Manuel Pichler

The Overview Pyramid is used to visualize a complete software system in a really compact manner. Therefor it collects a set of metrics from the categories Inheritance, Coupling and Size & Complexity, and puts them into relation. The following figure shows the base structure of the Overview Pyramid[ML06].

Base structure of the Overview Pyramid.

Metrics used by the Overview Pyramid

The following three lists contain all the metrics, which the Overview Pyramid uses.

Size and Complexity

The category Size & Complexity contributes the greatest and mostly used set of software metrics.

NOP

The Number Of Packages metric counts the packages within the analyzed software system.

NOC

The Number Of Classes metric counts the declared classes within the analyzed software system.

NOM

The Number Of Methods metric counts all declared methods, which in this context means class methods and simple functions.

LOC

The Lines Of Code metric shows the number of executable source lines within the analyzed software system. To calculate this value PHP_Depend counts all non whitespace lines and all non comment lines.

CYCLO

Cyclomatic complexity is a software metric (measurement). It was developed by Thomas McCabe and is used to measure the complexity of a program. It directly measures the number of linearly independent paths through a program's source code.

Source: Wikipedia

Coupling

This group of metrics informs about the coupling between different program parts in the analyzed application.

CALLS

This metric count the number of distinct function- and method-calls. Distinct means that one and the same method-call within a function- or method-body is only counted once.

FANOUT

The FANOUT provides information on types referenced by classes and interfaces. It only counts those types that are not part of the same Inheritance branch.

Inheritance

Both metrics in this group deal with the use of Inheritance and give a general overview of the use of Inheritance within the analyzed system.

ANDC

The Average Number of Derived Classes metric describes the average of derived classes. In a system of ten classes an ANDC-value of 0.5 means, that every second class is derived from another class.

AHH

The Average Hierarchy Height metric is a average depth of the inheritance hierarchy. In a system of ten classes, a AHH-value of 1 can be interpreted in different ways, for example: Five classes inherit from five other classes within the analyzed application or five classes inherit from a single root class.

Structure of the Overview Pyramid

Now that we know all metrics used for the Overview Pyramid, it is time to replace the placeholders with the measured informations. The figure below shows the filled Overview Pyramid.

The filled Overview Pyramid

In a second step, the previously independent metrics are set into relation. Therefor we calculate the average values of individual value pairs, these computed values provide us with new informations about the distributions within the application.

The following example figure of the Overview Pyramid contains a computed value for the measured LOC and NOM metric which shows us, that in the average each operation has 25 lines of code. This value can be described as very high, especially when you consider that most systems contain a variety of simple operation, like Getter and Setter, in addition to the main application logic.

Computed average values in the Overview Pyramid

To take reasonable conclusions from the computed values one important part is still missing, an adequate set of reference values. Without reference values, that say what values are low, average or high, it is not possible to classify these results. The current version of PHP_Depend supports a single set of reference values, this set was taken from [ML96].

Reference values

With these reference values PHP_Depend can classify the computed results. PHP_Depend uses this information for the generation of colored backgrounds, so that the color already supports the categorization.

The complete Overview Pyramid

The benefit of the Overview Pyramid

Of course, the final question is, which advantages offers the Overview Pyramid?

The Overview Pyramid provides a simple and size indipendent way to get a first impression of a software system, and this without an expensive source code analysis. Thus the Overview Pyramid is an effective tool for a first cost estimate for an unknown system. With the help of this tool and know-how, an experienced developer will quickly get a first impression and will know what can be expected from the analyzed application. And this knowledge could be a good help during the planning phase of a new project.

Bibliography

• Object-Oriented Metrics in Practice
  Using Software Metrics to Characterize, Evaluate, and Improve the Design of Object-Oriented Systems
  Michele Lanza, Radu Marinescu, 2006 © Springer-Verlag Berlin Heidelberg, ISBN 978-3-540-24429-5

Here a short follow up for the PHP_Depend release announcement, with hints on using PHP_Depend.

First I must explain that PHP_Depend is working package oriented. This means it heavily relies on @package and @subpackage annotations within the comment blocks. For these annotations PHP_Depend applies the following rules, these rules are borrowed from PEAR. Or was it PHPDocumentor?

1. function: The @package/@subpackage annotation in the file level comment is used for all functions declared within this file.
2. class/interface: The @package/@subpackage annotation within the class or interface comment is used for that type.
3. everything else: Everything that doesn't match on one of the previous rules is part of the +global Package.

With default settings PHP_Depend suppresses the +global Package, because it expects that this package contains classes from external libraries, like Zend Framework or eZComponents. If you use PHP_Depend for a project with less or no @package/@subpackage annotation, you should use the cli option --bad-documentation, which forces PHP_Depend to keep all classes, interfaces and functions within the +global package. This option was introduced with PHP_Depend - 0.8.0beta3.

The second important hint is related to exported charts and images. If your operation system, image viewer or browser supports SVG, you should use this format. The reason for this decision is really simple, SVG provides tooltips and so you get most informations from the generated graphics.

For the Overview-Pyramid this means, that you get a tooltip for the used acronyms.

PHP_Depend Overview-Pyramid with acronym tooltip

And within the JDepend-chart you get the package name as tooltip.

PHP_Depend JDepend-Chart with package tooltip

I hope this short introduction helps you while you are trying out PHP_Depend. If you are already using the metrics feature of PHPUnit, you should try out the new --phpunit-xml log option in the latest SVN-Revision.