const tam = require("./sources/tam");
const network = require("./network.json");

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

// Current information about courses
let currentCourses = null;

/**
 * Information about the course of a vehicle.
 * @typedef {Object} Course
 * @property {string} id Unique identifier for this course.
 * @property {string} line Transport line number.
 * @property {string} finalStop Final stop to which the course is headed.
 * @property {Object.<string,number>} nextPassings Next stations to which
 * the vehicle will stop, associated to the passing timestamp.
 */

/**
 * 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.
 * @returns {Object.<string,Course>} Mapping from active course IDs to
 * information about each course.
 */
const fetch = async() => {
    if (nextUpdate === null || Date.now() >= nextUpdate) {
        const courses = {};
        const passings = tam.fetchRealtime();
        const timing = (await passings.next()).value;

        nextUpdate = timing.nextUpdate;

        // Aggregate passings relative to the same course
        for await (const passing of passings) {
            const {
                course: id,
                routeShortName: line,
                stopId,
                destArCode: finalStop
            } = passing;

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

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

        // Filter courses to only keep those referring to known data
        for (const courseId of Object.keys(courses)) {
            const course = courses[courseId];

            if (!(course.line in network.lines)) {
                delete courses[courseId];
            } else {
                for (const stopId of Object.keys(course.nextPassings)) {
                    if (!(stopId in network.stops)) {
                        delete courses[courseId];
                        break;
                    }
                }
            }
        }

        currentCourses = courses;
    }

    return currentCourses;
};

exports.fetch = fetch;