Add method to compare form ID instances with each other

CHANGELOG.md

@@ -8,6 +8,7 @@ to [Semantic Versioning](http://semver.org/) and [Keep a CHANGELOG](http://keepa
 [2.0.0]: https://github.com/icehawk/forms/compare/v1.0.0...v2.0.0
 
 * Switched from Vagrant to docker-compose development environment, see [Contribution Guide](./.github/CONTRIBUTING.md)
+* Add method `FormIdInterface#equals( FormIdInterface $other ) : bool` in order to compare form ID instances with each other
 
 ### Backwards incompatible changes (BC breaks)
 

src/FormId.php

@@ -44,4 +44,9 @@ public function jsonSerialize() : string
 	{
 		return $this->toString();
 	}
+
+	public function equals( FormIdInterface $other ) : bool
+	{
+		return $this::class === $other::class && $this->toString() === $other->toString();
+	}
 }

src/Interfaces/FormIdInterface.php

@@ -8,4 +8,6 @@
 interface FormIdInterface extends Stringable, JsonSerializable
 {
 	public function toString() : string;
+
+	public function equals( FormIdInterface $other ) : bool;
 }

tests/Unit/FormIdTest.php

@@ -44,4 +44,26 @@ public function testInheritance() : void
 
 		self::assertSame( 'testForm', (string)$formId );
 	}
+
+	public function testEquals() : void
+	{
+		$formIdUnit  = FormId::new( 'unit' );
+		$formIdUnit2 = FormId::new( 'unit' );
+		$formIdTest  = FormId::new( 'test' );
+
+		$customFormIdUnit = TestFormId::new( 'unit' );
+		$customFormIdTest = TestFormId::new( 'test' );
+
+		self::assertTrue( $formIdUnit->equals( $formIdUnit2 ) );
+		self::assertTrue( $formIdUnit2->equals( $formIdUnit ) );
+
+		self::assertFalse( $formIdUnit->equals( $formIdTest ) );
+		self::assertFalse( $formIdTest->equals( $formIdUnit ) );
+
+		self::assertFalse( $formIdUnit2->equals( $formIdTest ) );
+		self::assertFalse( $formIdTest->equals( $formIdUnit2 ) );
+
+		self::assertFalse( $formIdUnit->equals( $customFormIdUnit ) );
+		self::assertFalse( $formIdTest->equals( $customFormIdTest ) );
+	}
 }