00. SelectableArray.js

import Circle from "Util/Circle"

const circleMap = new WeakMap();

export default class SelectableArray extends Array{
	static get [Symbol.species]() { return SelectableArray; }
	
	constructor(...args){
		super(...args);
	}
	
	random(){
		return this[Math.random()*this.length|0];
	}
	
	sequence(){
		let circle = circleMap.get(this);
		
		if(circle){
			circle.length = this.length;
		}
		else{
			circle = new Circle(this.length);
		}
		
		const value = this[circle.now];
		
		circle.succ;
		circleMap.set(this, circle);
		
		return value;
	}
}

01. Cicle.js

class Circle{
	constructor(length, offset=0){
		this._offset = offset;
		this.now = this._offset;
		this.length = length;
	}
	
	set now(value){
		this._cursor = value;
		this.succ; this.prev;
	}
	get now(){
		return this._cursor;
	}
	
	set length(value){
		this._length = value - 1;
		this.succ; this.prev;
	}
	get length(){
		return this._length + 1;
	}
	
	get succ(){
		this._cursor++;
		if(this._cursor > (this._length+this._offset)) this._cursor = this._offset;
		return this._cursor;
	}
	
	get prev(){
		this._cursor--;
		if(this._cursor < this._offset) this._cursor = (this._length+this._offset);
		return this._cursor;
	}
}

export default Circle;