TrainMotion.java

package com.codemanship.xmastrain;

public class TrainMotion {
    private static final int SECONDS_PER_HOUR = 3600;
    private static final int METRES_PER_KM = 1000;
    private final double acceleration;
    private final double topSpeed;
    private double velocity;
    private double distance;
    private final Sensor sensor;
    private double lastSensorDistance = 0;

    public TrainMotion(double accelerationMs2, double initialVelocityKph, double topSpeedKph, double distanceKm, Sensor sensor) {
        this.acceleration = accelerationMs2;
        this.velocity = (initialVelocityKph * METRES_PER_KM) / SECONDS_PER_HOUR;
        this.topSpeed = (topSpeedKph * METRES_PER_KM) / SECONDS_PER_HOUR;
        this.distance = distanceKm * METRES_PER_KM;
        this.sensor = sensor;
    }

    public void move(int timeMilliseconds) {
        distance += distanceAdded(timeMilliseconds);
        checkForSensorPass();
    }

    private void checkForSensorPass() {
        if (distance - lastSensorDistance >= METRES_PER_KM) {
            sensor.passed(distance / METRES_PER_KM, velocity);
            lastSensorDistance = distance;
        }
    }

    double velocityAfterAcceleration(int timeMilliseconds) {
        velocity = velocity + (acceleration * asSeconds(timeMilliseconds));
        if (velocity > topSpeed)
            velocity = velocity - (velocity % topSpeed);
        return velocity;
    }

    private int asSeconds(int timeMilliseconds) {
        return timeMilliseconds / 1000;
    }

    double distanceAdded(int timeMilliseconds) {
        double avgVelocity = (velocity + velocityAfterAcceleration(timeMilliseconds)) / 2;
        return avgVelocity * (asSeconds(timeMilliseconds));
    }

    double getDistanceTravelledKm() {
        return distance / METRES_PER_KM;
    }

    double getVelocity() {
        return velocity;
    }
}