Lua双指针队列：弹出零搬移的O(1)实现

flowchart LR
    subgraph data["_data 数组段"]
        d0["[0] nil"] --- d1["[1] nil"] --- d2["[2] 弹出中"] --- d3["[3] 活动"] --- d4["[4] 活动"]
    end
    I["头指针 _index = 2"] -.-> d2
    B["尾指针 _border = 5"] -.-> d4
    P["push：写 _data[_border]，_border+1"] --> d3
    O["pop：取 _data[_index] 置 nil，_index+1"] --> d2

local queue = class('queue')

function queue:ctor()
    self:reset()
end

function queue:push(d)
    self._data[self._border] = d
    self._border = self._border + 1
end

function queue:pop()
    local ret = nil
    if self._index < self._border then
        ret = self._data[self._index]
        self._data[self._index] = nil     -- 断引用，交给 GC
        self._index = self._index + 1
    else
        self:reset()                      -- 队列耗尽，重建表壳
    end
    return ret
end

function queue:front()
    if self._index < self._border then
        return self._data[self._index]
    end
    return nil
end

function queue:size()
    return self._border - self._index
end

function queue:empty()
    return self:size() == 0
end

function queue:reset()
    self._index  = 0
    self._border = 0
    self._data   = {}
end