PHP Depend - Software Metrics for PHP

Overview Pyramid

Contents

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 [1] .

Base structure of the Overview Pyramid.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
The Cyclomatic Complexity number [2] is a software metric (measurement). It was already developed in 1976 by Thomas J. McCabe and is used to calculate the complexity of a program. It directly measures the number of linearly independent paths through a program's source code.

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 PyramidThe 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 PyramidComputed 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 [1] .

Reference values

Metric

Low

Average

High

CYCLO/LOC

0.16

0.20

0.24

LOC/NOM

7

10

13

NOM/NOC

4

7

10

NOC/NOP

6

17

26

CALLS/NOM

2.01

2.62

3.2

FANOUT/CALLS

0.56

0.62

0.68

ANDC

0.25

0.41

0.57

AHH

0.09

0.21

0.32

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 PyramidThe 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.

1
Object-Oriented Metrics in Practice
© Springer-Verlag Berlin Heidelberg; ISBN 978-3-540-24429-5; Using Software Metrics to Characterize, Evaluate, and Improve the Design of Object-Oriented Systems; Michele Lanza, Radu Marinescu; 2006
2
http://www.literateprogramming.com/mccabe.pdf
IEEE Transactions on Software Enginerring; A Complexity Measure; Thomas J. McCabe; 1976

Abstraction Instability Chart

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