## Evolutionary vs Extensive: Refactorbot explained

Yesterday, I was discussing the Refactorbot with a friend of mine. Although he has been a developer for longer than me, he couldn’t grasp why the Refactorbot could not possibly test every solution and had to fall back to evolutionary design.

### A bit of maths

If you want to test every possible solution for a given design, you have to test every possible number of packages to hold classes and every possible combination of those classes in the packages.

If you have done some probabilities, statistics or enumerative combinatorics, you certainly will see where I am going to with that…

I have never been very good myself at mathematics, but my guess is that the number of possible solutions to our design problem can be represented with the following formula.

This is the sum of all combinations, without repetition, of *n* classes in *p* packages for *p* ranging from 1 (all classes in 1 packages) to *n* (each class in its own package):

with

*(for example, for 30 classes and 5 packages, this gives us 17100720 possibilities for this case alone)*

We then quickly understand that it would be impossible for a program to test each and every possible design for a sizeable system: the sheer number of possibilities would make it impossible to test in a lifetime!

### Refactorbot v0.0.0.3

If you want to have fun staring at an empty console while your CPU is hitting 100%, I have created an implementation of such a refactoring method.

You can download the Refactorbot v0.0.0.3 which includes this method. Good luck!

### The case for small systems

As much as my mathematical demonstration convinced me, myself and I, I had to tell you about my experiments on the design challenge (included in the test cases for the Refactorbot)…

It seems that for a small number of classes, say 8, the method is not only quick but very effective: I found the best ever solution to the design challenge!

ref model: 3 packages/8 classes (D=0.4444444444444445, R=0.8333333333333334)

-> p1 (3 classes): B A C

-> p2 (3 classes): D E F

-> p3 (2 classes): G H

```
```

`new model: 3 packages/8 classes (D=0.08333333333333333, R=1.0)`

-> p0 (4 classes): G C A B

-> p1 (3 classes): E H D

-> p2 (1 classes): F

However, don’t be fooled by this stunning result! It would become increasingly tedious to try and find a solution for bigger and bigger systems.

### The deal with evolutionary refactorings

I don’t think that the purpose of the Refactorbot is to come up with the best design - and I will write a piece soon about a definition of the BEST design - possible, but rather with a better design.

If we consider that the design of a software is its DNA, then you can probably try small improvements and keep them or discard them in an evolutionary fashion. Better design would emerge by trial and error; the Refactorbot should do just that.

What is perfect design anyway?

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March 4th, 2009 at 21:26

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