priorityQueue.js

import isArray from 'lodash/isArray';
import noop from 'lodash/noop';

import setImmediate from './setImmediate';

import queue from './queue';

/**
 * The same as [async.queue]{@link module:ControlFlow.queue} only tasks are assigned a priority and
 * completed in ascending priority order.
 *
 * @name priorityQueue
 * @static
 * @memberOf module:ControlFlow
 * @method
 * @see [async.queue]{@link module:ControlFlow.queue}
 * @category Control Flow
 * @param {Function} worker - An asynchronous function for processing a queued
 * task, which must call its `callback(err)` argument when finished, with an
 * optional `error` as an argument.  If you want to handle errors from an
 * individual task, pass a callback to `q.push()`. Invoked with
 * (task, callback).
 * @param {number} concurrency - An `integer` for determining how many `worker`
 * functions should be run in parallel.  If omitted, the concurrency defaults to
 * `1`.  If the concurrency is `0`, an error is thrown.
 * @returns {module:ControlFlow.QueueObject} A priorityQueue object to manage the tasks. There are two
 * differences between `queue` and `priorityQueue` objects:
 * * `push(task, priority, [callback])` - `priority` should be a number. If an
 *   array of `tasks` is given, all tasks will be assigned the same priority.
 * * The `unshift` method was removed.
 */
export default function(worker, concurrency) {
    // Start with a normal queue
    var q = queue(worker, concurrency);

    // Override push to accept second parameter representing priority
    q.push = function(data, priority, callback) {
        if (callback == null) callback = noop;
        if (typeof callback !== 'function') {
            throw new Error('task callback must be a function');
        }
        q.started = true;
        if (!isArray(data)) {
            data = [data];
        }
        if (data.length === 0) {
            // call drain immediately if there are no tasks
            return setImmediate(function() {
                q.drain();
            });
        }

        priority = priority || 0;
        var nextNode = q._tasks.head;
        while (nextNode && priority >= nextNode.priority) {
            nextNode = nextNode.next;
        }

        for (var i = 0, l = data.length; i < l; i++) {
            var item = {
                data: data[i],
                priority: priority,
                callback: callback
            };

            if (nextNode) {
                q._tasks.insertBefore(nextNode, item);
            } else {
                q._tasks.push(item);
            }
        }
        setImmediate(q.process);
    };

    // Remove unshift function
    delete q.unshift;

    return q;
}