tracktracker/src/data/simulation.js

import axios from "axios";
import turfAlong from "@turf/along";
import * as turfProjection from "@turf/projection";
import * as routing from "./routing.js";
import network from "./network.json";

const server = "http://localhost:4321";

// Time to stay at each stop (milliseconds)
const stopTime = 10000;

// Step used to compute the vehicle bearing (meters)
const angleStep = 10;

// Maximum speed of a vehicle (meters per millisecond)
const maxSpeed = 60 / 3600;

// Minimum speed of a vehicle (meters per millisecond)
const minSpeed = 10 / 3600;

// Normal speed of a vehicle
const normSpeed = (2 * maxSpeed + minSpeed) / 3;

/** Simulate the evolution of a vehicle course in the network. */
class Course {
    constructor(id) {
        // Unique identifier of this course
        this.id = id;

        // Line on which this vehicle operates
        this.line = null;

        // Line direction of this course
        this.direction = null;

        // Stop to which this course is headed
        this.finalStop = null;

        // Previous stops that this course left (stop id/timestamp pairs)
        this.prevPassings = [];

        // Next stops that this course will leave (stop id/timestamp pairs)
        this.nextPassings = [];

        // Stop that this course just left or will leave
        this.departureStop = null;

        // Time at which the last stop was left or will be left (timestamp)
        this.departureTime = 0;

        // Next stop that this course will reach
        this.arrivalStop = null;

        // Time at which the next stop will be left (timestamp)
        this.arrivalTime = 0;

        // Route between the current departure and arrival stops
        this.segment = null;

        // Distance already travelled between the two stops (meters)
        this.traveledDistance = 0;

        // Current vehicle speed (meters per millisecond)
        this.speed = 0;

        // Current vehicle latitude and longitude
        this.position = [0, 0];

        // Current vehicle bearing (clockwise degrees from north)
        this.angle = 0;
    }

    /** Find a route between the current departure and arrival stops. */
    updateSegment() {
        if (this.departureStop === null || this.arrivalStop === null) {
            this.segment = null;
            return;
        }

        const name = `${this.departureStop}-${this.arrivalStop}`;

        // Use predefined segment if it exists
        if (name in network.segments) {
            this.segment = network.segments[name];
            return;
        }

        if (!(this.departureStop in network.stops)) {
            console.warn(`Unknown stop: ${this.departureStop}`);
            this.segment = null;
            return;
        }

        if (!(this.arrivalStop in network.stops)) {
            console.warn(`Unknown stop: ${this.arrivalStop}`);
            this.segment = null;
            return;
        }

        // Compute a custom route between two stops
        this.segment = routing.findSegment(this.departureStop, this.arrivalStop);

        if (this.segment === null) {
            console.warn(`No route from ${this.departureStop} \
to ${this.arrivalStop}`);
        }
    }

    /** Merge passings data received from the server. */
    receiveData(data) {
        this.line = data.line;
        this.direction = data.direction;
        this.finalStop = data.finalStopId;

        const passings = Object.assign(
            Object.fromEntries(this.nextPassings),
            Object.fromEntries(data.passings),
        );

        // Remove older passings from next passings
        for (let [stop, _] of this.prevPassings) {
            delete passings[stop];
        }

        // Update departure time if still announced
        if (this.departureStop !== null) {
            if (this.departureStop in passings) {
                this.departureTime = passings[this.departureStop];
                delete passings[this.departureStop];
            }
        }

        // Update arrival time
        if (this.arrivalStop !== null) {
            if (this.arrivalStop in passings) {
                // Use announced time if available
                this.arrivalTime = passings[this.arrivalStop];
                delete passings[this.arrivalStop];
            } else {
                // Otherwise, arrive using a normal speed from current position
                const segment = this.segment;
                const distance = segment.properties.length - this.traveledDistance;
                const time = Math.floor(distance / normSpeed);
                this.arrivalTime = Date.now() + time;
            }
        }

        this.nextPassings = Object.entries(passings).sort(
            ([, time1], [, time2]) => time1 - time2
        );
    }

    /** Update the vehicle state. */
    update() {
        const now = Date.now();

        // When initializing, use the first available passing as start
        if (this.departureStop === null) {
            if (this.nextPassings.length > 0) {
                const [stopId, time] = this.nextPassings.shift();
                this.departureStop = stopId;
                this.departureTime = time;
                this.updateSegment();
            }
        }

        // …and the second one as the arrival
        if (this.arrivalStop === null) {
            if (this.nextPassings.length > 0) {
                const [stopId, time] = this.nextPassings.shift();
                this.arrivalStop = stopId;
                this.arrivalTime = time;
                this.updateSegment();
            }
        }

        if (this.segment !== null) {
            const segment = this.segment;
            const distance = segment.properties.length - this.traveledDistance;
            const duration = this.arrivalTime - stopTime - now;

            // Arrive to the next stop
            if (distance === 0) {
                this.prevPassings.push([this.departureStop, this.departureTime]);
                this.departureStop = this.arrivalStop;
                this.departureTime = this.arrivalTime;

                if (this.nextPassings.length > 0) {
                    const [stopId, time] = this.nextPassings.shift();
                    this.arrivalStop = stopId;
                    this.arrivalTime = time;
                } else {
                    this.arrivalStop = null;
                    this.arrivalTime = 0;
                }

                this.traveledDistance = 0;
                this.updateSegment();
            }

            if (this.departureTime > now) {
                // Wait for departure
                this.speed = 0;
            } else {
                if (this.traveledDistance === 0 && this.speed === 0) {
                    // We’re late, record the actual departure time
                    this.departureTime = now;
                }

                // Update current speed to arrive on time if possible
                this.speed = Course.computeSpeed(distance, duration);
            }
        }

        return true;
    }

    /** Integrate the current vehicle speed and update distance. */
    move(time) {
        if (this.segment === null) {
            return;
        }

        if (this.speed > 0) {
            this.traveledDistance = Math.min(
                this.traveledDistance + this.speed * time,
                this.segment.properties.length,
            );
        }

        // Compute updated position and angle based on a small step
        let positionBehind;
        let positionInFront;

        if (this.traveledDistance < angleStep / 2) {
            positionBehind = this.traveledDistance;
            positionInFront = angleStep;
        } else {
            positionBehind = this.traveledDistance - angleStep / 2;
            positionInFront = this.traveledDistance + angleStep / 2;
        }

        const positions = [
            positionBehind,
            this.traveledDistance,
            positionInFront,
        ].map(distance => turfProjection.toMercator(turfAlong(
            this.segment,
            distance / 1000
        )).geometry.coordinates);

        this.angle = Math.atan2(
            positions[0][1] - positions[2][1],
            positions[2][0] - positions[0][0]
        );

        this.position = positions[1];
    }

    /**
     * Compute the optimal speed to arrive on time.
     * @param {number} distance Distance to cover (meters)
     * @param {number} duration Remaining time (seconds)
     * @return {number} Optimal speed (meters per second)
     */
    static computeSpeed(distance, duration) {
        if (duration <= 0) {
            // Late: go to maximum speed
            return maxSpeed;
        }

        const speed = distance / duration;

        if (speed < minSpeed) {
            // Too slow: pause until speed is sufficient
            return 0;
        }

        return Math.min(maxSpeed, speed);
    }
}

/** Fetch passing data from the server and update simulation. */
const updateData = async courses => {
    const dataset = (await axios.get(`${server}/courses`)).data;

    // Update or create new courses
    for (const [id, data] of Object.entries(dataset)) {
        if (id in courses) {
            courses[id].receiveData(data);
        } else {
            const newCourse = new Course(data.id);
            newCourse.receiveData(data);
            courses[id] = newCourse;
        }
    }

    // Remove stale courses
    for (const id of Object.keys(courses)) {
        if (courses[id].departureStop === courses[id].finalStop) {
            delete courses[id];
        }
    }
};

export const start = () => {
    const courses = {};
    let lastFrame = null;
    let lastUpdate = null;

    const update = () => {
        const now = Date.now();

        if (lastUpdate === null || lastUpdate + 5000 <= now) {
            lastUpdate = now;
            updateData(courses);
        }

        const time = lastFrame === null ? 0 : now - lastFrame;
        lastFrame = now;

        for (const course of Object.values(courses)) {
            course.update();
            course.move(time);
        }
    };

    return { courses, update };
};