Lua均值方差：一次遍历的双统计

-- 错误：全样本存表再两遍算，内存与开销双高
local all = {}
for _, v in ipairs(battleHits) do
    all[#all + 1] = v
end
local m, s = avg(all), variance(all)

local function stats(samples)
    local n, mean, m2 = 0, 0, 0
    for _, v in ipairs(samples) do
        n = n + 1
        local d = v - mean
        mean = mean + d / n
        m2 = m2 + d * (v - mean)
    end
    if n == 0 then return 0, 0 end
    return mean, m2 / n
end
local label = panel:getChildByName("varText")
local m, var = stats({4, 8, 6, 2})
label:setString(m .. "/" .. var)