From bda5ad0f83b5e5709f3d24b938ab5a0686b549a2 Mon Sep 17 00:00:00 2001
From: Felix Arntz <felixarntz@google.com>
Date: Tue, 31 Oct 2023 13:20:46 -0500
Subject: [PATCH 1/8] Implement Check_Collection to separate concerns for check
 filtering from Check_Repository.

---
 includes/Checker/Abstract_Check_Runner.php    |   4 +-
 includes/Checker/Check_Collection.php         |  49 +++++
 includes/Checker/Check_Repository.php         |   7 +-
 includes/Checker/Default_Check_Collection.php | 187 ++++++++++++++++++
 includes/Checker/Default_Check_Repository.php |  43 ++--
 .../Checker/Check_Categories_Tests.php        |   3 +-
 .../Default_Check_Repository_Tests.php        |  29 ++-
 7 files changed, 268 insertions(+), 54 deletions(-)
 create mode 100644 includes/Checker/Check_Collection.php
 create mode 100644 includes/Checker/Default_Check_Collection.php

diff --git a/includes/Checker/Abstract_Check_Runner.php b/includes/Checker/Abstract_Check_Runner.php
index ce4b89af4..5651ad67e 100644
--- a/includes/Checker/Abstract_Check_Runner.php
+++ b/includes/Checker/Abstract_Check_Runner.php
@@ -368,7 +368,9 @@ final public function get_checks_to_run() {
 			$check_flags = $check_flags | Check_Repository::INCLUDE_EXPERIMENTAL;
 		}
 
-		$checks = $this->check_repository->get_checks( $check_flags, $check_slugs );
+		$checks = $this->check_repository->get_checks( $check_flags )
+			->include( $check_slugs )
+			->to_map();
 
 		// Filters the checks by specific categories.
 		$categories = $this->get_categories();
diff --git a/includes/Checker/Check_Collection.php b/includes/Checker/Check_Collection.php
new file mode 100644
index 000000000..f1874f4ba
--- /dev/null
+++ b/includes/Checker/Check_Collection.php
@@ -0,0 +1,49 @@
+<?php
+/**
+ * Class WordPress\Plugin_Check\Checker\Check_Collection
+ *
+ * @package plugin-check
+ */
+
+namespace WordPress\Plugin_Check\Checker;
+
+use ArrayAccess;
+use Countable;
+use IteratorAggregate;
+
+/**
+ * Check Collection interface.
+ *
+ * @since n.e.x.t
+ */
+interface Check_Collection extends ArrayAccess, Countable, IteratorAggregate {
+
+	/**
+	 * Returns the raw indexed array representation of this collection.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return array The indexed array of check objects.
+	 */
+	public function to_array(): array;
+
+	/**
+	 * Returns the raw map of check slugs and their check objects as a representation of this collection.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return array Map of `$check_slug => $check_obj` pairs.
+	 */
+	public function to_map(): array;
+
+	/**
+	 * Returns a new check collection containing the subset of checks based on the given check slugs.
+	 *
+	 * If the given list is empty, the same collection will be returned without any change.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return Check_Collection New check collection, effectively a subset of this one.
+	 */
+	public function include( array $check_slugs ): Check_Collection;
+}
diff --git a/includes/Checker/Check_Repository.php b/includes/Checker/Check_Repository.php
index 1c1141093..7c757d04b 100644
--- a/includes/Checker/Check_Repository.php
+++ b/includes/Checker/Check_Repository.php
@@ -63,9 +63,8 @@ public function register_check( $slug, Check $check );
 	 *
 	 * @since n.e.x.t
 	 *
-	 * @param int   $flags       The check type flag.
-	 * @param array $check_slugs An array of check slugs to return.
-	 * @return array An indexed array of check instances.
+	 * @param int $flags The check type flag.
+	 * @return Check_Collection Check collection providing an indexed array of check instances.
 	 */
-	public function get_checks( $flags = self::TYPE_ALL, array $check_slugs = array() );
+	public function get_checks( $flags = self::TYPE_ALL );
 }
diff --git a/includes/Checker/Default_Check_Collection.php b/includes/Checker/Default_Check_Collection.php
new file mode 100644
index 000000000..f5765fbd3
--- /dev/null
+++ b/includes/Checker/Default_Check_Collection.php
@@ -0,0 +1,187 @@
+<?php
+/**
+ * Class WordPress\Plugin_Check\Checker\Default_Check_Collection
+ *
+ * @package plugin-check
+ */
+
+namespace WordPress\Plugin_Check\Checker;
+
+use ArrayIterator;
+use Exception;
+use Traversable;
+
+/**
+ * Default Check Collection class.
+ *
+ * @since n.e.x.t
+ */
+class Default_Check_Collection implements Check_Collection {
+
+	/**
+	 * Map of `$check_slug => $check_obj` pairs.
+	 *
+	 * @since n.e.x.t
+	 * @var array
+	 */
+	private $checks;
+
+	/**
+	 * List of check slugs, in the same order as `$checks` - effectively the keys of that array.
+	 *
+	 * @since n.e.x.t
+	 * @var array
+	 */
+	private $slugs;
+
+	/**
+	 * Constructor.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param array $checks Map of `$check_slug => $check_obj` pairs for the collection.
+	 */
+	public function __construct( array $checks ) {
+		$this->checks = $checks;
+		$this->slugs  = array_keys( $this->checks );
+	}
+
+	/**
+	 * Returns the raw indexed array representation of this collection.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return array The indexed array of check objects.
+	 */
+	public function to_array(): array {
+		return array_values( $this->checks );
+	}
+
+	/**
+	 * Returns the raw map of check slugs and their check objects as a representation of this collection.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return array Map of `$check_slug => $check_obj` pairs.
+	 */
+	public function to_map(): array {
+		return $this->checks;
+	}
+
+	/**
+	 * Returns a new check collection containing the subset of checks based on the given check slugs.
+	 *
+	 * If the given list is empty, the same collection will be returned without any change.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return Check_Collection New check collection, effectively a subset of this one.
+	 */
+	public function include( array $check_slugs ): Check_Collection {
+		// Return unmodified collection if no check slugs to limit to are given.
+		if ( ! $check_slugs ) {
+			return $this;
+		}
+
+		$check_slugs = array_flip( $check_slugs );
+
+		$checks = array();
+		foreach ( $this->checks as $slug => $check ) {
+			if ( ! isset( $check_slugs[ $slug ] ) ) {
+				continue;
+			}
+
+			$checks[ $slug ] = $check;
+		}
+
+		return new static( $checks );
+	}
+
+	/**
+	 * Counts the checks in the collection.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return int Number of checks in the collection.
+	 */
+	public function count(): int {
+		return count( $this->checks );
+	}
+
+	/**
+	 * Returns an iterator for the checks in the collection.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @return Traversable Checks iterator.
+	 */
+	public function getIterator(): Traversable {
+		return new ArrayIterator( $this->checks );
+	}
+
+	/**
+	 * Checks whether a check exists with the given slug or index.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param string|int $offset Either a check slug (string) or index (integer).
+	 * @return bool True if a check exists at the given slug or index, false otherwise.
+	 */
+	public function offsetExists( $offset ) {
+		if ( is_string( $offset ) ) {
+			return isset( $this->checks[ $offset ] );
+		}
+
+		return isset( $this->slugs[ $offset ] );
+	}
+
+	/**
+	 * Retrieves the check with the given slug or index.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param string|int $offset Either a check slug (string) or index (integer).
+	 * @return Check|null Check with the given slug or index, or null if it does not exist.
+	 */
+	public function offsetGet( $offset ) {
+		if ( is_string( $offset ) ) {
+			if ( isset( $this->checks[ $offset ] ) ) {
+				return $this->checks[ $offset ];
+			}
+			return null;
+		}
+
+		if ( isset( $this->slugs[ $offset ] ) ) {
+			return $this->checks[ $this->slugs[ $offset ] ];
+		}
+
+		return null;
+	}
+
+	/**
+	 * Sets a check in the collection.
+	 *
+	 * This method does nothing as the collection is read-only.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param string|int $offset Either a check slug (string) or index (integer).
+	 * @param mixed      $value  Value to set.
+	 */
+	public function offsetSet( $offset, $value ) {
+		// Not implemented as this is a read-only collection.
+	}
+
+	/**
+	 * Removes a check from the collection.
+	 *
+	 * This method does nothing as the collection is read-only.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param string|int $offset Either a check slug (string) or index (integer).
+	 */
+	public function offsetUnset( $offset ) {
+		// Not implemented as this is a read-only collection.
+	}
+}
diff --git a/includes/Checker/Default_Check_Repository.php b/includes/Checker/Default_Check_Repository.php
index 761b0fee3..761d6f1d6 100644
--- a/includes/Checker/Default_Check_Repository.php
+++ b/includes/Checker/Default_Check_Repository.php
@@ -82,13 +82,10 @@ public function register_check( $slug, Check $check ) {
 	 *
 	 * @since n.e.x.t
 	 *
-	 * @param int   $flags       The check type flag.
-	 * @param array $check_slugs An array of check slugs to return.
-	 * @return array An array of check instances.
-	 *
-	 * @throws Exception Thrown when invalid flag is passed, or Check slug does not exist.
+	 * @param int $flags The check type flag.
+	 * @return Check_Collection Check collection providing an indexed array of check instances.
 	 */
-	public function get_checks( $flags = self::TYPE_ALL, array $check_slugs = array() ) {
+	public function get_checks( $flags = self::TYPE_ALL ) {
 		$checks = array();
 
 		if ( $flags & self::TYPE_STATIC ) {
@@ -99,37 +96,19 @@ public function get_checks( $flags = self::TYPE_ALL, array $check_slugs = array(
 			$checks += $this->runtime_checks;
 		}
 
-		// Filter out the specific check slugs requested.
-		if ( ! empty( $check_slugs ) ) {
-			$checks = array_map(
-				function ( $slug ) use ( $checks ) {
-					if ( ! isset( $checks[ $slug ] ) ) {
-						throw new Exception(
-							sprintf(
-								/* translators: %s: The Check slug. */
-								__( 'Check with the slug "%s" does not exist.', 'plugin-check' ),
-								$slug
-							)
-						);
-					}
-
-					return $checks[ $slug ];
-				},
-				$check_slugs
-			);
-		}
-
 		// Return all checks, including experimental if requested.
 		if ( $flags & self::INCLUDE_EXPERIMENTAL ) {
-			return $checks;
+			return new Default_Check_Collection( $checks );
 		}
 
 		// Remove experimental checks before returning.
-		return array_filter(
-			$checks,
-			static function ( $check ) {
-				return $check->get_stability() !== Check::STABILITY_EXPERIMENTAL;
-			}
+		return new Default_Check_Collection(
+			array_filter(
+				$checks,
+				static function ( $check ) {
+					return $check->get_stability() !== Check::STABILITY_EXPERIMENTAL;
+				}
+			)
 		);
 	}
 }
diff --git a/tests/phpunit/Checker/Check_Categories_Tests.php b/tests/phpunit/Checker/Check_Categories_Tests.php
index a6d26035b..7399a80e6 100644
--- a/tests/phpunit/Checker/Check_Categories_Tests.php
+++ b/tests/phpunit/Checker/Check_Categories_Tests.php
@@ -44,7 +44,8 @@ public function test_filter_checks_by_categories( array $categories, array $all_
 			$this->repository->register_check( $check[0], $check[1] );
 		}
 
-		$checks = $this->repository->get_checks();
+		$checks = $this->repository->get_checks()
+			->to_map();
 
 		$check_categories = new Check_Categories();
 		$filtered_checks  = $check_categories->filter_checks_by_categories( $checks, $categories );
diff --git a/tests/phpunit/Checker/Default_Check_Repository_Tests.php b/tests/phpunit/Checker/Default_Check_Repository_Tests.php
index 8822ce622..710f940ab 100644
--- a/tests/phpunit/Checker/Default_Check_Repository_Tests.php
+++ b/tests/phpunit/Checker/Default_Check_Repository_Tests.php
@@ -26,14 +26,14 @@ public function test_register_static_check() {
 		$check = new Static_Check();
 		$this->repository->register_check( 'static_check', $check );
 
-		$this->assertSame( array( 'static_check' => $check ), $this->repository->get_checks() );
+		$this->assertSame( array( 'static_check' => $check ), $this->repository->get_checks()->to_map() );
 	}
 
 	public function test_register_runtime_check() {
 		$check = new Runtime_Check();
 		$this->repository->register_check( 'runtime_check', $check );
 
-		$this->assertSame( array( 'runtime_check' => $check ), $this->repository->get_checks() );
+		$this->assertSame( array( 'runtime_check' => $check ), $this->repository->get_checks()->to_map() );
 	}
 
 	public function test_register_exception_thrown_for_invalid_check() {
@@ -86,7 +86,7 @@ public function test_get_checks_returns_all_checks() {
 			'runtime_check' => $runtime_check,
 		);
 
-		$this->assertSame( $expected, $this->repository->get_checks() );
+		$this->assertSame( $expected, $this->repository->get_checks()->to_map() );
 	}
 
 	public function test_get_checks_returns_static_checks_via_flag() {
@@ -96,7 +96,7 @@ public function test_get_checks_returns_static_checks_via_flag() {
 		$this->repository->register_check( 'static_check', $static_check );
 		$this->repository->register_check( 'runtime_check', $runtime_check );
 
-		$this->assertSame( array( 'static_check' => $static_check ), $this->repository->get_checks( Check_Repository::TYPE_STATIC ) );
+		$this->assertSame( array( 'static_check' => $static_check ), $this->repository->get_checks( Check_Repository::TYPE_STATIC )->to_map() );
 	}
 
 	public function test_get_checks_returns_runtime_checks_via_flag() {
@@ -106,7 +106,7 @@ public function test_get_checks_returns_runtime_checks_via_flag() {
 		$this->repository->register_check( 'static_check', $static_check );
 		$this->repository->register_check( 'runtime_check', $runtime_check );
 
-		$this->assertSame( array( 'runtime_check' => $runtime_check ), $this->repository->get_checks( Check_Repository::TYPE_RUNTIME ) );
+		$this->assertSame( array( 'runtime_check' => $runtime_check ), $this->repository->get_checks( Check_Repository::TYPE_RUNTIME )->to_map() );
 	}
 
 	public function test_get_checks_returns_checks_via_slug() {
@@ -116,14 +116,11 @@ public function test_get_checks_returns_checks_via_slug() {
 		$this->repository->register_check( 'static_check', $static_check );
 		$this->repository->register_check( 'runtime_check', $runtime_check );
 
-		$this->assertSame( array( $static_check ), $this->repository->get_checks( Check_Repository::TYPE_ALL, array( 'static_check' ) ) );
-	}
-
-	public function test_get_checks_throws_exception_for_invalid_check_slug() {
-		$this->expectException( 'Exception' );
-		$this->expectExceptionMessage( 'Check with the slug "invalid_check" does not exist.' );
+		$checks = $this->repository->get_checks( Check_Repository::TYPE_ALL )
+			->include( array( 'static_check' ) )
+			->to_array();
 
-		$this->repository->get_checks( Check_Repository::TYPE_ALL, array( 'invalid_check' ) );
+		$this->assertSame( array( $static_check ), $checks );
 	}
 
 	public function test_get_checks_returns_no_experimental_checks_by_default() {
@@ -142,7 +139,7 @@ public function test_get_checks_returns_no_experimental_checks_by_default() {
 			'runtime_check' => $runtime_check,
 		);
 
-		$this->assertSame( $expected, $this->repository->get_checks() );
+		$this->assertSame( $expected, $this->repository->get_checks()->to_map() );
 	}
 
 	public function test_get_checks_returns_experimental_checks_with_flag() {
@@ -163,7 +160,7 @@ public function test_get_checks_returns_experimental_checks_with_flag() {
 			'experimental_runtime_check' => $experimental_runtime_check,
 		);
 
-		$this->assertSame( $expected, $this->repository->get_checks( Check_Repository::TYPE_ALL | Check_Repository::INCLUDE_EXPERIMENTAL ) );
+		$this->assertSame( $expected, $this->repository->get_checks( Check_Repository::TYPE_ALL | Check_Repository::INCLUDE_EXPERIMENTAL )->to_map() );
 	}
 
 	public function test_get_checks_returns_experimental_static_checks_with_flag() {
@@ -182,7 +179,7 @@ public function test_get_checks_returns_experimental_static_checks_with_flag() {
 			'experimental_static_check' => $experimental_static_check,
 		);
 
-		$this->assertSame( $expected, $this->repository->get_checks( Check_Repository::TYPE_STATIC | Check_Repository::INCLUDE_EXPERIMENTAL ) );
+		$this->assertSame( $expected, $this->repository->get_checks( Check_Repository::TYPE_STATIC | Check_Repository::INCLUDE_EXPERIMENTAL )->to_map() );
 	}
 
 	public function test_get_checks_returns_experimental_runtime_checks_with_flag() {
@@ -201,6 +198,6 @@ public function test_get_checks_returns_experimental_runtime_checks_with_flag()
 			'experimental_runtime_check' => $experimental_runtime_check,
 		);
 
-		$this->assertSame( $expected, $this->repository->get_checks( Check_Repository::TYPE_RUNTIME | Check_Repository::INCLUDE_EXPERIMENTAL ) );
+		$this->assertSame( $expected, $this->repository->get_checks( Check_Repository::TYPE_RUNTIME | Check_Repository::INCLUDE_EXPERIMENTAL )->to_map() );
 	}
 }

From 3d193b68ed5c7416d94530c5181578c3fa05cb20 Mon Sep 17 00:00:00 2001
From: Felix Arntz <felixarntz@google.com>
Date: Tue, 31 Oct 2023 13:25:41 -0500
Subject: [PATCH 2/8] Fix PHPCS violations.

---
 includes/Checker/Check_Collection.php         | 1 +
 includes/Checker/Default_Check_Collection.php | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/includes/Checker/Check_Collection.php b/includes/Checker/Check_Collection.php
index f1874f4ba..78398e598 100644
--- a/includes/Checker/Check_Collection.php
+++ b/includes/Checker/Check_Collection.php
@@ -43,6 +43,7 @@ public function to_map(): array;
 	 *
 	 * @since n.e.x.t
 	 *
+	 * @param array $check_slugs List of slugs to limit to only those. If empty, the same collection is returned.
 	 * @return Check_Collection New check collection, effectively a subset of this one.
 	 */
 	public function include( array $check_slugs ): Check_Collection;
diff --git a/includes/Checker/Default_Check_Collection.php b/includes/Checker/Default_Check_Collection.php
index f5765fbd3..b72ad0c6f 100644
--- a/includes/Checker/Default_Check_Collection.php
+++ b/includes/Checker/Default_Check_Collection.php
@@ -8,7 +8,6 @@
 namespace WordPress\Plugin_Check\Checker;
 
 use ArrayIterator;
-use Exception;
 use Traversable;
 
 /**
@@ -75,6 +74,7 @@ public function to_map(): array {
 	 *
 	 * @since n.e.x.t
 	 *
+	 * @param array $check_slugs List of slugs to limit to only those. If empty, the same collection is returned.
 	 * @return Check_Collection New check collection, effectively a subset of this one.
 	 */
 	public function include( array $check_slugs ): Check_Collection {

From 6353199554735af2e8aff9b86154c5e03254d254 Mon Sep 17 00:00:00 2001
From: Felix Arntz <felixarntz@google.com>
Date: Tue, 31 Oct 2023 13:28:46 -0500
Subject: [PATCH 3/8] Fix PHPStan violation.

---
 includes/Checker/Default_Check_Collection.php | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/includes/Checker/Default_Check_Collection.php b/includes/Checker/Default_Check_Collection.php
index b72ad0c6f..8ff2ebef9 100644
--- a/includes/Checker/Default_Check_Collection.php
+++ b/includes/Checker/Default_Check_Collection.php
@@ -94,7 +94,7 @@ public function include( array $check_slugs ): Check_Collection {
 			$checks[ $slug ] = $check;
 		}
 
-		return new static( $checks );
+		return new self( $checks );
 	}
 
 	/**

From 6b9df7fbe80fca772eb13737a29e9a43a750e206 Mon Sep 17 00:00:00 2001
From: Felix Arntz <felixarntz@google.com>
Date: Thu, 2 Nov 2023 15:21:44 -0700
Subject: [PATCH 4/8] Implement filter method on check collection.

---
 includes/Checker/Default_Check_Collection.php | 18 ++++++++++++++++++
 includes/Checker/Default_Check_Repository.php | 11 ++++-------
 2 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/includes/Checker/Default_Check_Collection.php b/includes/Checker/Default_Check_Collection.php
index 8ff2ebef9..1c29fddd7 100644
--- a/includes/Checker/Default_Check_Collection.php
+++ b/includes/Checker/Default_Check_Collection.php
@@ -67,6 +67,24 @@ public function to_map(): array {
 		return $this->checks;
 	}
 
+	/**
+	 * Returns a new check collection containing the subset of checks based on the given check filter function.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param callable $filter_fn Filter function that accepts a single check object and should return a boolean for
+	 *                            whether to include the check in the new collection.
+	 * @return Check_Collection New check collection, effectively a subset of this one.
+	 */
+	public function filter( callable $filter_fn ): Check_Collection {
+		return new self(
+			array_filter(
+				$this->checks,
+				$filter_fn
+			)
+		);
+	}
+
 	/**
 	 * Returns a new check collection containing the subset of checks based on the given check slugs.
 	 *
diff --git a/includes/Checker/Default_Check_Repository.php b/includes/Checker/Default_Check_Repository.php
index 761d6f1d6..aa1670c14 100644
--- a/includes/Checker/Default_Check_Repository.php
+++ b/includes/Checker/Default_Check_Repository.php
@@ -102,13 +102,10 @@ public function get_checks( $flags = self::TYPE_ALL ) {
 		}
 
 		// Remove experimental checks before returning.
-		return new Default_Check_Collection(
-			array_filter(
-				$checks,
-				static function ( $check ) {
-					return $check->get_stability() !== Check::STABILITY_EXPERIMENTAL;
-				}
-			)
+		return ( new Default_Check_Collection( $checks ) )->filter(
+			static function ( $check ) {
+				return $check->get_stability() !== Check::STABILITY_EXPERIMENTAL;
+			}
 		);
 	}
 }

From 62d98c6f7305fdd2dd229d2a2e5a1ce1c776e464 Mon Sep 17 00:00:00 2001
From: Felix Arntz <felixarntz@google.com>
Date: Fri, 3 Nov 2023 11:07:55 -0700
Subject: [PATCH 5/8] Introduce Check_Collection::require() method to bring
 back the functionality to fail if an invalid check slug is given.

---
 includes/Checker/Abstract_Check_Runner.php    |  3 ++-
 includes/Checker/Check_Collection.php         | 24 +++++++++++++++++
 includes/Checker/Default_Check_Collection.php | 27 +++++++++++++++++++
 .../Default_Check_Repository_Tests.php        |  8 ++++++
 4 files changed, 61 insertions(+), 1 deletion(-)

diff --git a/includes/Checker/Abstract_Check_Runner.php b/includes/Checker/Abstract_Check_Runner.php
index 5651ad67e..59138e1bd 100644
--- a/includes/Checker/Abstract_Check_Runner.php
+++ b/includes/Checker/Abstract_Check_Runner.php
@@ -369,7 +369,8 @@ final public function get_checks_to_run() {
 		}
 
 		$checks = $this->check_repository->get_checks( $check_flags )
-			->include( $check_slugs )
+			->require( $check_slugs ) // Ensures all of the given slugs are valid.
+			->include( $check_slugs ) // Ensures only the checks with the given slugs are included.
 			->to_map();
 
 		// Filters the checks by specific categories.
diff --git a/includes/Checker/Check_Collection.php b/includes/Checker/Check_Collection.php
index 78398e598..c5698cbe3 100644
--- a/includes/Checker/Check_Collection.php
+++ b/includes/Checker/Check_Collection.php
@@ -9,6 +9,7 @@
 
 use ArrayAccess;
 use Countable;
+use Exception;
 use IteratorAggregate;
 
 /**
@@ -36,6 +37,17 @@ public function to_array(): array;
 	 */
 	public function to_map(): array;
 
+	/**
+	 * Returns a new check collection containing the subset of checks based on the given check filter function.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param callable $filter_fn Filter function that accepts a single check object and should return a boolean for
+	 *                            whether to include the check in the new collection.
+	 * @return Check_Collection New check collection, effectively a subset of this one.
+	 */
+	public function filter( callable $filter_fn ): Check_Collection;
+
 	/**
 	 * Returns a new check collection containing the subset of checks based on the given check slugs.
 	 *
@@ -47,4 +59,16 @@ public function to_map(): array;
 	 * @return Check_Collection New check collection, effectively a subset of this one.
 	 */
 	public function include( array $check_slugs ): Check_Collection;
+
+	/**
+	 * Throws an exception if any of the given check slugs are not present, or returns the same collection otherwise.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param array $check_slugs List of slugs to limit to only those. If empty, the same collection is returned.
+	 * @return Check_Collection The unchanged check collection.
+	 *
+	 * @throws Exception Thrown when any of the given check slugs is not present in the collection.
+	 */
+	public function require( array $check_slugs ): Check_Collection;
 }
diff --git a/includes/Checker/Default_Check_Collection.php b/includes/Checker/Default_Check_Collection.php
index 1c29fddd7..de81e85b9 100644
--- a/includes/Checker/Default_Check_Collection.php
+++ b/includes/Checker/Default_Check_Collection.php
@@ -8,6 +8,7 @@
 namespace WordPress\Plugin_Check\Checker;
 
 use ArrayIterator;
+use Exception;
 use Traversable;
 
 /**
@@ -115,6 +116,32 @@ public function include( array $check_slugs ): Check_Collection {
 		return new self( $checks );
 	}
 
+	/**
+	 * Throws an exception if any of the given check slugs are not present, or returns the same collection otherwise.
+	 *
+	 * @since n.e.x.t
+	 *
+	 * @param array $check_slugs List of slugs to limit to only those. If empty, the same collection is returned.
+	 * @return Check_Collection The unchanged check collection.
+	 *
+	 * @throws Exception Thrown when any of the given check slugs is not present in the collection.
+	 */
+	public function require( array $check_slugs ): Check_Collection {
+		foreach ( $check_slugs as $slug ) {
+			if ( ! isset( $this->checks[ $slug ] ) ) {
+				throw new Exception(
+					sprintf(
+						/* translators: %s: The Check slug. */
+						__( 'Check with the slug "%s" does not exist.', 'plugin-check' ),
+						$slug
+					)
+				);
+			}
+		}
+
+		return $this;
+	}
+
 	/**
 	 * Counts the checks in the collection.
 	 *
diff --git a/tests/phpunit/Checker/Default_Check_Repository_Tests.php b/tests/phpunit/Checker/Default_Check_Repository_Tests.php
index 710f940ab..b2b1ddfb6 100644
--- a/tests/phpunit/Checker/Default_Check_Repository_Tests.php
+++ b/tests/phpunit/Checker/Default_Check_Repository_Tests.php
@@ -123,6 +123,14 @@ public function test_get_checks_returns_checks_via_slug() {
 		$this->assertSame( array( $static_check ), $checks );
 	}
 
+	public function test_get_checks_throws_exception_for_invalid_check_slug() {
+		$this->expectException( 'Exception' );
+		$this->expectExceptionMessage( 'Check with the slug "invalid_check" does not exist.' );
+
+		$this->repository->get_checks( Check_Repository::TYPE_ALL )
+			->require( array( 'invalid_check' ) );
+	}
+
 	public function test_get_checks_returns_no_experimental_checks_by_default() {
 		$static_check               = new Static_Check();
 		$runtime_check              = new Runtime_Check();

From 541a16f220e2463e1f772ca6ab8297287fbdff3b Mon Sep 17 00:00:00 2001
From: Felix Arntz <felixarntz@google.com>
Date: Fri, 3 Nov 2023 11:24:40 -0700
Subject: [PATCH 6/8] Implement proper test coverage for Check_Collection.

---
 .../Default_Check_Collection_Tests.php        | 93 +++++++++++++++++++
 .../Default_Check_Repository_Tests.php        | 24 +----
 2 files changed, 95 insertions(+), 22 deletions(-)
 create mode 100644 tests/phpunit/Checker/Default_Check_Collection_Tests.php

diff --git a/tests/phpunit/Checker/Default_Check_Collection_Tests.php b/tests/phpunit/Checker/Default_Check_Collection_Tests.php
new file mode 100644
index 000000000..152e27370
--- /dev/null
+++ b/tests/phpunit/Checker/Default_Check_Collection_Tests.php
@@ -0,0 +1,93 @@
+<?php
+/**
+ * Tests for the Default_Check_Collection class.
+ *
+ * @package plugin-check
+ */
+
+use WordPress\Plugin_Check\Checker\Default_Check_Repository;
+use WordPress\Plugin_Check\Checker\Runtime_Check as Runtime_Check_Interface;
+use WordPress\Plugin_Check\Test_Data\Runtime_Check;
+use WordPress\Plugin_Check\Test_Data\Static_Check;
+
+class Default_Check_Collection_Tests extends WP_UnitTestCase {
+
+	private $checks;
+	private $collection;
+
+	public function set_up() {
+		parent::set_up();
+
+		$this->checks = array(
+			'static_check'  => new Static_Check(),
+			'runtime_check' => new Runtime_Check(),
+		);
+
+		$repository = new Default_Check_Repository();
+		foreach ( $this->checks as $slug => $check ) {
+			$repository->register_check( $slug, $check );
+		}
+
+		$this->collection = $repository->get_checks();
+	}
+
+	public function test_to_array() {
+		$this->assertSame(
+			array_values( $this->checks ),
+			$this->collection->to_array()
+		);
+	}
+
+	public function test_to_map() {
+		$this->assertSame(
+			$this->checks,
+			$this->collection->to_map()
+		);
+	}
+
+	public function test_filter() {
+		$this->assertSame(
+			array( $this->checks['runtime_check'] ),
+			$this->collection->filter(
+				function ( $check ) {
+					return $check instanceof Runtime_Check_Interface;
+				}
+			)->to_array()
+		);
+	}
+
+	public function test_include() {
+		$this->assertSame(
+			array( $this->checks['static_check'] ),
+			$this->collection->include( array( 'static_check' ) )->to_array()
+		);
+	}
+
+	public function test_include_with_empty() {
+		$this->assertSame(
+			array_values( $this->checks ),
+			$this->collection->include( array() )->to_array()
+		);
+	}
+
+	public function test_include_with_invalid() {
+		$this->assertSame(
+			array( $this->checks['runtime_check'] ),
+			$this->collection->include( array( 'runtime_check', 'invalid_check' ) )->to_array()
+		);
+	}
+
+	public function test_require() {
+		$this->assertSame(
+			array_values( $this->checks ),
+			$this->collection->require( array( 'static_check' ) )->to_array()
+		);
+	}
+
+	public function test_require_with_invalid() {
+		$this->expectException( 'Exception' );
+		$this->expectExceptionMessage( 'Check with the slug "invalid_check" does not exist.' );
+
+		$this->collection->require( array( 'static_check', 'invalid_check' ) );
+	}
+}
diff --git a/tests/phpunit/Checker/Default_Check_Repository_Tests.php b/tests/phpunit/Checker/Default_Check_Repository_Tests.php
index b2b1ddfb6..b1a8898d9 100644
--- a/tests/phpunit/Checker/Default_Check_Repository_Tests.php
+++ b/tests/phpunit/Checker/Default_Check_Repository_Tests.php
@@ -16,6 +16,8 @@
 
 class Default_Check_Repository_Tests extends WP_UnitTestCase {
 
+	private $repository;
+
 	public function set_up() {
 		parent::set_up();
 
@@ -109,28 +111,6 @@ public function test_get_checks_returns_runtime_checks_via_flag() {
 		$this->assertSame( array( 'runtime_check' => $runtime_check ), $this->repository->get_checks( Check_Repository::TYPE_RUNTIME )->to_map() );
 	}
 
-	public function test_get_checks_returns_checks_via_slug() {
-		$static_check  = new Static_Check();
-		$runtime_check = new Runtime_Check();
-
-		$this->repository->register_check( 'static_check', $static_check );
-		$this->repository->register_check( 'runtime_check', $runtime_check );
-
-		$checks = $this->repository->get_checks( Check_Repository::TYPE_ALL )
-			->include( array( 'static_check' ) )
-			->to_array();
-
-		$this->assertSame( array( $static_check ), $checks );
-	}
-
-	public function test_get_checks_throws_exception_for_invalid_check_slug() {
-		$this->expectException( 'Exception' );
-		$this->expectExceptionMessage( 'Check with the slug "invalid_check" does not exist.' );
-
-		$this->repository->get_checks( Check_Repository::TYPE_ALL )
-			->require( array( 'invalid_check' ) );
-	}
-
 	public function test_get_checks_returns_no_experimental_checks_by_default() {
 		$static_check               = new Static_Check();
 		$runtime_check              = new Runtime_Check();

From 99bf7a5c0db12950323ad01cdf50c5d1e5fc7cde Mon Sep 17 00:00:00 2001
From: Felix Arntz <felixarntz@google.com>
Date: Fri, 3 Nov 2023 11:34:47 -0700
Subject: [PATCH 7/8] Update phpmd.xml to ignore public methods that are for
 special interfaces.

---
 phpmd.xml | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/phpmd.xml b/phpmd.xml
index 0dc175f0d..1f07359e3 100644
--- a/phpmd.xml
+++ b/phpmd.xml
@@ -19,6 +19,12 @@
 		</properties>
 	</rule>
 
+	<rule ref="rulesets/codesize.xml/TooManyPublicMethods">
+		<properties>
+			<property name="ignorepattern" value="(^(getIterator|offsetExists|offsetGet|offsetSet|offsetUnset))i" />
+		</properties>
+	</rule>
+
 	<rule ref="rulesets/cleancode.xml">
 		<exclude name="ElseExpression" />
 		<exclude name="StaticAccess" />

From edbf4d27f2c03b3b879e36693467a909dc9b94c3 Mon Sep 17 00:00:00 2001
From: Felix Arntz <felixarntz@google.com>
Date: Fri, 3 Nov 2023 11:44:57 -0700
Subject: [PATCH 8/8] Update phpmd.xml to ignore public methods that are for
 special interfaces.

---
 phpmd.xml | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/phpmd.xml b/phpmd.xml
index 1f07359e3..6e6f9cc61 100644
--- a/phpmd.xml
+++ b/phpmd.xml
@@ -11,6 +11,7 @@
 
 	<rule ref="rulesets/codesize.xml">
 		<exclude name="CyclomaticComplexity" />
+		<exclude name="TooManyPublicMethods" />
 	</rule>
 
 	<rule ref="rulesets/codesize.xml/CyclomaticComplexity">
@@ -21,7 +22,7 @@
 
 	<rule ref="rulesets/codesize.xml/TooManyPublicMethods">
 		<properties>
-			<property name="ignorepattern" value="(^(getIterator|offsetExists|offsetGet|offsetSet|offsetUnset))i" />
+			<property name="ignorepattern" value="(^(__construct|getIterator|offsetExists|offsetGet|offsetSet|offsetUnset))i" />
 		</properties>
 	</rule>