Object Mentor Blog: The Bloat at the Edge of Duplication Removal (The Orange Model)

The Bloat at the Edge of Duplication Removal (The Orange Model)

Michael Feathers — Tue, 17 Feb 2009 21:51:00 -0600

There are times when I sit down to work on some piece of code and I see a bit of duplication. I remove it. Then I see a bit more, and a bit more. When I’m done with all of my extractions, I look back at the code and notice that it’s increased in size about 20 or 30 percent. If I’m working with someone else, it’s often a shock for them. After all, we are used to thinking that code gets smaller when we remove duplication. Often it does, but not always.

Let’s take a look at an example:


private void adjustPaths() {
  pathNew = pathNew.removeFirstSegments(1).removeFileExtension()
                  .addFileExtension("class");
  pathOld = pathOld.removeFirstSegments(1).removeFileExtension()
                  .addFileExtension("class");
}

That’s six lines of code with obvious duplication. Now let’s remove it.


private void adjustPaths() {
  pathNew = adjustPath(pathNew);
  pathOld = adjustPath(pathOld);
}

private IPath adjustPath(IPath path) {
  return path.removeFirstSegments(1).removeFileExtension()
               .addFileExtension("class");
}

Now we have 9 lines.

What’s going on here? Well, when we extract duplication, we have to put it someplace, and that operation isn’t free. In all languages, there is a cost in space for separate functions. In Java, the cost includes method declaration line and lines for the curly braces. In Python, the cost is (again) the declaration line and some whitespace to separate the function from other functions.

Code is like an Orange

The mental model I have for this is that code can be separated into two parts. One part is structural. It gives form to the code. It consists of all of the cruft that we use to declare a method. The stuff inside is the meat — the pure computational code which is held in place by the structural bits.


/* cruft */ private IPath adjustPath(IPath path) { 
/* meat  */    return path.removeFirstSegments(1).removeFileExtension()
/* meat  */                .addFileExtension("class");
/* cruft */ }

This pattern is rather common. In fact, you’ll see it in most living things. The muscle tissue in your body (the meat) is held in place by a sheath called fascia. The same pattern occurs in fruit and vegetables. The pulp in an orange is held in place by fascia too, it’s the white part between the sections:

Here’s what duplication removal does, structurally. It allows you to pull out redundant bits of pulp from big sections, yielding smaller sections, but the side effect is that you end up with more fascia. Duplication removal increases the ratio of fascia to pulp. If the amount of pulp you are able to remove exceeds the size of the fascia you introduce, the net amount of code decreases, otherwise it might increase.

In general, I think that a high fascia to pulp ratio is better for maintenance. It gives us is a higher surface area to volume ratio for our code. This can enhance testability and make it easier to compose new software – we already have smaller more understandable pieces.

The cost of fascia, however, can be disturbing. In the normal formatting convention for C++ a new method will give you at least 5 lines. You need the line for the method declaration in the header, one for the definition in the source file, a line apiece for the beginning and ending braces and a newline to separate the method from next one. In Haskell, you can get away with 2 if the method is a one-liner and you forgo type declarations. The thing to notice is that it is never zero. There’s always some space overhead when we create a new method. The question is whether the amount of code that we remove as duplication swamps the addition overhead or not.

We can model duplication removal this way — the number of lines T’ after removing n stretches of x contiguous lines is:

T’ = T – (n – 1)x + c + n

We start with T, the original number of lines in the program and we remove n – 1 stretches of x lines. We leave in the nth x (as the method we extract) and we add in c which is the number of lines of fascia for a method in our language. We also add in n again since we will need to leave in a line for the call at each of the n sites where the code was originally duplicated.

According to the equation, we can end up with more code if c is large or if n and x are small. Aggressive refactoring with a c of 3 or 4 and and an n of 2 and an x of 1 will definitely increase code size.

Again, is this bad? No, I think it’s just something to be aware of. Removing duplication increases the ratio of fascia to pulp. To me, it’s only annoying when I’m working in a language with a high c value.

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