Lua权重随机：加权抽取的累进区间

-- 错误：均分概率，权重形同虚设
local it = POOL[math.random(#POOL)]

local POOL = {
    {id = "a", w = 50},
    {id = "b", w = 30},
    {id = "c", w = 20},
}
local function pick()
    local total = 0
    for _, it in ipairs(POOL) do
        total = total + it.w
    end
    local r = math.random(1, total)
    for _, it in ipairs(POOL) do
        r = r - it.w
        if r <= 0 then
            return it.id
        end
    end
end