Implemented __len

local map = {}
local function mod_pairs(_t)
    local t = {}
    for k, v in pairs(_t) do
        t[#t + 1] = k
    end
    local i = 0
    return function()
        i = i + 1
        return t[i], _t[t[i]]
    end
end
local copy = function(_t)
    local t = {}
    for k, v in pairs(_t) do
        t[k] = type(v) == "table" and copy(v) or v
    end
    return t
end
local sm = function(_t, _mt)
    map[_t] = setmetatable(copy(_t), _mt)
    for k, v in mod_pairs(_t) do
        _t[k] = nil
    end
    for i = 1, _mt.__len or #_t do
        _t[i] = {}
    end
    return setmetatable(_t, {
        __index = map[_t];
        __newindex = function(self, key, value)
            map[_t][key] = value
        end;
    })
end

-- Testing
local lentest = sm({1, 3, 4, 3, 43, 34, 3}, {__len = 0}) -- setting the __len to 0
print(#lentest) --> 0
lentest[5] = 7
print(lentest[5]) --> 7
print(#lentest) --> 0