#!/usr/bin/lua

Set = {}
local mt = {} -- 集合的元表
 
-- 根据参数列表中的值创建一个新的集合
function Set.new(l)
    local set = {}
     setmetatable(set, mt)
    for _, v in pairs(l) do set[v] = true end
     return set
end
 
-- 并集操作
function Set.union(a, b)
	 if getmetatable(a) ~= mt or getmetatable(b) ~= mt then
        error("metatable error.")
     end

    local retSet = Set.new{} -- 此处相当于Set.new({})
    for v in pairs(a) do retSet[v] = true end
    for v in pairs(b) do retSet[v] = true end
    return retSet
end
 
-- 交集操作
function Set.intersection(a, b)
    local retSet = Set.new{}
    for v in pairs(a) do retSet[v] = b[v] end
    return retSet
end
 
-- 打印集合的操作
function Set.toString(set)
     local tb = {}
     for e in pairs(set) do
          tb[#tb + 1] = e
     end
     return "{" .. table.concat(tb, ", ") .. "}"
end
 
function Set.print(s)
     print(Set.toString(s))
end

local set1 = Set.new({10, 20, 30})
local set2 = Set.new({1, 2})
print(getmetatable(set1))
print(getmetatable(set2))
assert(getmetatable(set1) == getmetatable(set2))

mt.__add = Set.union

local set1 = Set.new({10, 20, 30})
local set2 = Set.new({1, 2})
local set3 = set1 + set2
Set.print(set3)

mt.__tostring = Set.toString

print(set3)

do
function Set.new(l)
    local set = {}
    setmetatable(set, mt)
    for _, v in pairs(l) do set[v] = true end
    mt.__metatable = "You cannot get the metatable" -- 设置完我的元表以后，不让其他人再设置
    return set
end

local tb = Set.new({1, 2})
print(tb)

print(getmetatable(tb))
setmetatable(tb, {})
end