Kill the mutants
Pick a module. Write tests against it. The engine breaks the code underneath them one edit at a time and counts how many of those breaks your tests notice.
One or many
Choose a word based on a count, and get the boundary right.
Rounding money
Round to the cent and split a bill without losing a penny.
Overlapping ranges
Decide whether two half-open date ranges collide.
Paging through results
Slice a list into pages and describe where you are in it.
Empty, one, many
Chunk a list and de-duplicate it by a key.
Sorting by several keys
Build a comparator that sorts by one field, then another.
Truncating text
Shorten a string to a limit without cutting a word in half.
Building a cache key
Turn a query into a key, and don't let two different queries collide.
Deciding who may act
Answer whether a user may perform an action on a document.
Parsing a query string
Turn key=value pairs into an object, repeats and all.
Retrying with backoff
Retry a failing operation, waiting longer each time, then give up.
How the score works
Your score is killed mutants over killable mutants. Some mutants change the code without changing what it does — those are shown greyed, labelled equivalent, and left out of the denominator rather than quietly divided away. Line coverage is shown too, always beside the mutation score and never on its own.