local Expression = {}
local _expFunc = {}
local _expEnvMT = {}
local _expEnv = setmetatable({}, _expEnvMT)
local pcall = pcall
local setfenv = setfenv

local _expContextStack = {}
function Expression.Execute(expStr, expContext)
    local expFunc = _expFunc[expStr]
    if not expFunc then
        expFunc = loadstring(expStr)
        _expFunc[expStr] = expFunc
    end
    if expFunc then
        _expContextStack[#_expContextStack + 1] = _expEnvMT.__index
        _expEnvMT.__index = expContext
        setfenv(expFunc, _expEnv)
        -- simply just support 3 return values
        local suc, r1, r2, r3, r4 = pcall(expFunc)
        _expEnvMT.__index = _expContextStack[#_expContextStack]
        _expContextStack[#_expContextStack] = nil
        if suc then
            if r4 ~= nil then
                error('too many return values, should less than 3')
            else
                return r1, r2, r3
            end
        else
            error(r1)
        end
    else
        error('load expression failed')
    end
end

function Expression.Release(expStr)
    if expStr then
        _expFunc[expStr] = nil
    else
        _expFunc = {}
    end
end

return Expression