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Coding
Coding
(2 posts)
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Code Quality
Beyond test coverage: nine numbers, each worked out by hand
Coverage
78% → 100%
same new tests, a small jump
Mutation score
57% → 97%
same new tests, a big jump
Coverage says the code ran.
Mutation score says a bug gets caught.
7 more numbers
Cyclomatic
Cognitive
CRAP
Hotspots
Duplication
Coupling
Code smells
Code Quality Beyond Test Coverage
Coding
Sep 18, 2026 · 28 min
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Design Principles
Seven change requests reach one small shop
CHANGE REQUEST
PRINCIPLE
WARNING NUMBER
1
Senior age moves to 65
DRY
duplication
2
Add a coupon code
KISS
cognitive complexity
3
Euro launch that never came
YAGNI
dead code
4
Change the invoice layout
Separation of concerns
churn × complexity
5
Switch the email provider
Coupling
the import graph
6
Add a tax rule per country
Single responsibility
cyclomatic complexity
7
Make discounts configurable
Level of abstraction
lines for the same rules
Design Principles Behind the Code Quality Numbers
Coding
Sep 18, 2026 · 18 min