Traffic Lights

This is a simulation of a traffic light system, with two timers: X (by default, and in the spec: 5 minutes) which is the time interval that the lights automatically change directions and Y (by default, and in the spec: 30 seconds) which is the time that a light direction must stay yellow before changing directions.

This application contains a number of things

1. A traffic lights controller system that utilises a set interval and a second count to coordinate the changing of lights and directions
2. A simple Node.js server using Express that contains some simple API routes and a server sent event (SSE) socket, and the traffic lights controller in the backend
3. A front end application which utilises the Node.js server and its API
4. Unit tests for the backend system for controlling lights using Mocha

The application works as a standalone Node.js command line application run using: node run, or as a whole Node.js web application using: node server which will be listening on port 3000.

Installation

Make sure you have node installed. Clone or download the package, then on the command line while in the directory run npm install.

Running the application

To run the application bare bones just to see it working on the command line, simply run the command node run.

Running the interactive web application

To run the web app, run node server on the command line. If all works well, you can access the application at http://localhost:3000 and there are instructions on how to run the application on the page itself.

Testing

Running the test

Make sure you have mocha installed on your machine by using the command npm install -g mocha. To test simply run the command mocha

Tests

All tests are straightforward functional tests, with the exception of the last test which will take approximately 6 seconds to complete.

The logic of the last test is to test the flow of the traffic light controller system. We pass in a small value of X and Y (3 and 1) and then the test waits for changes to be signalled and checks that the lights have changed correctly.

The Code

The crux of the application is in the LightsController type. It takes in two Lights each representing the lights opposite each other in a particular direction (e.g. North South). The LightsController has functions to coordinate the changing of lights from green to yellow, and changing directions.

The main function is the run function which takes in X and Y as mentioned in this readme's introduction. The code of the function is below:

this.mainTimer = setInterval(() => {
	this.secondsElapsed++;
	if(this.secondsElapsed % timeToChangeDirection /*X*/ == timeToChangeDirection /*X*/ - timeToWaitOnYellow /*Y*/) {
		this.changeToYellow();
	}
	if(this.secondsElapsed % timeToChangeDirection /*X*/ == 0) {
		this.changeDirection();
	}
}, 1000)

The function starts an interval which ticks every second. When the seconds counted reaches Y seconds before X seconds has passed, the lights change to yellow, then once the seconds counted reaches X (i.e. Y seconds have passed since he lights have changed to yellow), the lights switch directions. We mod the seconds elapsed with X simply to reset to 0 each time the seconds reach X for easier comparison.

Room for improvement

I am happy with the application, but there are some things that could be looked at for improvement and refactoring:

• In lightscontroller.js, there is a second counter which counts all the seconds that have elapsed since running. If left running for a very long time, this counter may overflow which may cause some strange behaviour.
• The timing component in lightscontroller.js may be better as a separate function on its own.
• The Lights Controller should probably own the traffic lights it's been given and the lights themselves shouldn't be able to be accessed by outside functions - this would take some refactoring
• The front end application could be written in a better framework
• The output of the console application could be formatted a bit more nicely