mark" value="X" />X
+      <input type="radio" name="player_mark" value="O" />O
+    </div>
+
+    <% if params[:player_mark] == nil %>
+      <p>*Please select a mark in order to continue.</p>
+    <% end %>
+
+    <div class="form-section">
+      Would you like to play against the computer?:
+      <input type="radio" name="opponent" value="yes" />Yes
+      <input type="radio" name="opponent" value="no" />No
+    </div>
+
+    <% if params[:opponent] == nil %>
+      <p>*Please indicate if you would like to play against the computer in order to continue.</p>
+    <% end %>
+
+    <div class="form-section">
+      <input id="setup-submit" type ="submit" value="Submit" />
+    </div>
+  </form>
+</div>
+