Retrieves the maximum character lengths for the comment form fields.

function wp_get_comment_fields_max_lengths() {
	global $wpdb;

	$lengths = array(
		'comment_author'       => 245,
		'comment_author_email' => 100,
		'comment_author_url'   => 200,
		'comment_content'      => 65525,
	);

	if ( $wpdb->is_mysql ) {
		foreach ( $lengths as $column => $length ) {
			$col_length = $wpdb->get_col_length( $wpdb->comments, $column );
			$max_length = 0;

			// No point if we can't get the DB column lengths.
			if ( is_wp_error( $col_length ) ) {
				break;
			}

			if ( ! is_array( $col_length ) && (int) $col_length > 0 ) {
				$max_length = (int) $col_length;
			} elseif ( is_array( $col_length ) && isset( $col_length['length'] ) && (int) $col_length['length'] > 0 ) {
				$max_length = (int) $col_length['length'];

				if ( ! empty( $col_length['type'] ) && 'byte' === $col_length['type'] ) {
					$max_length = $max_length - 10;
				}
			}

			if ( $max_length > 0 ) {
				$lengths[ $column ] = $max_length;
			}
		}
	}

	/**
	 * Filters the lengths for the comment form fields.
	 *
	 * @since 4.5.0
	 *
	 * @param int[] $lengths Array of maximum lengths keyed by field name.
	 */
	return apply_filters( 'wp_get_comment_fields_max_lengths', $lengths );
}