diff --git a/bot.py b/bot.py
index cbe28c3..38afbef 100644
--- a/bot.py
+++ b/bot.py
@@ -7,172 +7,88 @@ from telepot.namedtuple import KeyboardButton, ReplyKeyboardMarkup, ReplyKeyboar
 from pprint import pprint
 # kCOJ API
 import access
+from interface import kuser
 # configurations
 import config
 
-users = {}
-class kuser:
-    def __init__(self, un='', pw='', st=0):
-        self.username = un
-        self.password = pw
-        self.status = st
-        self.api = access.kuser_api()
-
-    def test_login(self, chat_id):
-        self.api.login(self.username, self.password)
-        if self.api.check_online() == True:
-            return True
-        else:
-            self.status = 1
-            bot.sendMessage(chat_id, "哇...登入失敗，讓我們重新開始一次", reply_markup=ReplyKeyboardRemove())
-            time.sleep(0.6)
-            bot.sendMessage(chat_id, "請輸入您的學號：", reply_markup=ReplyKeyboardRemove())
-            return False
-    def display_main(self, chat_id):
-        self.status = 3
-        q_list = self.api.list_questions()
-        q_available = "📝<i>可繳交的作業</i>\n\n"
-        q_unavailable = "📝<i>沒有可繳交的作業哦！</i>\n"
-        if q_list == {}:
-            q_str = q_unavailable
-        else:
-            q_str = q_available
-            for key in q_list.keys():
-                if q_list[key][1] == '期限未到':
-                    q_str += "📗<b>" + key + "</b> (到 " + q_list[key][0] + ")\n [" + q_list[key][2] + "] /question_" + key + "\n\n"
-            if q_str == q_available:
-                q_str = q_unavailable
-        bot.sendMessage(chat_id, "💁 <b>" + self.username + "</b> /logout\n"
-                                 "➖➖➖➖➖\n" + q_str + "➖➖➖➖➖\n"
-                                 "你今天寫扣了嗎？", parse_mode='HTML',
-                                 reply_markup=ReplyKeyboardMarkup(keyboard=[
-                                     [KeyboardButton(text="查看題庫📝"), KeyboardButton(text="重新整理🔃")],
-                                     [KeyboardButton(text="登出帳號🚪"), KeyboardButton(text="修改密碼💱"), KeyboardButton(text="提供幫助📚")]
-                                 ]))
-    def display_questions(self, chat_id):
-        q_list = self.api.list_questions()
-        q_str = ''
-        for key in q_list.keys():
-            if q_list[key][1] == '期限未到':
-                q_str += "📗<b>" + key + "</b> (到 " + q_list[key][0] + ")\n [" + q_list[key][2] + "] /question_" + key + "\n\n"
-            else:
-                q_str += "📕<b>" + key + "</b> (到 " + q_list[key][0] + ")\n [" + q_list[key][2] + "] /question_" + key + "\n\n"
-        bot.sendMessage(chat_id, "💁 <b>" + self.username + "</b> /logout\n"
-                                 "➖➖➖➖➖\n"
-                                 "📝<i>所有作業</i>\n\n" + q_str + 
-                                 "➖➖➖➖➖\n"
-                                 "你今天寫扣了嗎？", parse_mode='HTML',
-                                 reply_markup=ReplyKeyboardMarkup(keyboard=[
-                                     [KeyboardButton(text="回主畫面🏠"), KeyboardButton(text="重新載入🔃")],
-                                     [KeyboardButton(text="登出帳號🚪"), KeyboardButton(text="修改密碼💱"), KeyboardButton(text="提供幫助📚")]
-                                 ]))
-    def display_question(self, chat_id, number):
-        content = self.api.show_question(number)
-        q = self.api.list_questions()[number]
-        q_str = "📗" if q[1] == '期限未到' else "📕"
-        q_str += "<b>" + number + "</b> (到 " + q[0] + ")"
-        bot.sendMessage(chat_id, q_str + "\n<code>" + content + "</code>",
-                         parse_mode='HTML',
-                         reply_markup=ReplyKeyboardMarkup(keyboard=[
-                                     [KeyboardButton(text="回主畫面🏠"), KeyboardButton(text="回到題庫📝")],
-                                     [KeyboardButton(text="上傳解答📮"), KeyboardButton(text="查看結果☑️"), KeyboardButton(text="通過名單🌐")] if q[1] == '期限未到' else 
-                                     [KeyboardButton(text="查看結果☑️"), KeyboardButton(text="通過名單🌐")],
-                                     [KeyboardButton(text="登出帳號🚪"), KeyboardButton(text="修改密碼💱"), KeyboardButton(text="提供幫助📚")]
-                         ]))
-    def help_you(self, chat_id):
-        bot.sendMessage(chat_id, "這裡是 kC Online Judge Bot！\n"
-                                 "可以簡稱我為 kCOJ Bot，目前定居於 `@kcoj_bot`\n"
-                                 "作用是讓大家可以方便的透過我使用郭老程設課的 OJ\n"
-                                 "操作很簡單（？）我想大家應該都不會有問題吧～\n\n"
-                                 "不過還是有些需要注意的地方：\n"
-                                 "1. 學號與密碼將以「明文」方式儲存。\n"
-                                 "2. 反正郭老的 OJ 也是「明文」存您的帳號密碼。\n"
-                                 "3. 我以我的人格擔保，不會使用您提供的資訊侵害您的權利", parse_mode="Markdown",
-                                 reply_markup=ReplyKeyboardMarkup(keyboard=[
-                                     [KeyboardButton(text="回主畫面🏠")]
-                                 ]))
-        time.sleep(0.6)
-        bot.sendMessage(chat_id, "\n原始碼被託管於 GitHub，網址如下：\n"
-                                 "https://github.com/PinLin/kcoj_bot")
-
-def split_cmd(text):
-    if text[0] != '/':
-        return [text]
-    else:
-        cmd = text.replace('_', ' ').strip().lower().split(' ')
-        return cmd
-
-
 def on_chat(msg):
-    pprint(msg)
     content_type, chat_type, chat_id = telepot.glance(msg)
-    print(content_type, chat_type, chat_id)
+    pprint(msg) # for debug
+    print(content_type, chat_type, chat_id) # for debug
 
-    me = kuser()
+    # create a user object
+    user = kuser(chat_id, bot)
     if str(chat_id) in users:
-        me = users[str(chat_id)]
+        user = users[str(chat_id)]
     else:
-        users[str(chat_id)] = me
+        users[str(chat_id)] = user
 
     if content_type == 'text':
-        command = split_cmd(msg['text'])
+        # pre-treat the command
+        command = [msg['text']]
+        if msg['text'][0] == '/':
+            command = msg['text'].replace('_', ' ').lower().split(' ')
+
+        # first-time user
+        if user.status == '第一次用':
+            user.new_user()
+
+        # press password
+        elif user.status == '輸入學號':
+            user.press_password(msg['text'])
+
+        # login
+        elif user.status == '輸入密碼':
+            user.login_kcoj(msg['text'])
+
+        elif user.status == '舊的密碼':    
+            if user.check_online() == True:
+                user.press_newpassword(msg['text'])
+
+        elif user.status == '修改密碼':
+            if user.check_online() == True:
+                user.change_password(msg['text'])
 
-        if me.status == 1:
-            me.status = 2
-            me.username = msg['text']
-            bot.sendMessage(chat_id, "請輸入您的密碼：")
-        elif me.status == 2:
-            me.password = msg['text']
-            bot.sendMessage(chat_id, "登入中...")
-            if me.test_login(chat_id) == True:
-                me.display_main(chat_id)
-        elif me.status == 4:
-            me.password = msg['text']
-            bot.sendMessage(chat_id, "修改成功" if me.api.change_password(me.password) == True else "修改失敗")
-            if me.test_login(chat_id) == True:
-                me.display_main(chat_id)
         elif command[0] == '/start' or command[0] == '重新整理🔃' or command[0] == '回主畫面🏠':
-            if me.status == 0:
-                me.status = 1
-                bot.sendMessage(chat_id, "是初次見面的朋友呢，設定一下吧！", reply_markup=ReplyKeyboardRemove())
-                time.sleep(0.6)
-                bot.sendMessage(chat_id, "請輸入您的學號：", reply_markup=ReplyKeyboardRemove())
-            else:
-                if me.test_login(chat_id) == True:
-                    me.display_main(chat_id)
+            if user.check_online() == True:
+                user.display_main()
+
         elif command[0] == '/question' or command[0] == '查看題庫📝' or command[0] == '回到題庫📝' or command[0] == '重新載入🔃':
-            if me.test_login(chat_id) == True:
+            if user.check_online() == True:
                 if len(command) > 1:
-                    me.display_question(chat_id, command[1])
+                    user.display_question(command[1])
                 else:
-                    me.display_questions(chat_id)
+                    user.display_questions()
+
         elif command[0] == '/help' or command[0] == '提供幫助📚':
-            if me.test_login(chat_id) == True:
-                me.help_you(chat_id)
+            if user.check_online() == True:
+                user.help_you()
+
         elif command[0] == '/password' or command[0] == '修改密碼💱':
-            if me.test_login(chat_id) == True:
-                me.status = 4
-                bot.sendMessage(chat_id, "請輸入要設定的新密碼：", reply_markup=ReplyKeyboardRemove())
+            if user.check_online() == True:
+                user.press_oldpassword()
+
         elif command[0] == '/logout' or command[0] == '登出帳號🚪':
-            bot.sendMessage(chat_id, "您現在已經是登出的狀態。", reply_markup=ReplyKeyboardRemove())
-            me = kuser()
-            me.status = 1
-            users[str(chat_id)] = me
-            time.sleep(0.6)
-            bot.sendMessage(chat_id, "請輸入您的學號：", reply_markup=ReplyKeyboardRemove())
+            user = kuser(chat_id, bot)
+            users[str(chat_id)] = user
+            user.logout_system()
+
+    elif False:
+        # receive a document
+        pass
 
     else:
         bot.sendMessage(chat_id, "我不是來看這些的。")
 
+users = {}
 bot = telepot.Bot(config.TOKEN)
 MessageLoop(bot, on_chat).run_as_thread()
 
 # for debug
 def main():
     pass
-    # over 20MB will stop this bot!
-    # bot.download_file('BQADBQADHQADPAc5VEZQejGkM9C4Ag', '../wa.txt')
-main()
 
-while True:
-    time.sleep(10)
\ No newline at end of file
+if __name__ == '__main__':
+    main()
+    while True:
+        time.sleep(10)
\ No newline at end of file
diff --git a/interface.py b/interface.py
new file mode 100644
index 0000000..b2a873d
--- /dev/null
+++ b/interface.py
@@ -0,0 +1,148 @@
+#! /usr/bin/env python3
+
+# necessary modules
+import requests, telepot
+from telepot.loop import MessageLoop
+from telepot.namedtuple import KeyboardButton, ReplyKeyboardMarkup, ReplyKeyboardRemove
+from pprint import pprint
+# kCOJ API
+import access
+# configurations
+import config
+
+class kuser:
+    def __init__(self, chat_id, bot):
+        self.chat_id = chat_id
+        self.bot = bot
+        self.username = ''
+        self.password = ''
+        self.status = '第一次用'
+        self.api = access.kuser_api()
+
+    def new_user(self):
+        self.bot.sendMessage(self.chat_id, "是初次見面的朋友呢，設定一下吧！", reply_markup=ReplyKeyboardRemove())
+        self.press_username()
+    
+    def press_username(self):
+        self.status = '輸入學號'
+        self.bot.sendMessage(self.chat_id, "請輸入您的學號：", reply_markup=ReplyKeyboardRemove())
+
+    def press_password(self, text):
+        self.status = '輸入密碼'
+        self.username = text
+        self.bot.sendMessage(self.chat_id, "輸入完可刪除訊息以策安全！\n請輸入您的密碼：", reply_markup=ReplyKeyboardRemove())
+
+    def press_oldpassword(self):
+        self.status = '舊的密碼'
+        self.bot.sendMessage(self.chat_id, "輸入錯誤將取消操作\n"
+                                           "請輸入要原本的舊密碼：", reply_markup=ReplyKeyboardRemove())
+
+    def press_newpassword(self, text):
+        self.status = '修改密碼'
+        if text != self.password:
+            self.display_main()
+        else:
+            self.bot.sendMessage(self.chat_id, "使用此功能請務必小心！\n"
+                                               "請輸入要設定的新密碼：", reply_markup=ReplyKeyboardRemove())
+        
+    def change_password(self, text):
+        self.status = '正常使用'
+        self.password = text
+        self.bot.sendMessage(self.chat_id, "修改成功" if self.api.change_password(self.password) == True else "修改失敗",
+            reply_markup=ReplyKeyboardMarkup(keyboard=[
+                [KeyboardButton(text="回主畫面🏠")]
+            ]))
+
+    def login_kcoj(self, text):
+        self.status = '正常使用'
+        self.password = text
+        self.bot.sendMessage(self.chat_id, "登入中...", reply_markup=ReplyKeyboardRemove())
+        if self.check_online() == True:
+            self.display_main()
+
+    def fail_login(self):
+        self.bot.sendMessage(self.chat_id, "哇...登入失敗，讓我們重新開始一次", reply_markup=ReplyKeyboardRemove())
+        self.press_username()
+
+    def check_online(self):
+        self.api.login_kcoj(self.username, self.password)
+        if self.api.check_online() == True:
+            return True
+        else:
+            self.fail_login()
+            return False
+
+    def logout_system(self):
+        self.bot.sendMessage(self.chat_id, "您現在已經是登出的狀態。", reply_markup=ReplyKeyboardRemove())
+        self.press_username()
+
+    def display_main(self):
+        self.status = '正常使用'
+        q_dict = self.api.list_questions()
+        q_available = "📝<i>可繳交的作業</i>\n\n"
+        q_unavailable = "📝<i>沒有可繳交的作業哦！</i>\n"
+        if q_dict == {}:
+            q_str = q_unavailable
+        else:
+            q_str = q_available
+            for key in q_dict.keys():
+                if q_dict[key][1] == '期限未到':
+                    q_str += "📗<b>" + key + "</b> (到 " + q_dict[key][0] + ")\n [" + q_dict[key][2] + "] /question_" + key + "\n\n"
+            if q_str == q_available:
+                q_str = q_unavailable
+        self.bot.sendMessage(self.chat_id, "💁 <b>" + self.username + "</b> /logout\n"
+                                           "➖➖➖➖➖\n" + q_str + "➖➖➖➖➖\n"
+                                           "你今天寫扣了嗎？",
+                                           parse_mode='HTML',
+                                           reply_markup=ReplyKeyboardMarkup(keyboard=[
+                                               [KeyboardButton(text="查看題庫📝"), KeyboardButton(text="重新整理🔃")],
+                                               [KeyboardButton(text="登出帳號🚪"), KeyboardButton(text="修改密碼💱"), KeyboardButton(text="提供幫助📚")]
+                                           ]))
+
+    def display_questions(self):
+        self.status = '正常使用'
+        q_dict = self.api.list_questions()
+        q_str = ''
+        for key in q_dict.keys():
+            q_str += "📗" if q_dict[key][1] == '期限未到' else "📕"
+            q_str += "<b>" + key + "</b> (到 " + q_dict[key][0] + ")\n [" + q_dict[key][2] + "] /question_" + key + "\n\n"
+        self.bot.sendMessage(self.chat_id, "💁 <b>" + self.username + "</b> /logout\n"
+                                           "➖➖➖➖➖\n📝<i>所有作業</i>\n\n" + q_str + "➖➖➖➖➖\n"
+                                           "你今天寫扣了嗎？", 
+                                           parse_mode='HTML',
+                                           reply_markup=ReplyKeyboardMarkup(keyboard=[
+                                               [KeyboardButton(text="回主畫面🏠"), KeyboardButton(text="重新載入🔃")],
+                                               [KeyboardButton(text="登出帳號🚪"), KeyboardButton(text="修改密碼💱"), KeyboardButton(text="提供幫助📚")]
+                                           ]))
+
+    def display_question(self, number):
+        self = '正常使用'
+        content = self.api.show_question(number)
+        q = self.api.list_questions()[number]
+        q_str = "📗" if q[1] == '期限未到' else "📕"
+        q_str += "<b>" + number + "</b> (到 " + q[0] + ")"
+        self.bot.sendMessage(self.chat_id, q_str + "\n<code>" + content + "</code>",
+            parse_mode='HTML',
+            reply_markup=ReplyKeyboardMarkup(keyboard=[
+                [KeyboardButton(text="回主畫面🏠"), KeyboardButton(text="回到題庫📝")],
+                [KeyboardButton(text="上傳解答📮"), KeyboardButton(text="查看結果☑️"), KeyboardButton(text="通過名單🌐")] if q[1] == '期限未到' else 
+                [KeyboardButton(text="查看結果☑️"), KeyboardButton(text="通過名單🌐")],
+                [KeyboardButton(text="登出帳號🚪"), KeyboardButton(text="修改密碼💱"), KeyboardButton(text="提供幫助📚")]
+            ]))
+
+    def help_you(self):
+        self.status = '正常使用'
+        self.bot.sendMessage(self.chat_id, "這裡是 kC Online Judge Bot！\n"
+                                           "可以簡稱我為 kCOJ Bot，目前定居於 `@kcoj_bot`\n"
+                                           "作用是讓大家可以方便的透過我使用郭老程設課的 Online Judge\n"
+                                           "操作很簡單（？）我想大家應該都不會有問題吧～\n\n"
+                                           "不過還是有些需要注意的地方：\n"
+                                           "1. 學號與密碼將以「明文」方式儲存在記憶體裡，不會儲存在我的硬碟中。\n"
+                                           "2. 反正郭老的 Online Judge 也是「明文」存您的帳號密碼。\n"
+                                           "3. 我以我的人格擔保，不會使用您提供的資訊侵害您的權利。\n\n"
+                                           "然後，附個[郭老 Online Judge 傳送門](" + config.URL + ")", parse_mode="Markdown",
+                                           reply_markup=ReplyKeyboardMarkup(keyboard=[
+                                               [KeyboardButton(text="回主畫面🏠")]
+                                           ]))
+        self.bot.sendMessage(self.chat_id, "\n原始碼被託管於 GitHub，網址如下：\n"
+                                           "https://github.com/PinLin/kcoj_bot")
\ No newline at end of file