tracktracker/src/tam/realtime.js

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

/**
 * Comparison function between two stop passings.
 *
 * @param passing1 First stop passing.
 * @param passing2 Second stop passing.
 * @return Negative value if passing1 is sooner than passing2, positive
 * otherwise, zero if they occur at the same time.
 */
const passingCompare = ({arrivalTime: time1}, {arrivalTime: time2}) => (
    time1 - time2
);

// Time at which the course data needs to be updated next
let nextUpdate = null;

// Current information about courses
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.
 * - `finalStop`: The final stop to which the course is headed.
 * - `nextPassings`: Next passings of the vehicle, sorted by increasing
 *   arrival time, containing both the stop identifier (`stopId`) and the
 *   expected arrival timestamp (`arrivalTime`).
 *
 * @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))
        {
            // End of courses information stream. Sort next stops by increasing
            // arrival time in each course then save result in memory cache
            for (let course of Object.values(courses))
            {
                course.nextPassings.sort(passingCompare);
            }

            currentCourses = courses;
            res(currentCourses);
            return;
        }

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

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

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

        if (!(id in courses))
        {
            courses[id] = {
                id, line, finalStop,
                nextPassings: [{stopId, arrivalTime}],
            };
        }
        else
        {
            courses[id].nextPassings.push({stopId, arrivalTime});
        }
    });
});

exports.getCourses = getCourses;