【高级技巧】Lua小顶堆：沙巴克修理请求的优先出队

local function popMin(q)
    local mi = 1
    for i = 2, #q do                  -- 每次出队全表扫 O(n)
        if q[i].urgency < q[mi].urgency then mi = i end
    end
    return table.remove(q, mi)
end

-- 沙巴克修理请求队列，按urgency小顶堆化
local function push(q, item)
    q[#q + 1] = item
    local i = #q
    while i > 1 do
        local p = math.floor(i / 2)
        if q[p].urgency <= q[i].urgency then break end
        q[p], q[i] = q[i], q[p]       -- 上浮
        i = p
    end
end