wp_list_comments( string|array $args = array(), array $comments = null )

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 = $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,
	);

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

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

	// Figure out what comments we'll be looping through ($_comments)
	if ( null !== $comments ) {
		$comments = (array) $comments;
		if ( empty($comments) )
			return;
		if ( 'all' != $r['type'] ) {
			$comments_by_type = separate_comments($comments);
			if ( empty($comments_by_type[$r['type']]) )
				return;
			$_comments = $comments_by_type[$r['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 ( $r['page'] || $r['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 ( $r['page'] != $current_cpage || $r['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'] = get_current_user_id();
				} else {
					$commenter = wp_get_current_commenter();
					if ( $commenter['comment_author_email'] ) {
						$comment_args['include_unapproved'] = $commenter['comment_author_email'];
					}
				}

				$comments = get_comments( $comment_args );

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

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

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

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

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

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

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

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

	if ( '' === $r['page'] ) {
		if ( empty($overridden_cpage) ) {
			$r['page'] = get_query_var('cpage');
		} else {
			$threaded = ( -1 != $r['max_depth'] );
			$r['page'] = ( 'newest' == get_option('default_comments_page') ) ? get_comment_pages_count($_comments, $r['per_page'], $threaded) : 1;
			set_query_var( 'cpage', $r['page'] );
		}
	}
	// Validation check
	$r['page'] = intval($r['page']);
	if ( 0 == $r['page'] && 0 != $r['per_page'] )
		$r['page'] = 1;

	if ( null === $r['reverse_top_level'] )
		$r['reverse_top_level'] = ( 'desc' == get_option('comment_order') );

	wp_queue_comments_for_comment_meta_lazyload( $_comments );

	if ( empty( $r['walker'] ) ) {
		$walker = new Walker_Comment;
	} else {
		$walker = $r['walker'];
	}

	$output = $walker->paged_walk( $_comments, $r['max_depth'], $r['page'], $r['per_page'], $r );

	$in_comment_loop = false;

	if ( $r['echo'] ) {
		echo $output;
	} else {
		return $output;
	}
}