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mutant-rspec

Before starting with mutant its recommended to understand the nomenclature.

Setup

To add mutant to your rspec code base you need to:

  1. Add mutant-rspec as development dependency to your Gemfile or .gemspec

    This may look like:

    # A gemfile
    gem 'mutant-rspec'
  2. Run mutant against the rspec integration via the --use rspec flag.

Run through example

This uses mbj/auom a small library that has 100% mutation coverage. Its tests execute very fast and do not have any IO so its a good playground example to interact with.

All the setup described above is already done.

git clone https://github.com/mbj/auom
cd auom
bundle install # gemfile references mutant-rspec already
bundle exec mutant --include lib --require auom --use rspec -- 'AUOM*'

This prints a report like:

Mutant environment:
Matcher:         #<Mutant::Matcher::Config match_expressions: [AUOM*]>
Integration:     Mutant::Integration::Rspec
Jobs:            8
Includes:        ["lib"]
Requires:        ["auom"]
Subjects:        23
Mutations:       1003
Results:         1003
Kills:           1003
Alive:           0
Runtime:         51.52s
Killtime:        200.13s
Overhead:        -74.26%
Mutations/s:     19.47
Coverage:        100.00%

Now lets try adding some redundant (or unspecified) code:

patch -p1 <<'PATCH'
--- a/lib/auom/unit.rb
+++ b/lib/auom/unit.rb
@@ -170,7 +170,7 @@ module AUOM
     # TODO: Move defaults coercions etc to .build method
     #
     def self.new(scalar, numerators = nil, denominators = nil)
-      scalar = rational(scalar)
+      scalar = rational(scalar) if true

       scalar, numerators   = resolve([*numerators], scalar, :*)
       scalar, denominators = resolve([*denominators], scalar, :/)
PATCH

Running mutant again prints the following:

evil:AUOM::Unit.new:/home/mrh-dev/example/auom/lib/auom/unit.rb:172:45e17
@@ -1,9 +1,7 @@
 def self.new(scalar, numerators = nil, denominators = nil)
-  if true
-    scalar = rational(scalar)
-  end
+  scalar = rational(scalar)
   scalar, numerators = resolve([*numerators], scalar, :*)
   scalar, denominators = resolve([*denominators], scalar, :/)
   super(scalar, *[numerators, denominators].map(&:sort)).freeze
 end
-----------------------
Mutant configuration:
Matcher:         #<Mutant::Matcher::Config match_expressions: [AUOM*]>
Integration:     Mutant::Integration::Rspec
Jobs:            8
Includes:        ["lib"]
Requires:        ["auom"]
Subjects:        23
Mutations:       1009
Results:         1009
Kills:           1008
Alive:           1
Runtime:         50.93s
Killtime:        190.09s
Overhead:        -73.21%
Mutations/s:     19.81
Coverage:        99.90%

This shows mutant detected the alive mutation. Which shows the conditional we deliberately added above is redundant.

Feel free to also remove some tests. Or do other modifications to either test or code.

Test-Selection

Mutation testing is slow. The key to making it fast is selecting the correct set of tests to run. Mutant currently supports the following built-in strategy for selecting tests/specs:

Mutant uses the "longest rspec example group descriptions prefix match" to select the tests to run.

Example for a subject like Foo::Bar#baz it will run all example groups with description prefixes in Foo::Bar#baz, Foo::Bar and Foo. The order is important, so if mutant finds example groups in the current prefix level, these example groups must kill the mutation.

RAILS_ENV=test bundle exec mutant -r ./config/environment --use rspec User
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