add function to generate unique username if already exist

lib/diskuy_web/auth/google_auth.ex

 defmodule DiskuyWeb.Auth.GoogleAuth do
   alias DiskuyWeb.Auth.Guardian
   alias Diskuy.Account
+  alias Diskuy.Utility.Utility
+
   def get_access_token(code, conn) do
     case ElixirAuthGoogle.get_token(code, conn) do
       {:ok, profile} ->
@@ -16,9 +18,31 @@ defmodule DiskuyWeb.Auth.GoogleAuth do
     case Account.get_by_email(email) do
       {:ok, user} ->
         Guardian.create_token(user)
+      {:error, :not_found} ->
+        create_user(username, email, picture)
+    end
+  end
+
+  defp create_user(username, email, picture) do
+    case Account.get_by_username(username) do
+      {:ok, _user} ->
+        new_username = create_username(username)
+        {:ok, user} = Account.create_user(%{username: new_username, email: email, picture: picture})
+        Guardian.create_token(user)
       {:error, :not_found} ->
         {:ok, user} = Account.create_user(%{username: username, email: email, picture: picture})
         Guardian.create_token(user)
     end
   end
+
+  defp create_username(username) do
+    new_username = "#{username}-#{Utility.random_string(6)}"
+    case Account.get_by_username(new_username) do
+      {:ok, _user} ->
+        create_username(username)
+      {:error, :not_found} ->
+        new_username
+    end
+  end
+
 end