Create a Not expression

This will reduce the amount of expressions needed
and allow for complex expressions (within an OR
or an AND) to be negated.

src/Expression/ForClasses/Not.php

@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Expression\ForClasses;
+
+use Arkitect\Analyzer\ClassDescription;
+use Arkitect\Expression\Description;
+use Arkitect\Expression\Expression;
+use Arkitect\Rules\Violation;
+use Arkitect\Rules\ViolationMessage;
+use Arkitect\Rules\Violations;
+
+final class Not implements Expression
+{
+    /** @var Expression */
+    private $expression;
+
+    public function __construct(Expression $expression)
+    {
+        $this->expression = $expression;
+    }
+
+    public function describe(ClassDescription $theClass, string $because): Description
+    {
+        return new Description('must NOT (' . $this->expression->describe($theClass, '')->toString() . ')', $because);
+    }
+
+    public function evaluate(ClassDescription $theClass, Violations $violations, string $because): void
+    {
+        $newViolations = new Violations();
+        $this->expression->evaluate($theClass, $newViolations, $because);
+        if (0 !== $newViolations->count()) {
+            return;
+        }
+
+        $violations->add(Violation::create(
+            $theClass->getFQCN(),
+            ViolationMessage::selfExplanatory($this->describe($theClass, $because))
+        ));
+    }
+}

tests/Unit/Expressions/ForClasses/NotTest.php

@@ -0,0 +1,59 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses;
+
+use Arkitect\Analyzer\ClassDescription;
+use Arkitect\Analyzer\FullyQualifiedClassName;
+use Arkitect\Expression\ForClasses\IsInterface;
+use Arkitect\Expression\ForClasses\Not;
+use Arkitect\Rules\Violations;
+use PHPUnit\Framework\TestCase;
+
+class NotTest extends TestCase
+{
+    public function test_it_should_return_violation_error(): void
+    {
+        $isNotInterface = new Not(new IsInterface());
+        $classDescription = new ClassDescription(
+            FullyQualifiedClassName::fromString('HappyIsland'),
+            [],
+            [],
+            null,
+            false,
+            false,
+            true,
+            false,
+            false
+        );
+        $because = 'we want to add this rule for our software';
+        $violationError = $isNotInterface->describe($classDescription, $because)->toString();
+
+        $violations = new Violations();
+        $isNotInterface->evaluate($classDescription, $violations, $because);
+        self::assertNotEquals(0, $violations->count());
+
+        $this->assertEquals('must NOT (HappyIsland should be an interface) because we want to add this rule for our software', $violationError);
+    }
+
+    public function test_it_should_return_true_if_is_not_interface(): void
+    {
+        $isNotInterface = new Not(new IsInterface());
+        $classDescription = new ClassDescription(
+            FullyQualifiedClassName::fromString('HappyIsland'),
+            [],
+            [],
+            null,
+            false,
+            false,
+            false,
+            false,
+            false
+        );
+        $because = 'we want to add this rule for our software';
+        $violations = new Violations();
+        $isNotInterface->evaluate($classDescription, $violations, $because);
+        self::assertEquals(0, $violations->count());
+    }
+}