Make generate_code_verifier a class method

README.md

@@ -119,7 +119,8 @@ get('/authorize') do
   user_id = request.session['user_id']
   # User needs to take care of generating the code_verifier and storing it in
   # the session.
-  request.session['code_verifier'] ||= authorizer.generate_code_verifier
+  request.session['code_verifier'] ||= Google::Auth::WebUserAuthorizer.generate_code_verifier
+  authorizer.code_verifier = request.session['code_verifier']
   credentials = authorizer.get_credentials(user_id, request)
   if credentials.nil?
     redirect authorizer.get_authorization_url(login_hint: user_id, request: request)

lib/googleauth/user_authorizer.rb

@@ -260,11 +260,9 @@ def code_verifier= new_code_verifier
 
       # Generate the code verifier needed to be sent while fetching
       # authorization URL.
-      def code_verifier
-        @code_verifier ||= begin
-          random_number = rand 32..96
-          SecureRandom.alphanumeric random_number
-        end
+      def self.generate_code_verifier
+        random_number = rand 32..96
+        SecureRandom.alphanumeric random_number
       end
 
       private

spec/googleauth/web_user_authorizer_spec.rb

@@ -90,7 +90,7 @@
     end
 
     it "should include code_challenge and code_challenge_method" do
-      authorizer.generate_code_verifier
+      authorizer.code_verifier = Google::Auth::WebUserAuthorizer.generate_code_verifier
       url = authorizer.get_authorization_url(request: request)
       expect(url).to match(/code_challenge=/)
       expect(url).to match(/code_challenge_method=S256/)