'use strict';

class Pool {

constructor() {

this.instances = [];

this.free = [];

this.current = 0;

this.size = 0;

this.available = 0;

}

acquire() {

if (this.available === 0) return null;

let instance = null;

let free = false;

do {

instance = this.instances[this.current];

free = this.free[this.current];

this.current++;

if (this.current === this.size) this.current = 0;

} while (!instance || !free);

return instance;

}

add(instance) {

if (this.instances.includes(instance)) {

throw new Error('Pool: add duplicates');

}

this.size++;

this.available++;

this.instances.push(instance);

this.free.push(true);

}

capture() {

const instance = this.acquire();

if (!instance) return null;

const index = this.instances.indexOf(instance);

this.free[index] = false;

this.available--;

return instance;

}

release(instance) {

const index = this.instances.indexOf(instance);

if (index < 0) throw new Error('Pool: release unexpected instance');

if (this.free[index]) throw new Error('Pool: release not captured');

this.free[index] = true;

this.available++;

}

}

// Usage

const pool = new Pool();

pool.add({ instance: 1 });

pool.add({ instance: 2 });

const last = { instance: 3 };

pool.add(last);

for (let i = 0; i < 10; i++) {

console.log(pool.capture());

if (i === 5) pool.release(last);

}