tracktracker/back/data/realtime.js

const tam = require('./sources/tam');
const util = require('../util');

let nextUpdate = null;
let currentCourses = null;

/**
 * Fetch real-time information about active courses in the TaM network.
 *
 * New data will only be fetched from the TaM server once every minute,
 * otherwise pulling from the in-memory cache.
 *
 * The following information is provided for each active course:
 *
 * - `id`: Unique identifier for the course.
 * - `line`: Line number.
 * - `nextStop`: Identifier of the next stop of the course.
 * - `arrivalTime`: Timestamp at which the vehicle is predicted to arrive
 *                  to the next stop.
 * - `finalStop`: The final stop to which the course is headed.
 *
 * @return Mapping from active course IDs to information about each course.
 */
const getCourses = () => new Promise((res, rej) =>
{
    if (nextUpdate !== null && Date.now() < nextUpdate)
    {
        res(currentCourses);
        return;
    }

    const courses = {};
    let lastUpdate = null;

    tam.fetchRealtime((err, entry) =>
    {
        if (err)
        {
            rej(err);
            return;
        }

        if (!util.isObject(entry))
        {
            currentCourses = courses;
            res(currentCourses);
            return;
        }

        if ('lastUpdate' in entry)
        {
            lastUpdate = entry.lastUpdate;
            nextUpdate = entry.nextUpdate;
            return;
        }

        const {
            course: id,
            routeShortName: line,
            stopId: nextStop,
            destArCode: finalStop,
        } = entry;

        const arrivalTime = lastUpdate + parseInt(entry.delaySec, 10) * 1000;

        if (!(id in courses))
        {
            courses[id] = {id, line, nextStop, arrivalTime, finalStop};
        }
        else if (arrivalTime < courses[id].arrivalTime)
        {
            // The stop where the next passing is soonest is assumed
            // to be the next stop
            courses[id].nextStop = nextStop;
            courses[id].arrivalTime = arrivalTime;
        }
    });
});

exports.getCourses = getCourses;