Interactive visualisation of Pi and friends with D3.js

Inspiration

I recently stumbled onto the magnificent posters created by Martin Krzywinski over at http://mkweb.bcgsc.ca/pi/. He’s come up with some truly original ways to illustrate the appearance and complexity of various irrational numbers.

Specifically, I really liked the minimalism and simplicity of the 2013 edition, shown here:

pi-dots-01

The rules used for generating the coloured dots is fascinatingly simple. Each digit 0-9 is assigned a unique colour. The i’th circle is then coloured according to the value of the i’th digit of Pi. The smaller circle inside is coloured based on the value of the following digit.

Simple rules, complex outcome.

Martin created other versions as well, e.g. one where adjacent equals are connected by same coloured lines. They’re all fascinating and you can even buy them as posters!

Interactive

Looking at these posters I couldn’t help wonder, what lay beyond the chosen boundaries for each poster. What if some great pattern or sequence was lurking just outside of view? If only there was a way to go explore further digits and other constellations using the same visualisation format.

Always the tinkerer, this got me thinking about how to create something like that. One thing led to another and before long I had a rough prototype cobbled together with D3.js.

I spent a bit more time adding a few input controls and polishing it into a neat little demo. I’ve put it up at:

winski.rhardih.io

Go try it out! Instructions are at the bottom.

As an example, this is how the Feynman Point looks, at a column width of 31:

Screen Shot 2016-06-10 at 12.30.29

For the more curious out there, I’ve put the code on GitHub at github.com/rhardih/winski.

Happy π hunting!

Dead simple concurrency limitation in Go

Backgrounding long running tasks is a classic and ubiquitous problem for web applications.

E.g. Something triggered the need to download and manipulate a file, but we don’t want to hold up the main thread responsible for bringing a response back to the client.

Most likely you’d want to offload this task to a background worker or another service, but sometimes it’s nice to be able to just handle the processing right then and there.

Go makes concurrency incredibly simple with the go keyword. So simple in fact, that you might quickly run into problems if you are handling files in the this manner and just spin up new goroutines for everything.

File descriptors

Herein lies the problem. Most systems don’t have an unlimited number of file descriptors for a process to use. Probing the two machines within my current reach yields the following:

$ sw_vers
ProductName:    Mac OS X
ProductVersion: 10.11.4
BuildVersion:   15E65
$ ulimit -n
256
~$ cat /etc/issue
Ubuntu 14.04.4 LTS \n \l
 
~$ ulimit -n
1024

Evidently there isn’t all that many available on either system. Opening a ton of sockets and files at once, will inevitably lead to errors; “Too many open files” or similar.

Limitation

Go’s concurrency primitives lends itself to definitions laid out by Tony Hoare in Communicating sequential processes and as such Go has the concept of channels.

Channels provides the necessary “blocking” mechanism, to allow a collection of goroutines to handle a common workload. Instead of creating a lot of goroutines all at once, rather a handful can be created, each waiting to handle incoming items on a channel.

Below is a very simple example, where a, (buffered channel with five slots*), channel is created as a work queue. Subsequently five goroutines are created and each set to wait on incoming work from the queue. In the main thread, the queue is then filled up all at once with work for the goroutines.

Once all the work have been loaded onto the queue, the channel is closed, which is Go’s way of telling consuming goroutines that nothing more will appear on the channel. This works in tandem with the range keyword to keep receiving on the channel until it is closed.

Note, the WaitGroup is effectively a Monitor; another concurrency construct that allows the main thread to wait for all the goroutines to finish.

Running the program produces the following output:

$ go run conc.go
10:25:37 Work a enqueued
10:25:37 Work b enqueued
10:25:37 Work c enqueued
10:25:37 Work d enqueued
10:25:37 Work e enqueued
10:25:37 Work f enqueued
10:25:39 Worker 1 working on a
10:25:39 Worker 3 working on d
10:25:39 Work g enqueued
10:25:39 Work h enqueued
10:25:39 Worker 2 working on b
10:25:39 Work i enqueued
10:25:39 Worker 4 working on e
10:25:39 Work j enqueued
10:25:39 Worker 0 working on c
10:25:39 Work k enqueued
10:25:41 Worker 0 working on j
10:25:41 Worker 3 working on g
10:25:41 Worker 2 working on h
10:25:41 Worker 1 working on f
10:25:41 Worker 4 working on i
10:25:41 Work l enqueued
10:25:41 Work m enqueued
10:25:41 Work n enqueued
10:25:41 Work o enqueued
10:25:43 Worker 0 working on k
10:25:43 Worker 1 working on n
10:25:43 Worker 3 working on l
10:25:43 Worker 2 working on m
10:25:43 Worker 4 working on o

Go’s concurrency primitives is that rare combination of easy and powerful, making it effortless to write threaded code.

It doesn’t save you from inherent limitations of the host system however, which is a good thing. Awareness of what the code actually does on the machine, is a virtue to strive for.

Edit: Thanks to Jemma for pointing out that buffering the channel isn’t needed afterall.

This post was included in the Go Newsletter issue 110.

Page specific Javascript in Rails 3

Premise

One of the neat features from Rails 3.1 and up is the asset pipeline:

The asset pipeline provides a framework to concatenate and minify or compress JavaScript and CSS assets. It also adds the ability to write these assets in other languages such as CoffeeScript, Sass and ERB.

This means that in production, you will have one big Javascript file and also one big CSS file. This reduces the number of request the browser has to make and generally loads the page faster.

In the case of Javascript concatenation however, it does bring about a problem. Executing code when the DOM has loaded is commonplace in most web applications today, but if everything is included in one big file, and more importantly the same file, for all actions on all controllers, how do you run code that is specific to just a single view?

Solution(s)

Obviously there is more than one way of solving this problem, and rather unlike Rails, there doesn’t seem to be any “best practice” dictated. The closest I found is this excerpt from section 2 of the Rails Guide about the Asset Pipeline:

You should put any JavaScript or CSS unique to a controller inside their respective asset files, as these files can then be loaded just for these controllers with lines such as <%= javascript_include_tag params[:controller] %> or <%= stylesheet_link_tag params[:controller] %>.

And it isn’t even followed by an example, which seems more of an indication, that this isn’t something you should do at all.

Let’s start by this example nonetheless.

Per controller inclusion

By default Rails has only one top level Javascript manifest file, namely app/assets/javascripts/application.js which has the following content:

// This is a manifest file that'll be compiled into including all the files listed below.
// Add new JavaScript/Coffee code in separate files in this directory and they'll automatically
// be included in the compiled file accessible from http://example.com/assets/application.js
// It's not advisable to add code directly here, but if you do, it'll appear at the bottom of the
// the compiled file.
//
//= require jquery
//= require jquery_ujs
//= require_tree .

And this is included in the default layout with:

<%= javascript_include_tag "application" %>

N.B. When testing production on localhost, with e.g. rails s -e production, rails by default wont serve static assets, which application.js becomes after pre-compilation, so to avoid any problems when locally testing production, the following setting needs to be changed from false to true in config/environments/production.rb:

# Disable Rails's static asset server (Apache or nginx will already do this)
config.serve_static_assets = true

Now let’s say we have a controller, let’s call it ApplesController, and its corresponding Coffescript file, apples.js.coffee. We might try to include it as per the Rails Guide suggestion like so:

<%= javascript_include_tag params[:controller] %>

And this will work just fine in development mode, but in production produce the following error:

ActionView::Template::Error (apples.js isn't precompiled):

To remedy this, we need to do a couple of things. First off we should remove the require_tree . part from application.js, so we don’t wind up including the same script twice. Just removing the equal sign is enough:

//  require_tree .

To avoid a name clash rename apples.js.coffee to something else, e.g. apples.controller.js.coffee. Then create a new manifest file named apples.js, which includes your coffeescript file:

//= require apples.controller

Lastly, the default configuration of Rails only includes and pre-compiles application.js, so we need to tell the pre-compiler to now also include apples.js. This is also in config/environments/production.rb. Uncomment the following setting, and change search.js to apples.js:

# Precompile additional assets (application.js, application.css, and all non-JS/CSS are already added)
config.assets.precompile += %w( apples.js )

Note that this is a match, so it could also be something like '*.js' in case you have more manifests, which would be the case for per controller inclusion.

Views

The same concept as above could be extended to target individual actions/views of each controller, by having the actions be part of the manifest name. Individual javascript files could then be included like so:

<%= javascript_include_tag "#{params[:controller]}.#{params[:action}" %>

This makes an assumption that all actions on all controllers have a dedicated Javascript file. An assumption which most likely won’t be true in most cases. Another option could be an conditional include like so:

<%= yield :action_specific_js if content_for?(:action_specific_js %>

And then move the include tag to the specific views that need it.

Testing for existence of a page element or class

The DOM loaded event handler could look something like this:

jQuery ->
  if $('#some_element').length > 0
    // Do some stuff here

This could also be a class on body eg.:

jQuery ->
  if $('body.controller_name_action_name').length > 0
    // Do some stuff here

And then then the erb would be like this:

<!DOCTYPE html>
<html>
<head>
  <title>AppName</title>
  <%= stylesheet_link_tag    "application" %>
  <%= javascript_include_tag "application" %>
  <%= csrf_meta_tags %>
</head>
<body class="<%= "#{params[:controller]}_#{params[:action]}" %>">
 
<%= yield %>
 
</body>
</html>

Function encapsulation and on-page triggering

Instead of registering the handlers for DOM loaded, wrap the necessary code in a function that can be called later and then trigger that function directly in the respective view.

There is one thing we need to consider though. All Coffeescript sources for each controller get wrapped in it’s own closed scope, i.e this Coffeescript in apples.js.coffee:

apples_index = ->
  console.log("Hello! Yes, this is Apples.")

Becomes:

((
  function(){
    var a;
    a=function(){
        return console.log("Hello! Yes, this is Apples.")
    }
  }
)).call(this);

So in order for us to have a globally callable function, we must first expose it somehow. We can do this by attaching the function to the window object. Changing the above code like so:

window.exports ||= {}
window.exports.apples_index = ->
  console.log("Hello! Yes, this is Apples.")

If we insert this line in application.html.erb layout just before the closing body tag:

<!DOCTYPE html>                                                                           
<html>                                                                                    
<head>                                                                                    
  <title>AppName</title>                                                                   
  <%= stylesheet_link_tag    "application" %>                                             
  <%= javascript_include_tag "application" %>                                             
  <%= csrf_meta_tags %>                                                                   
</head>                                                                                   
<body>                                                                                    
 
<%= yield %>                                                                              
 
<%= yield :action_specific_js if content_for?(:action_specific_js) %>                     
</body>                                                                                   
</html>

We can now call the exposed function directly from our view like so:

<% content_for :action_specific_js do %>
<script type="text/javascript" language="javascript">
  $(function() { window.exports.apples_index(); });
</script>
<% end %>

Wrap up

Neither of these three examples is a “one-fit-all” solution I would say. Dividing up the Javascript source will start to make sense as soon as the Javascript codebase grows past a certain size. It might be interesting to test out, just how big that size is on a certain bandwidth, but I think that’s out of the scope for this post.

Given the fact that there isn’t really a defined best practice yet, perhaps the ruby community will come up with something better than the examples I presented here. In my opinion I think this is definitely something that could be better thought out.