"Mutation Testing" by Igor Gajowiak

Mutation testing
Igor Gajowiak
July 2019
Igor Gajowiak 1 / 13

Why do we test our code ?
To ensure that code works properly
To have regression base for future development

What does test the tests ?
Code coverage index
Mutation testing ! ! !

Mutation testing
1 Prerequisites : Code base, test suite.
2 Run the test suite
3 Introduce small change to the code base - mutation
4 Run test suite
5 If the ﬁrst test run succeeds and the second one not, then
it’s great !

Examples
Before :
1 f o r ( int i = 0; i < 100; ++i )
2 {
3 . . .
4 }
After :
1 f o r ( int i = 0; i > 100; ++i )
2 {
3 . . .
4 }

Examples
Before :
1 i f ( a && b)
2 {
3 c = 1;
4 }
After :
1 i f ( a | | b)
2 {
3 c = 1;
4 }

Result interpretation
Survived - tests did not detect change
Killed - tests detected change in the code
Incompetent - mutation produced ﬂawed code

What is the goal ?
KILL THEM ALL

Code coverage vs mutation testing
Was a line executed ?
VS
Was a functionality veriﬁed ?

Examples of mutations
+ −
∗ /
< >
<
+ + i − − i
! = ==
continue break
|| &&
true false

It is time-consuming process
EXECUTION_TIME =
NUMBER_OF_MUTANTS * TEST_SUITE_EXECUTION_TIME
Solutions :
Parallelize
Write fast tests

Detecting incompetent mutants
1 while ( true )
2 {
3 . . .
4 i f ( condition )
5 {
6 break ;
7 }
8 . . .
9 }
1 while ( true )
2 {
3 . . .
4 i f ( condition )
5 {
6 continue ;
7 }
8 . . .
9 }

Stryker
Stryker - https://stryker-mutator.io
Supported mutations - https://github.com/stryker-
mutator/stryker-net/blob/master/docs/Mutators.md

"Mutation Testing" by Igor Gajowiak

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