How to convert Directed Acyclic Graph (DAG) to Tree

Question

I have been looking for C# examples to transform a DAG into a Tree.

Does anyone have an examples or pointers in the right direction?

Clarification Update

I have a graph that contains a list of modules that my application is required to load. Each module has a list of modules it depends on. For example here are my modules, A, B C, D and E

• A has no dependencies
• B depends on A, C and E
• C depends on A
• D depends on A
• E depends on C and A

I want resolve dependencies and generate a tree that looks like this...

--A

--+--B

-----+--C

---------+--D

--+--E

Topological Sort

Thanks for the information, if I perform a Topological sort and reverse the output i will have the following order

• A
• B
• C
• D
• E

I want to maintain the hierarchical structure so that my modules are loaded into the correct context, for example... module E should be in the same container as B

Thanks

Rohan

Answer 1

There's the graph theoretical answer and the programmer's answer to this. I assume you can handle the programmers part yourself. For the graph theorethical answer:

• A DAG is a set of modules where it never happens that A needs B, and at the same time, B (or one of the modules B needs) needs A, in modules-speak: no circular dependency. I've seen circular dependencies happen (search the Gentoo forums for examples), so you can't even be 100% sure you have a DAG, but let's assume you have. It it not very hard to do a check on circular dependencies, so I'd recommend you do so somewhere in your module loader.
• In a tree, something that never can happen is that A depends on B and C and that both B and C depend on D (a diamond), but this can happen in a DAG.
• Also, a tree has exactly one root node, but a DAG can have multiple "root" nodes (i.e. modules that nothing depends on). For example a program like the GIMP, the GIMP program will be the root node of the set of modules, but for GENTOO, almost any program with a GUI is a "root" node, while the libraries etc are dependencies of them. (I.E. both Konqueror and Kmail depend on Qtlib, but nothing depends on Konqueror, and nothing depends on Kmail)

The Graph theorethical answer to your question, as others pointed out, is that a DAG can't be converted to a tree, while every tree is a DAG.

However, (high-level programmers answer) if you want the tree for graphical representations, you're only interested in the dependencies of a specific module, not what's depending on that module. Let me give an example:

A depends on B and C
B depends on D and E
C depends on D and F

I can't show this as an ASCII-art tree, for the simple reason that this can't be converted into a tree. However, if you want to show what A depends on, you can show this:

A
+--B
|  +--D
|  +--E
+--C
   +--D
   +--F

As you see, you get double entries in your tree - in this case "only" D but if you do an "expand all" on the Gentoo tree, I guarantee you that your tree will have at least 1000 times the amount of nodes as there are modules. (there are at least 100 packages that depend on Qt, so everything Qt depends on will be present at least 100 times in the tree).

If you have a "large" or "complex" tree, It might be best to expand the tree dynamically, not in advance, otherwise you might have a very memory-intensive process.

The disadvantage of the tree above is that if you click open B, then D, you see that A and B depend on D, but not that also C depends on D. However, depending on your situation, this might not be important at all - if you maintain a list of loaded modules, on loading C you see that you have loaded D already, and it doesn't matter it wasn't loaded for C, but for B. It is loaded, that's all that matters. If you dynamically maintain what directly depends on a certain module, you can handle the opposite problem (unloading) as well.

However, what you can't do with a tree is what's in your final sentence: preserve topological order, i.e. if B gets loaded in the same container as C, you'll never get to load C in the same container as well. Or you might have to be put up with putting everything in one container (not that I fully understand what you mean with "loading into the same container")

Good luck!