mutant

mutant
======

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## What is Mutant?

**AI writes your code. AI writes your tests. But who tests the tests?**

Copilot, Claude, and ChatGPT generate code faster than humans can review it. They'll even
write tests that pass. But passing tests aren't the same as *meaningful* tests.

Mutant is mutation testing for Ruby. It systematically modifies your code and verifies your
tests actually catch each change.

The more code AI writes for you, the more you need verification you can trust.

## What is an Alive Mutation?

Each surviving (alive) mutation is a call to action with exactly one of two options:

- **Keep the mutated code** - your tests already specify the correct semantics, and the
  original code is redundant. Accept the mutation as a simplification.
- **Add the missing test** - the original code is correct, but the tests don't verify the
  behavior the mutation removed.

## Author

Mutant was created and is developed by [Markus Schirp](https://schirp-dso.com).
It is the subject of [IEEE-published research](https://ieeexplore.ieee.org/document/7107453/)
and is included in the [Trail of Bits Ruby Security Field Guide](https://trailofbits.github.io/rubysec/mutant/index.html).

## Quick Start

```ruby
# lib/person.rb
class Person
  def initialize(age:)
    @age = age
  end

  def adult?
    @age >= 18
  end
end
```

```ruby
# spec/person_spec.rb
RSpec.describe Person do
  describe '#adult?' do
    it 'returns true for age 19' do
      expect(Person.new(age: 19).adult?).to be(true)
    end

    it 'returns false for age 17' do
      expect(Person.new(age: 17).adult?).to be(false)
    end
  end
end
```

Tests pass. But run mutant:

```bash
gem install mutant-rspec
mutant run --use rspec --usage opensource --require ./lib/person 'Person#adult?'
```

Mutant finds a surviving mutation indicating a shallow test:

```diff
 def adult?
-  @age >= 18
+  @age > 18
 end
```

Your tests don't cover `age == 18`. The mutation from `>=` to `>` doesn't break them.

This is just one of many mutation operators. Mutant also mutates arithmetic, logical,
bitwise operators, removes statements, modifies return values, and more.

A full working example is available in the [quick_start](quick_start/) directory.

## Session History

Mutant records every run to `.mutant/results/`, and keeps the 100 most recent.
You can recall past results without re-running mutation testing:

```bash
# List past sessions (most recent first)
mutant session list

#