Keep Cognitive Complexity Down

Cognitive complexity tells us, how difficult code is to understand by a reader.

How is cognitive complexity measured?

function get_words_from_number(int $number): string
{
    $amountInWords = '';

    if ($number === 1) {            // + 1
        $amountInWords = 'one';
    } elseif ($number === 2) {      // + 1
        $amountInWords = 'couple';
    } elseif ($number === 3) {      // + 1
        $amountInWords = 'a few';
    } else {                        // + 1
        $amountInWords = 'a lot';
    }

    return $amountInWords;
}

This function uses nesting, conditions and continue back and forth. It's hard to read and results in cognitive complexity of 4.

How to keep cognitive complexity on 1? Read Keep Cognitive Complexity Low with PHPStan post to learn it.

Install

composer require tomasvotruba/cognitive-complexity --dev

The package is available on PHP 7.2-8.1 versions in tagged releases.

Usage

With PHPStan extension installer, everything is ready to run.

Enable each item on their own with simple configuration:

# phpstan.neon
parameters:
    cognitive_complexity:
        class: 50
        function: 8

Detect complex Class Dependency Trees

In classes like controllers, Rector rules, PHPStan rules or other services of specific type, the complexity can be hidden in the __construct() dependencies. Simple class with 10 dependencies is more complex than complex class with 2 dependencies.

That's why there is a rule to detect these dependency trees. It checks:

• complexity of current class
• constructor dependencies and their class complexity together

Final number is compared and used as a final complexity:

# phpstan.neon
parameters:
    cognitive_complexity:
        dependency_tree: 150
        dependency_tree_types:
            # only these explicit types are checked, nothing else
            - Rector\Contract\Rector\RectorInterface

Happy coding!