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Measure twice, cut once
SOFTWARE METRICS

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@matthiasmullie
Software metrics

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Make software measurable
instead of relying on intuition.
Software metrics

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Presentation titleSoftware metrics

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Presentation titleSoftware metrics

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Presentation titleSoftware metrics

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Software metrics
COMPLEXITY

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Every code path adds complexity
Counts # of code paths
Software metrics » Complexity
Cyclomatic complexity

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• Analyze control flow
• Start at 1
• Increase at every decision path
(if, for, case, …)
Software metrics » Complexity
Cyclomatic complexity

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if ($user->isLoggedIn()) {
echo 'Welcome back, ' . $user->getName();
} else {
echo 'Hi there, stranger!';
}
Cyclomatic complexity = 2
Software metrics » Complexity
Cyclomatic complexity

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Every code path adds complexity
Counts # possible outcomes
Software metrics » Complexity
NPath complexity

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Software metrics » Complexity
NPath complexity
if ($user->getSalutation()) {
echo $user->getSalutation().' ';
}
if ($user->getName()) {
echo $user->getName();
} else {
echo 'anon';
}
Outcomes: • $salutation anon
• anon
• $salutation $name
• $name

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Every operation adds complexity
Software metrics » Complexity
Halstead measures

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• Analyze vocabulary
• Count amount of operators
(+, -, =, &&, ... and reserved words,
like if and for)
• Count amount of operands
(values, variables & function names)
Software metrics » Complexity
Halstead measures

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if ($user->isLoggedIn()) {
echo 'Welcome back, ' . $user->getName();
} else {
echo 'Hi there, stranger!';
}
Operands = 4 (4 unique)
Software metrics » Complexity
Halstead measures

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if ($user->isLoggedIn()) {
echo 'Welcome back, ' . $user->getName();
} else {
echo 'Hi there, stranger!';
}
Operators = 11 (7 unique)
Software metrics » Complexity
Halstead measures

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Engineered to match human analysis
Software metrics » Complexity
Maintainability index

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Combination of:
• Cyclomatic complexity
• Halstead measures
• Lines of code
Software metrics » Complexity
Maintainability index

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Metrics for matthiasmullie/minify, MatthiasMullieMinifyJS::stripWhitespace
See https://www.cauditor.org/matthiasmullie/minify/c17eb048daa44b43fa98bfa405147e77a040df76/metrics
Software metrics » Complexity
Maintainability index

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Questions?
Software metrics » Complexity

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Software metrics
COUPLING

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Amount of incoming dependencies
= other classes using this one
Common for core functionality
Software metrics » Coupling
Afferent coupling

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class Afferent
{
function __construct() {}
}
class Efferent
{
function __construct(Afferent $a) {}
}
Afferent: 1, Efferent: 0
Software metrics » Coupling
Afferent coupling

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Changes here may break things elsewhere
Must be stable & thoroughly tested
Software metrics » Coupling
Afferent coupling

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Amount of outgoing dependencies
= other classes being used
Common for controllers
Software metrics » Coupling
Efferent coupling

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class Afferent
{
function __construct() {}
}
class Efferent
{
function __construct(Afferent $a) {}
}
Afferent: 0, Efferent: 1
Software metrics » Coupling
Efferent coupling

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• Harder to read/maintain
• Harder to reuse
• Harder to test in isolation
• Brittle
Software metrics » Coupling
Efferent coupling

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Changes elsewhere may break things here
Probably violates single responsibility
principle
Software metrics » Coupling
Efferent coupling

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= efferent coupling / total coupling
= efferent coupling / (efferent + afferent)
Resilience to change
Software metrics » Coupling
Instability

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High efferent coupling
= likely to be impacted by changes in
= other classes
= not stable
Software metrics » Coupling
Instability

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Low afferent coupling
= change is easy because there isn’t much
= that depends on this
= not stable
Software metrics » Coupling
Instability

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Stable = good
… as long as you don’t need to change
the implementation!
Software metrics » Coupling
Instability

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Aim for extremes, make classes as stable
or unstable as possible!
Unstables can be refactored/decomposed
later. Better than having dependencies
everywhere.
Software metrics » Coupling
Instability

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Counts # of ancestors
Software metrics » Coupling
Depth of inheritance

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class Ancestor {}
DIT: 0
class Descendant1 extends Ancestor {}
DIT: 1
class Descendant2 extends Descendant1 {}
DIT: 2
Software metrics » Coupling
Depth of inheritance

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Inheritance is not bad!
• Code reuse
• Polymorphism
Software metrics » Coupling
Depth of inheritance

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But too much is!
• Less stable: change in ancestor can
affect descendants
• More complexity: methods of all
ancestors are inherited
Software metrics » Coupling
Depth of inheritance

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Questions?
Software metrics » Coupling

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WHY WOULD I CARE?
Software metrics

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Presentation titleSoftware metrics » Usage
Which would you rather maintain?

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Presentation titleSoftware metrics » Usage

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Software metrics » Usage
Complexity
This guy has
something going on!
Let’s see…

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It’s not too crazy! But:
• Recursion
• 3 loops
• 4 conditions
• Callback function
Software metrics » Usage
Complexity

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Method does too much:
• Data transformation
• Processing
• Cleanup
Hard to test & prove right!
Software metrics » Usage
Complexity

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Software metrics » Usage
Complexity
This guy
has a lot
going on…

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Presentation titleSoftware metrics

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Software metrics » Usage
Coupling
• Lots of outgoing
dependencies
• Some inheritance
Very brittle!

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Changes in any of these
could affect this class!
Software metrics » Usage
Coupling

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Software metrics » Usage
Refactoring
• 1000+ LOC class
• Pretty complex
• Very unstable
• Self-contained, barely
reusable

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Software metrics » ObservationsSoftware metrics » Usage
Reduced
complexity
everywhere
Decreased +
increased
stability

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Software metrics » Observations
Refactoring
• Greatly reduced
“monster class”
• Split into multiple
reusable pieces
Software metrics » Usage

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Questions?
Software metrics » Usage

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OBSERVATIONS
Software metrics

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Software metrics » Observations
Spikes!

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Complexity often caused by a few commits!
Watch out with new features & rewrites.
Software metrics » Observations
Spikes!

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Presentation titleSoftware metrics » Observations

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Software metrics » Observations
No evolution

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• No change over time
• No change across projects
Do developers have a “signature”?
Software metrics » Observations
No evolution

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Software metrics » Observations
Personalities?
2 guys working on same project:
• One was very fast (but messier)
• One was a lot more thorough
Who’s who?

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Software metrics » Observations
Personalities?

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Software metrics » Observations
Personalities?
M
essy
Thorough

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Software metrics » Observations
Personalities?
Can metrics show personality traits?
Please help me figure that out!
https://www.cauditor.org/user/feedback

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Presentation title

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Questions?
Software metrics

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mullie.eu
• mullie.eu/measuring-software-complexity
• mullie.eu/measuring-software-coupling
• cauditor.org/help/metrics
Software metrics
Resources