The writings of Peter Stuifzand

Archive for March 2012

No network joins an internetwork smaller than itself.


Yesterday I was working on my realtime feed reader (and writer). I can read feeds with this, but I can also write posts. With it you can write a small post with a title, description and link.

When I was looking around for websites that support WebFinger I found a similar thing on Twitter. It contained the host-meta file, but instead of showing WebFinger related links, it showed something called an OExchange link. So I took a look what it was.

OExchange is a protocol that allows sharing of URLs with any service. It supports discovery of share URLs. When you have a service that allows sharing of URLs, then OExchange allows tool builders to share URLs with your service.

I created such a service, so I thought how about using this to share URLs. So I added these files. Now I couldn't find tools, that allow me to use this with my own service. Every tool I found could only share with pre-discovered websites. I couldn't even put my own domain name in there. That sucks for a tool that is about discovery.

Someone should write a Firefox extension that uses this. Maybe Firefox Share could do this?

Ok, but lets get back to the point I was trying to make. What about using a similar thing to allow tool developers to discover URLs to subscribe to feeds?

Such a tool or extension could include a list of pre configured domains, but allow you to add your own domains. The discovery process takes care of the rest.

It important to know that this file could be cached. It shouldn't be downloaded every time you like to subscribe to a feed.

This could make subscribing to feeds as easy as it is in Twitter. How about that?

Tonight I wrote this parser in Perl with Marpa::XS. The nice thing about Marpa is that it will tell you which terminals (or tokens) it expects at the current position. So instead of guessing what the next token will be you can just try to parse a token and try it.

I got the general idea from the TAP::Spec::Parser. The difference is that it will try all alternatives at once, while mine tries each after the other until one works.

I hope this will help people to understand this kind of parsing better. Let me know what you think.

I was thinking about implicit algorithms and implicit datastructures and thought how this applied to templates in web applications. I found that there are a few places where I write the same template code with just a few different pieces.

One of the examples is a piece of code for showing a text field with a few labels around it. This piece of template code is used for almost every field in the product edit (and new) form. This is repeated code, but we should not do this, right?

There are a few parts to one of these pieces of code. It has a textual name, a short description, an input field (which depends on the type of the value) and the actual value in the field.

<p><label for="sku">SKU</label> <span class='description'>Stock keeping unit</span>
<input type="text" id="sku" name="sku" value="[% product.sku %]"></p>

It could look like the above. To get an idea about what an implicit algorithms is, you could say that you're the computer and the data structure is in your head.

Write 'p' start tag
Write 'label' start tag with 'for' attribute with value 'id of field'
Write name of field
Write 'label' end tag
Write a space
If field has a description
    Write 'span' start tag with 'class' attribute with value 'description'
    Write description of field
    Write 'span' end tag
Write 'input' tag with type, id, name and value.
Write 'p' end tag

This piece of pseudocode shows how we run the piece of code in our head. The strange thing is that computers are actually really good at following instructions. A computer could this piece of code better than you could.

This piece of code is implicit in the template. Why didn't I write it as a piece of code?

There is also an implicit datastructure in there. Maybe something like this:

Field {
    string title
    string name
    string id
    string description
    string type

Field { "SKU", "sku", "sku", "Stock keeping unit", "text" }

If I create another line like this field description I can let the computer create a piece of HTML code for me, without thinking about it.

Field { "Name", "name", "name", Name of the product", "text" }

The thing is that by starting to use datastructures and algorithms for this, we can start to write more general functions that can be used for even more parts of your web app. Imagine the possibilities.

I was working on a small game that displays some blocks. These blocks need to move. The way to make these blocks move is by changing the x and y position over time.

The way I did this in the game was by calling a tick() method every 1/60th of a second on every block that could be moving. Then the block can change it's position according to some algorithm. I just wrote code that makes these changes to the position.

But if you take a closer look at the code you can see that some lines are connected. These lines are part of the same algorithm, but there is no place where all the parts come together in the same place.

The blocks in the first level have three states.

  1. Do nothing. Stay in the same position.
  2. Falling. When falling the block makes sure its position returns to the default position.
  3. Shaking. Some blocks shake, other don't. When shaking the position is changed randomly around its current position, the position from state 1.

Let's look at shaking. We shake the block by changing the difference from the current position of the block by a small random amount. In the code it looks like this.

if (selected) {
    this.dx = rnd.nextInt(3) - 1;
    this.dy = rnd.nextInt(3) - 1;

The fields dx and dy are the difference in pixel position that this object is drawn on screen.

The Level class contains a small part of this algorithm. It makes sure that the object is drawn (dx,dy) pixels from its normal position. Without this line of code the algorithm wouldn't work.

In a way there is also an implicit datastructure that contains the difference position. The code doesn't make that obvious.

Making it obvious

The is a piece of code contained in the game that shakes an Entity. By extracting this into a class we can help other programmers and reuse this piece of code. But we have to look at the concepts that are used in this code.

A few concepts that I think are in this code are: shake, tick, difference in position and entity. There are as always many ways to do the same thing.

public class Shake implements Mover {
    private static final Random rnd = new Random();
    private Movable object;

    public Shake(Movable dm)  { this.object=dm; }

    public void tick() {
        object.moveBy(rnd.nextInt(3)-1, rnd.nextInt(3)-1);

Now we need to write multiple objects that do a similar thing. This way we can show that the concept is useful.

public class Falling implements Mover {
    private Movable object;

    public Falling(Movable m) { this.object=dm; }

    public void tick() {
        if (m.y() < 0) {
            m.moveBy(0, 3);

We can see here a falling block. This code will make the block fall automatically. Because of the way the game works a block will only fall by a certain amount. Then it needs to move to the next position in the level.

I will write some more about implicit algorithms (and datastructures) later.

I have an article in the pipeline about implicit algorithms (and datastructures), which is a continuation of a few articles I wrote last year and the year before. It explains the same idea, but from another point of view: a small game that I started to write.

The thing is that the article isn't that good. Especially the example is too simple. Now it just hangs there. It doesn't move and I don't want to touch it anymore.

I do think that it about an idea that is really important. I will publish it after publishing this.