export default class Queue { //队列：指针移动 (最优)。   数据量大的时候比用Map速度快
    constructor() {
        this.buffer = [];
        this.head = 0;
        this.tail = 0;
    }
    getHead(){
       return this.buffer[this.head];
    }
    dequeue() {//从头部取出
        if (this.head >= this.tail) return null;
        const item = this.buffer[this.head];
        this.buffer[this.head] = undefined; // 帮助GC
        this.head++;
        
        // 定期清理前部空间
        if (this.head > 10000 && this.head > this.buffer.length / 2) {
            this.buffer = this.buffer.slice(this.head);
            this.tail -= this.head;
            this.head = 0;
        }
        return item;
    }
    
    enqueue(item) {//从尾部追加
        this.buffer[this.tail] = item;
        this.tail++;
    }
}