Create a IsA expression

This will allow for testing if a class extends or
implements another code unit, anywhere in the inheritance tree.

src/Expression/ForClasses/IsA.php

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+<?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 IsA implements Expression
+{
+    /** @var string[] */
+    private $allowedFqcnList;
+
+    public function __construct(string ...$allowedFqcnList)
+    {
+        $this->allowedFqcnList = $allowedFqcnList;
+    }
+
+    public function describe(ClassDescription $theClass, string $because): Description
+    {
+        $allowedFqcnList = implode(', ', $this->allowedFqcnList);
+
+        return new Description("should inherit from one of: $allowedFqcnList", $because);
+    }
+
+    public function evaluate(ClassDescription $theClass, Violations $violations, string $because): void
+    {
+        if (!$this->isA($theClass, ...$this->allowedFqcnList)) {
+            $violation = Violation::create(
+                $theClass->getFQCN(),
+                ViolationMessage::selfExplanatory($this->describe($theClass, $because))
+            );
+
+            $violations->add($violation);
+        }
+    }
+
+    private function isA(ClassDescription $theClass, string ...$allowedFqcnList): bool
+    {
+        foreach ($allowedFqcnList as $allowedFqcn) {
+            if (\is_a($theClass->getFQCN(), $allowedFqcn, true)) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+}

tests/Unit/Expressions/ForClasses/DummyClasses/Banana.php

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+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses;
+
+class Banana implements FruitInterface
+{
+
+}

tests/Unit/Expressions/ForClasses/DummyClasses/CavendishBanana.php

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+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses;
+
+class CavendishBanana extends Banana
+{
+
+}

tests/Unit/Expressions/ForClasses/DummyClasses/Dog.php

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+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses;
+
+final class Dog
+{
+
+}

tests/Unit/Expressions/ForClasses/DummyClasses/DwarfCavendishBanana.php

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+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses;
+
+final class DwarfCavendishBanana extends CavendishBanana
+{
+
+}

tests/Unit/Expressions/ForClasses/DummyClasses/FruitInterface.php

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+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses;
+
+interface FruitInterface
+{
+
+}

tests/Unit/Expressions/ForClasses/IsATest.php

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+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses;
+
+use Arkitect\Analyzer\ClassDescription;
+use Arkitect\Analyzer\FullyQualifiedClassName;
+use Arkitect\Expression\ForClasses\IsA;
+use Arkitect\Rules\Violations;
+use Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses\Banana;
+use Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses\CavendishBanana;
+use Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses\Dog;
+use Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses\DwarfCavendishBanana;
+use Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses\FruitInterface;
+use PHPUnit\Framework\TestCase;
+
+final class IsATest extends TestCase
+{
+    /**
+     * @test
+     */
+    public function it_should_have_no_violation_when_it_implements(): void
+    {
+        $interface = FruitInterface::class;
+        $isA = new IsA($interface);
+        $classDescription = new ClassDescription(
+            FullyQualifiedClassName::fromString(CavendishBanana::class),
+            [],
+            [FullyQualifiedClassName::fromString($interface)],
+            null,
+            false,
+            false,
+            false,
+            false,
+            false
+        );
+
+        $violations = new Violations();
+        $isA->evaluate($classDescription, $violations, '');
+
+        self::assertEquals(0, $violations->count());
+    }
+
+    /**
+     * @test
+     */
+    public function it_should_have_no_violation_when_it_extends(): void
+    {
+        $class = Banana::class;
+        $isA = new IsA($class);
+        $classDescription = new ClassDescription(
+            FullyQualifiedClassName::fromString(DwarfCavendishBanana::class),
+            [],
+            [],
+            FullyQualifiedClassName::fromString($class),
+            false,
+            false,
+            false,
+            false,
+            false
+        );
+
+        $violations = new Violations();
+        $isA->evaluate($classDescription, $violations, '');
+
+        self::assertEquals(0, $violations->count());
+    }
+
+    /**
+     * @test
+     */
+    public function it_should_have_violation_if_it_doesnt_extend_nor_implement(): void
+    {
+        $interface = FruitInterface::class;
+        $class = Banana::class;
+        $isA = new IsA($class, $interface);
+        $classDescription = new ClassDescription(
+            FullyQualifiedClassName::fromString(Dog::class),
+            [],
+            [],
+            null,
+            false,
+            false,
+            false,
+            false,
+            false
+        );
+
+        $violations = new Violations();
+        $isA->evaluate($classDescription, $violations, '');
+
+        self::assertEquals(1, $violations->count());
+        self::assertEquals(
+            "should inherit from one of: $class, $interface",
+            $isA->describe($classDescription, '')->toString()
+        );
+    }
+}