让我们从头做一个 MUD 吧！

--- Timer System
HeartOfWorld = {
  rate = 1,     -- beat times per second
  members = {}, -- all hearts in here
  last_beat_time = 0
}

function HeartOfWorld:Add(heart)
   ......
end

function HeartOfWorld:Del(heart_or_idx)
  ......
end

function HeartOfWorld:Tick()
  ......
      -- Make hearts beating
    for idx, obj in pairs(self.members) do
      if obj.HeartBeat ~= nil and type(obj.HeartBeat) == 'function' then
        obj:HeartBeat(now)
      end
    end
  ......
end

--- A TCP server can be set a recieving handler.
TcpServer = {
  num2client = {}, -- 通过玩家 ID 找到客户端对象的索引表
  client2num = {}, -- 通过客户端对象找到玩家 ID 的索引表
  clients = {},    -- 客户端列表
  conn_count = 0   -- 当前连接总数
}

function TcpServer.Start(self, bind_addr, handler)
  ......

  -- 游戏主循环 --
  while true do
    -- Processing network events
    ......

    -- Processing heartbeat timer
    HeartOfWorld:Tick()
  end
end

function TcpServer.SendTo(self, client_id, message, no_ret)
  ......
end

function TcpServer.CloseClient(self, client_id)
  ......
end

...
  print("Starting TCP server ...")
  TcpServer:Start({}, handler)
...

-- Sessions pool --
SessionPool = {}
...
--- A command system which you can set a command to it.
CommandSystem = {
  cmd_tab = {}, -- 存放所有命令的容器

  ......

  ProcessCommand = function(self, user_id, command_line)
    .......
    -- 命令行方式解析输入的文字
    string.gsub(command_line, "[^ ]+", function(w) table.insert(cmds, w) end)
    ......
    local cmd_fun = self.cmd_tab[cmd] -- 查找命令
    ......
    THIS_PLAYER = SessionPool[user_id] -- Shotcut: this_player
    ......
      local ret = cmd_fun(cmds)  -- 运行命令
      Reply(PROMPT, true)
      return ret
    ......
  end
}

--- Broadcast system
Channel = {
  members = {}
}

......

function Channel:Join(user_id, member)
...
end

function Channel:Leave(user_id)
...
end

function Channel:Say(message, ...)
  for user_id, member in pairs(self.members) do
    local ignore = false
    for i, sender in ipairs { ... } do
      if member == sender then
        ignore = true
      end
    end

    if ignore == false then
      TcpServer:SendTo(user_id, message)
    end
  end
end

--- Save/Load user data
require("MudOS/serialize")
local md5 = require("MudOS/md5")
......

UserData = {
  user_name = nil,
  pass_token = nil,

  .......

  Load = function(user_name, password)
    ......
  end,

  Save = function(self)
    .......
    local save_obj_name = 'player_' .. self.user_name
    io.output(save_file)
    save(save_obj_name, self)
    io.close(save_file)
    return true
  end,

  .......
}

-- Load GameLib level code
print("Start to load GameLib ...")
require("MudLib/index")

-- Start up network procedule
...
  print("Starting TCP server ...")
  TcpServer:Start({}, handler)
...

...
print("正在构建空间系统 ...")
require("MudLib/space")

print("正在构建房间系统 ...")
require("MudLib/room")
require("MudLib/map")

print("正在构建角色系统 ...")
require("MudLib/char")

-- TODO 构建“道具系统”

print("正在构建战斗系统 ...")
require("MudLib/combat")

print("正在构建命令系统 ...")
dofile(MUD_LIB_PATH .. "cmds.lua")
...

---代表一个物理空间物体
--@param environment 所处环境
--@param content 内容
SpaceObject = {
  environment = nil, 
  content = {}, 

  New = function(self, value)
    ...
  end,

  --查找本身包含的内容物
  --@param #table key 内容物的属性名,如果是nil则对比整个内容物体
  --@param #table value 要查找的属性值或者内容物本身
  --@param #function fun是找到后的处理函数，形式fun(pos, con_obj)
  --@return #table 返回fun()的返回值（仅限第一个返回值）数值，或者是找到的对象数组
  Search = function(self, key, value, fun)
    ...
  end,

  Leave = function(self)
    ...
  end,

  Put = function(self, env)
    ...
  end,

  Dispose = function(self)
    ...
  end
}

World = SpaceObject:New()  -- 所有物理空间存放的位置
World.channel = Channel:New() -- 构建一个世界频道

Room = {
  title = "虚空",
  desc = "这里一片白茫茫",
  exits = {}, --east="xxx", west="yyy", ...
  channel = World.channel
}

function Room:New(value)
  local ret = NewInstance(self, value, SpaceObject)
  ret:Put(World)
  ret.channel =  Channel:New()
  return ret
end

function Room:ToStr()
  local output = [[
--%s--  
%s
这里的出口：
%s
这里有：
%s]]

  ...
  
  return string.format(output, self.title, self.desc, exits_str, content_str)
end

BornPoint = Room:New({
  title = "出生点",
  desc = "这里是一片空地，周围站着很多刚注册的新手玩家。",
  exits = {
    east = "NewbiePlaza",
    west = "SmallRoad"
  }
})

NewbiePlaza = Room:New({
  title = "新手广场",
  desc = "光秃秃的黄土地上，有几棵小树。",
  exits = {
    west = "BornPoint"
  }
})

SmallRoad = Room:New({
  title = "小路",
  desc = "这条小路荒草蔓延。似乎是通往外界的唯一道路。",
  exits = {
    east = "BornPoint"
  }
})

CommandList.kill = function(cmds)
  local target = nil
  local target_id = cmds[2] -- cmds[1]是指令本身，cmds[2]才是参数
  if target_id == nil then
    Reply("你怒气冲冲的瞪着空气，不知道要攻击谁。")
    return
  else
    if target_id == THIS_PLAYER.id then
      Reply("你狠狠用左脚踢了一下自己的右脚，发现这个行为很傻，于是就停止了。")
      return
    end
    local targets = THIS_PLAYER.environment:Search('id', target_id)
    if #targets == 0 then
      Reply(string.format("没有%s这个东西", target_id))
      return
    elseif targets[1].hp ~= nil and targets[1].hp > 0 then
      target = targets[1]
    else
      Reply("你不能攻击一个死物。")
      return
    end
  end

  if target ~= nil then
    table.insert(THIS_PLAYER.fright_list, target)
    Reply(string.format("你对着%s大喝一声：“納命来！”", target.name))

    --反击
    table.insert(target.fright_list, THIS_PLAYER)
    Reply(string.format("%s对你一瞪眼，一跺脚，狠狠道：“竟敢在太岁头上动土？”", target.name))
    if target.user_id ~= nil then
      target:Reply(string.format("%s向你发起了攻击！", THIS_PLAYER.name))
    end
  end
end

if a_skill ~= nil then
    -- 有招攻无招
    if d_skill == nil then
      damage = double_power
    else
      --这种复杂判断其实应该用哈系表查询，但是if写法更容易表达内在含义
      --tiger>monkey>crane>tiger
      if a_skill == d_skill then
        damage = normal_power
      elseif a_skill == "tiger" then
        if d_skill == "monkey" then
          damage = double_power
        elseif d_skill == "crane" then
          damage = lease_power
        end
      elseif a_skill == "monkey" then
        if d_skill == "tiger" then
          damage = lease_power
        elseif d_skill == "crane" then
          damage = double_power
        end
      elseif a_skill == "crane" then
        if d_skill == "monkey" then
          damage = lease_power
        elseif d_skill == "tiger" then
          damage = double_power
        end
      end

    end
  end