const PQ = require('p-q');

// so many parallel promises should run at the same time
const concurrency = 3;

// simulate work with delay function
function delay(times, ms = 1000) {
    return new Promise((resolve) => {
        setTimeout(resolve, times * ms);
    });
}

// somewhat official function for returning random integer in range
function getRandomInt(min, max) {
    min = Math.ceil(min);
    max = Math.floor(max);
    return Math.floor(Math.random() * (max - min)) + min; //The maximum is exclusive and the minimum is inclusive
}

// initialize local queue - set pre-defined function, that returns a promise
const queue = new PQ(async(data) => {
    await delay(getRandomInt(0, 5));
    console.log(`finished ${data.i}`);
});

// the heart of the operation
//   add one item to queue and block until one finishes,
//   so there is no more than 3 running
async function add(data) {
    console.log(`add ${data.i}`);
    queue.add(data);
    if (queue.length() >= concurrency) {
        await new Promise(resolve => {
            queue.once('processed', () => {
                resolve();
            });
        });
    }
}

async function main() {
    // let's simulate rabbitmq - should not accept more than 3 items at a time,
    //   so that other instances can fetch new events
    for (let i = 0; i < 100; i++) {
        if (i < 10) {
            await add({i});
        } else if (i > 30 && i < 40) {
            await add({i});
        } else {
            await delay(1);
        }
    }
}

main()
    .then(() => {
        console.log('END');
    })
    .catch(err => {
        throw err;
    });