@@ -0,0 +1,124 @@
+<?php
+
+/**
+ * Tests for separate_comments().
+ *
+ * @group comment
+ *
+ * @covers ::separate_comments
+ */
+class Tests_Comment_SeparateComments extends WP_UnitTestCase {
+
+	/**
+	 * Post the comments belong to.
+	 *
+	 * @var int
+	 */
+	protected static $post_id;
+
+	public static function wpSetUpBeforeClass( WP_UnitTest_Factory $factory ) {
+		self::$post_id = $factory->post->create();
+	}
+
+	/**
+	 * Creates a comment of the given type and returns it.
+	 *
+	 * @param string $type Comment type.
+	 * @return WP_Comment The created comment.
+	 */
+	private function create_comment( $type ) {
+		return self::factory()->comment->create_and_get(
+			array(
+				'comment_post_ID' => self::$post_id,
+				'comment_type'    => $type,
+			)
+		);
+	}
+
+	/**
+	 * Tests that an empty list returns the four default groups, all empty.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_empty_list_returns_default_groups() {
+		$comments = array();
+
+		$this->assertSame(
+			array(
+				'comment'   => array(),
+				'trackback' => array(),
+				'pingback'  => array(),
+				'pings'     => array(),
+			),
+			separate_comments( $comments )
+		);
+	}
+
+	/**
+	 * Tests that comments are grouped by type, and trackbacks and pingbacks are also grouped as pings.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_groups_comments_by_type() {
+		$comment   = $this->create_comment( 'comment' );
+		$trackback = $this->create_comment( 'trackback' );
+		$pingback  = $this->create_comment( 'pingback' );
+		$comments  = array( $comment, $trackback, $pingback );
+
+		$grouped = separate_comments( $comments );
+
+		$this->assertSame( array( $comment ), $grouped['comment'], 'The comment should be in the comment group only.' );
+		$this->assertSame( array( $trackback ), $grouped['trackback'], 'The trackback should be in the trackback group.' );
+		$this->assertSame( array( $pingback ), $grouped['pingback'], 'The pingback should be in the pingback group.' );
+		$this->assertSame( array( $trackback, $pingback ), $grouped['pings'], 'Trackbacks and pingbacks should both be in the pings group, in order.' );
+	}
+
+	/**
+	 * Tests that a comment with an empty type is grouped as a comment.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_empty_type_is_grouped_as_comment() {
+		$comment               = $this->create_comment( 'comment' );
+		$comment->comment_type = '';
+		$comments              = array( $comment );
+
+		$grouped = separate_comments( $comments );
+
+		$this->assertSame( array( $comment ), $grouped['comment'], 'A comment with an empty type should be in the comment group.' );
+		$this->assertArrayNotHasKey( '', $grouped, 'There should be no group for an empty type.' );
+	}
+
+	/**
+	 * Tests that a custom comment type gets its own group and is not added to pings.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_custom_type_gets_its_own_group() {
+		$note     = $this->create_comment( 'note' );
+		$comments = array( $note );
+
+		$grouped = separate_comments( $comments );
+
+		$this->assertSame( array( $note ), $grouped['note'], 'A custom type should get its own group.' );
+		$this->assertSame( array(), $grouped['comment'], 'A custom type should not be in the comment group.' );
+		$this->assertSame( array(), $grouped['pings'], 'A custom type should not be in the pings group.' );
+	}
+
+	/**
+	 * Tests that the groups hold references to the elements of the passed array.
+	 *
+	 * @ticket 65819
+	 */
+	public function test_groups_hold_references_to_the_passed_array() {
+		$comments = array( $this->create_comment( 'pingback' ) );
+
+		$grouped = separate_comments( $comments );
+
+		$replacement            = $this->create_comment( 'comment' );
+		$grouped['pingback'][0] = $replacement;
+
+		$this->assertSame( $replacement, $comments[0], 'Replacing an element in a group should replace it in the passed array.' );
+		$this->assertSame( $replacement, $grouped['pings'][0], 'The pings group should reference the same element.' );
+	}
+}