wp_list_comments

 *
 * @param string|array $args {
 *     Optional. Formatting options.
 *
 *     @type object   $walker            Instance of a Walker class to list comments. Default null.
 *     @type int      $max_depth         The maximum comments depth. Default empty.
 *     @type string   $style             The style of list ordering. Accepts 'ul', 'ol', or 'div'.
 *                                       'div' will result in no additional list markup. Default 'ul'.
 *     @type callable $callback          Callback function to use. Default null.
 *     @type callable $end-callback      Callback function to use at the end. Default null.
 *     @type string   $type              Type of comments to list. Accepts 'all', 'comment',
 *                                       'pingback', 'trackback', 'pings'. Default 'all'.
 *     @type int      $page              Page ID to list comments for. Default empty.
 *     @type int      $per_page          Number of comments to list per page. Default empty.
 *     @type int      $avatar_size       Height and width dimensions of the avatar size. Default 32.
 *     @type bool     $reverse_top_level Ordering of the listed comments. If true, will display
 *                                       newest comments first. Default null.
 *     @type bool     $reverse_children  Whether to reverse child comments in the list. Default null.
 *     @type string   $format            How to format the comments list. Accepts 'html5', 'xhtml'.
 *                                       Default 'html5' if the theme supports it.
 *     @type bool     $short_ping        Whether to output short pings. Default false.
 *     @type bool     $echo              Whether to echo the output or return it. Default true.
 * }
 * @param WP_Comment[] $comments Optional. Array of WP_Comment objects.
 * @return void|string Void if 'echo' argument is true, or no comments to list.
 *                     Otherwise, HTML list of comments.
 */
function wp_list_comments( $args = array(), $comments = null ) {
	global $wp_query, $comment_alt, $comment_depth, $comment_thread_alt, $overridden_cpage, $in_comment_loop;

	$in_comment_loop = true;

	$comment_alt        = 0;
	$comment_thread_alt = 0;
	$comment_depth      = 1;

	$defaults = array(
		'walker'            => null,
		'max_depth'         => '',
		'style'             => 'ul',
		'callback'          => null,
		'end-callback'      => null,
		'type'              => 'all',
		'page'              => '',
		'per_page'          => '',
		'avatar_size'       => 32,
		'reverse_top_level' => null,
		'reverse_children'  => '',
		'format'            => current_theme_supports( 'html5', 'comment-list' ) ? 'html5' : 'xhtml',
		'short_ping'        => false,
		'echo'              => true,
	);

	$parsed_args = wp_parse_args( $args, $defaults );

	/**
	 * Filters the arguments used in retrieving the comment list.
	 *
	 * @since 4.0.0
	 *
	 * @see wp_list_comments()
	 *
	 * @param array $parsed_args An array of arguments for displaying comments.
	 */
	$parsed_args = apply_filters( 'wp_list_comments_args', $parsed_args );

	// Figure out what comments we'll be looping through ($_comments).
	if ( null !== $comments ) {
		$comments = (array) $comments;
		if ( empty( $comments ) ) {
			return;
		}
		if ( 'all' !== $parsed_args['type'] ) {
			$comments_by_type = separate_comments( $comments );
			if ( empty( $comments_by_type[ $parsed_args['type'] ] ) ) {
				return;
			}
			$_comments = $comments_by_type[ $parsed_args['type'] ];
		} else {
			$_comments = $comments;
		}
	} else {
		/*
		 * If 'page' or 'per_page' has been passed, and does not match what's in $wp_query,
		 * perform a separate comment query and allow Walker_Comment to paginate.
		 */
		if ( $parsed_args['page'] || $parsed_args['per_page'] ) {
			$current_cpage = get_query_var( 'cpage' );
			if ( ! $current_cpage ) {
				$current_cpage = 'newest' === get_option( 'default_comments_page' ) ? 1 : $wp_query->max_num_comment_pages;
			}

			$current_per_page = get_query_var( 'comments_per_page' );
			if ( $parsed_args['page'] != $current_cpage || $parsed_args['per_page'] != $current_per_page ) {
				$comment_args = array(
					'post_id' => get_the_ID(),
					'orderby' => 'comment_date_gmt',
					'order'   => 'ASC',
					'status'  => 'approve',
				);

				if ( is_user_logged_in() ) {
					$comment_args['include_unapproved'] = array( get_current_user_id() );
				} else {
					$unapproved_email = wp_get_unapproved_comment_author_email();

					if ( $unapproved_email ) {
						$comment_args['include_unapproved'] = array( $unapproved_email );
					}
				}

				$comments = get_comments( $comment_args );

				if ( 'all' !== $parsed_args['type'] ) {
					$comments_by_type = separate_comments( $comments );
					if ( empty( $comments_by_type[ $parsed_args['type'] ] ) ) {
						return;
					}

					$_comments = $comments_by_type[ $parsed_args['type'] ];
				} else {
					$_comments = $comments;
				}
			}

			// Otherwise, fall back on the comments from `$wp_query->comments`.
		} else {
			if ( empty( $wp_query->comments ) ) {
				return;
			}
			if ( 'all' !== $parsed_args['type'] ) {
				if ( empty( $wp_query->comments_by_type ) ) {
					$wp_query->comments_by_type = separate_comments( $wp_query->comments );
				}
				if ( empty( $wp_query->comments_by_type[ $parsed_args['type'] ] ) ) {
					return;
				}
				$_comments = $wp_query->comments_by_type[ $parsed_args['type'] ];
			} else {
				$_comments = $wp_query->comments;
			}

			if ( $wp_query->max_num_comment_pages ) {
				$default_comments_page = get_option( 'default_comments_page' );
				$cpage                 = get_query_var( 'cpage' );
				if ( 'newest' === $default_comments_page ) {
					$parsed_args['cpage'] = $cpage;

					/*
					* When first page shows oldest comments, post permalink is the same as
					* the comment permalink.
					*/
				} elseif ( 1 == $cpage ) {
					$parsed_args['cpage'] = '';
				} else {
					$parsed_args['cpage'] = $cpage;
				}

				$parsed_args['page']     = 0;
				$parsed_args['per_page'] = 0;
			}
		}
	}

	if ( '' === $parsed_args['per_page'] && get_option( 'page_comments' ) ) {
		$parsed_args['per_page'] = get_query_var( 'comments_per_page' );
	}

	if ( empty( $parsed_args['per_page'] ) ) {
		$parsed_args['per_page'] = 0;
		$parsed_args['page']     = 0;
	}

	if ( '' === $parsed_args['max_depth'] ) {
		if ( get_option( 'thread_comments' ) ) {
			$parsed_args['max_depth'] = get_option( 'thread_comments_depth' );
		} else {
			$parsed_args['max_depth'] = -1;
		}
	}

	if ( '' === $parsed_args['page'] ) {
		if ( empty( $overridden_cpage ) ) {
			$parsed_args['page'] = get_query_var( 'cpage' );
		} else {
			$threaded            = ( -1 != $parsed_args['max_depth'] );
			$parsed_args['page'] = ( 'newest' === get_option( 'default_comments_page' ) ) ? get_comment_pages_count( $_comments, $parsed_args['per_page'], $threaded ) : 1;
			set_query_var( 'cpage', $parsed_args['page'] );
		}
	}
	// Validation check.