/**
 * Represents a queue data structure.
 * @class
 */
export class Queue {
    /**
     * Creates an instance of Queue.
     * @constructor
     */
    constructor() {
        /**
         * The data array to store elements.
         * @type {Array}
         * @private
         */
        this.data = [];
        /**
         * The index of the front element in the queue.
         * @type {number}
         * @private
         */
        this.head = 0;
    }

    /**
     * Retrieves the front element of the queue.
     * @returns {*} The front element of the queue.
     */
    front() {
        return this.data[this.head];
    }

    /**
     * Retrieves the back element of the queue.
     * @returns {*} The back element of the queue.
     */
    back() {
        return this.data[this.data.length - 1];
    }

    /**
     * Removes the back element from the queue.
     * @throws {QueueEmptyError} Throws an error if the queue is empty.
     */
    popBack() {
        if (this.data.length == this.head) {
            throw new QueueEmptyError();
        }
        this.data.length -= 1;
    }

    /**
     * Checks if the queue is empty.
     * @returns {boolean} True if the queue is empty, otherwise false.
     */
    empty() {
        return this.data.length == this.head;
    }

    /**
     * Returns the size of the queue.
     * @returns {number} The size of the queue.
     */
    size() {
        return this.data.length - this.head;
    }

    /**
     * Alias for the size() method.
     * @type {number}
     */
    get length() {
        return this.size();
    }

    /**
     * Pushes elements into the queue.
     * @param {...*} elements The elements to push into the queue.
     */
    push() {
        this.data.push.apply(this.data, arguments);
    }

    /**
     * Removes the front element from the queue.
     * @throws {QueueEmptyError} Throws an error if the queue is empty.
     */
    popFront() {
        if (this.empty()) { throw new QueueEmptyError(); }

        this.head += 1;
        if (this.head == this.data.length) {
            this.data = [];
            this.head = 0;
            return;
        }
        if (this.head >= this.data.length >> 1 && this.head >= 16) {
            this.data = this.data.slice(this.head);
            this.head = 0;
        }
    }

    /**
     * Returns an iterator that allows iterating through the elements of the queue.
     * @returns {*} elements of the queue.
     * @generator
     */
    *[Symbol.iterator]() {
        for (let i = this.head; i < this.data.length; i += 1) {
            yield this.data[i];
        }
    }
}

/**
 * Represents an error thrown when attempting operations on an empty queue.
 * @class
 */
export class QueueEmptyError extends Error {
    /**
     * Creates an instance of QueueEmptyError.
     * @constructor
     */
    constructor() {
        super('Queue is empty');
    }
}