[gecode-users] How do you debug your progam?

[Guido Tack]

Hi Roberto.

> I'm not referring to the C++ code, but to the set of constraint that
> describe your problem:
> I don't know if my question will be relevant for most of you, anyway,
> I would ask how do you debug your constraint program?

As you have already noticed, debugging constraint programs requires  
different tools than debugging C++ programs.

> Of course, from a theoretical point of view, thinking before doing is
> a best-practice, but a bug could always occur and maybe you are not
> able to find it even after a deep analysis.

In many cases, there isn't even a real bug in your model, but it just  
doesn't propagate enough and the search space is huge.

> [...]
>
> Any idea where to look?
>
> I would also ask if it is somewhat possible to "follow" the variable
> assignment procedure step-by-step (of course, on very simple example
> with few variables) in order to "visualize" the propagation of
> constraints and the backtracking points.

This is basically what one would use a graphical tool such as the  
Mozart Explorer for. You get a visualization of (a part of) the  
search tree, and you can click on the nodes and get information about  
the state of the search at that point.
We don't have an explorer for Gecode yet, but we have one for our  
Java interface Gecode/J. This is not officially released yet, but  
quite functional. It has been used for teaching CP courses, and the  
documentation and source code are available from the course pages, e.g.:
http://www.imit.kth.se/courses/2G1515/software.html

We are setting up web pages for Gecode/J at
http://www.gecode.org/gecodej

These are pretty preliminary right now, but you can expect to find  
more content in a few days.

The downside of using Gecode/J is that you would have to re-model  
your problem in Java. This is however not too hard, as the interface  
is nearly the same as in C++.

> In other words, I would like to print out all the variable assignment,
> one by one, along with a message saying "Hey, I'm backtracking now!"
> or something like that.

If you just want some debugging output during search, and you don't  
want to switch to Java for debugging, you can write your own  
instrumented search engine. Take one of the available search engines  
as a starting point, and simply add debugging output at the  
appropriate places.

For instance, take the file gecode/search/dfs.icc and add some  
debugging output to the DfsEngine::explore(void) function - you'll  
immediately see the relevant parts.

> Do you think it would be possible (easily) with Gecode?

Gecode offers quite some flexibility, as it is easy to write your own  
search engines. After all, something like the Gecode/J explorer is  
written completely on top of the existing abstractions for search in  
Gecode. I hope we can offer more debugging tools in the future, but  
currently we are focusing on other issues.

Hope this helps a bit,

	Guido