Results and the score

A mutant is your code with one small change in it, >= becoming > say. mutastic runs the tests that reach the changed line. If one fails, the tests noticed: the mutant is caught. If they all pass, nothing checks what that line does: the mutant is missed, and that’s a gap in the tests.

What happens to a mutant

Result	Symbol	Means
Caught	✓	A test failed with the mutant in.
Missed	✗	Every test that reaches it still passes: a gap in the tests.
Timed out	⏱	The tests stopped finishing in time, usually a loop that no longer ends. Counts as caught.
Not covered	○	No test reaches its line.
Skipped		Never run, for the reason given with it. Left out of the score.

Colour is never the only signal: every result also has its symbol or word. With --vocabulary=classic (or MUTASTIC_VOCABULARY=classic) mutastic says killed and survived instead of caught and missed. The JSON report and the event stream always use the standard names: Killed, Survived, Timeout, NoCoverage and Ignored.

The score

The score is the share of the mutants that ran which the tests noticed:

score = (caught + timed out) / (every mutant − skipped) × 100

A mutant no test reaches counts against the score, as one no test checks does. A run passes when the score reaches the threshold, 80 unless --threshold says otherwise; a run with nothing to score says so, rather than claiming 100%.

Skipped mutants

Some changes would tell you nothing, so mutastic doesn’t run them. Each skip says why, with a kind first:

Kind	Means	For example
arid:	Code whose change no test should have to notice	A call that only writes output, a trivial getter or setter, toString() and hashCode()
generated:	Code the compiler wrote, not you	Closing a try-with-resources resource, the hashing in a switch on a String
equivalent:	The change does nothing	A method that already returns its type’s default, “changed” to return it
user:	Your decision	@DoNotMutate, a rule in mutastic.yaml, or mutastic ignore

@DoMutate on a method or class brings back what the arid filters skip there, and MUTASTIC_ARID=audit runs every arid mutant anyway, to check the filters.

mutastic never mutates constructors, static initialisers, float and double arithmetic, an enum’s values(), or a record’s generated equals, hashCode and toString.