'use strict';

const _ = require('lodash');
const Q = require('q');

function promiseQueue(options) {
    options = _.assign({
        concurrency: 1
    }, options);

    let queue = [];

    let running = 0;

    function processQueue() {
        while (running < options.concurrency && queue.length) {
            let state = queue.shift();

            running += 1;

            let fnPromise = Q.try(state.fn);

            state.d.resolve(fnPromise);

            fnPromise.finally(() => {
                running -= 1;
                processQueue();
            });
        }
    }

    return function enqueue(fn) {
        let d = Q.defer();

        queue.push({ fn, d });
        processQueue();

        return d.promise;
    };
}

module.exports = promiseQueue;