private function retrieve_userid() {
		// The user ID is an integer, let's cast to string.
		$replacement = ! empty( $this->args->post_author ) ? (string) $this->args->post_author : (string) get_query_var( 'author' );

		return $replacement;
	}

	/**
	 * Retrieve the post/page/cpt's published year for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_post_year() {
		if ( empty( $this->args->ID ) ) {
			return null;
		}

		return \get_the_date( 'Y', $this->args->ID );
	}

	/**
	 * Retrieve the post/page/cpt's published month for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_post_month() {
		if ( empty( $this->args->ID ) ) {
			return null;
		}

		return \get_the_date( 'F', $this->args->ID );
	}

	/**
	 * Retrieve the post/page/cpt's published day for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_post_day() {
		if ( empty( $this->args->ID ) ) {
			return null;
		}

		return \get_the_date( 'd', $this->args->ID );
	}

	/**
	 * Retrieve the post/page/cpt author's first name for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_author_first_name() {
		$replacement = null;

		$user_id = (int) $this->retrieve_userid();
		$name    = get_the_author_meta( 'first_name', $user_id );
		if ( $name !== '' ) {
			$replacement = $name;
		}

		return $replacement;
	}

	/**
	 * Retrieve the post/page/cpt author's last name for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_author_last_name() {
		$replacement = null;

		$user_id = (int) $this->retrieve_userid();
		$name    = get_the_author_meta( 'last_name', $user_id );
		if ( $name !== '' ) {
			$replacement = $name;
		}

		return $replacement;
	}

	/**
	 * Retrieve the post/page/cpt permalink for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_permalink() {
		if ( empty( $this->args->ID ) ) {
			return null;
		}

		return \get_permalink( $this->args->ID );
	}

	/**
	 * Retrieve the post/page/cpt content for use as replacement string.
	 *
	 * @return string|null
	 */
	private function retrieve_post_content() {
		$replacement = null;

		// The check `post_password_required` is because content must be hidden for a post with a password.
		if ( ! empty( $this->args->ID ) && $this->args->post_content !== '' && ! post_password_required( $this->args->ID ) ) {
			$content     = strip_shortcodes( $this->args->post_content );
			$replacement = wp_strip_all_tags( $content );
		}

		return $replacement;
	}

	/**
	 * Retrieve the current or first category title. To be used for import data from AIOSEO.
	 * The code derives from AIOSEO's way of dealing with that var, so we can ensure 100% seamless transition.
	 *
	 * @return string|null
	 */
	private function retrieve_category_title() {
		if ( empty( $this->args ) || empty( $this->args->ID ) ) {
			return null;
		}
		$post_id = $this->args->ID;

		$post       = get_post( $post_id );
		$taxonomies = get_object_taxonomies( $post, 'objects' );

		foreach ( $taxonomies as $taxonomy_slug => $taxonomy ) {
			if ( ! $taxonomy->hierarchical ) {
				continue;
			}
			$post_terms = get_the_terms( $post_id, $taxonomy_slug );
			if ( is_array( $post_terms ) && count( $post_terms ) > 0 ) {
				// AiOSEO takes the name of whatever the first hierarchical taxonomy is.
				$term = $post_terms[0];
				if ( $term ) {
					return $term->name;
				}
			}
		}

		return null;
	}

	/* *********************** HELP TEXT RELATED ************************** */

	/**
	 * Set the help text for a user/plugin/theme defined extra variable.
	 *
	 * @param string                     $type                 Type of variable: 'basic' or 'advanced'.
	 * @param WPSEO_Replacement_Variable $replacement_variable The replacement variable to register.
	 */
	private static function register_help_text( $type, WPSEO_Replacement_Variable $replacement_variable ) {
		$identifier = $replacement_variable->get_variable();

		if ( ( is_string( $type ) && in_array( $type, [ 'basic', 'advanced' ], true ) )
			&& ( $identifier !== '' && ! isset( self::$help_texts[ $type ][ $identifier ] ) )
		) {
			self::$help_texts[ $type ][ $identifier ] = $replacement_variable;
		}
	}

	/**
	 * Generates a list of replacement variables based on the help texts.
	 *
	 * @return array List of replace vars.
	 */
	public function get_replacement_variables_with_labels() {
		self::setup_statics_once();

		$custom_variables = [];
		foreach ( array_merge( WPSEO_Custom_Fields::get_custom_fields(), WPSEO_Custom_Taxonomies::get_custom_taxonomies() ) as $custom_variable ) {
			$custom_variables[ $custom_variable ] = new WPSEO_Replacement_Variable( $custom_variable, $this->get_label( $custom_variable ), '' );
		}

		$replacement_variables = array_filter(
			array_merge( self::$help_texts['basic'], self::$help_texts['advanced'] ),
			[ $this, 'is_not_prefixed' ],
			ARRAY_FILTER_USE_KEY
		);

		$hidden = $this->get_hidden_replace_vars();

		return array_values(
			array_map(
				static function ( WPSEO_Replacement_Variable $replacement_variable ) use ( $hidden ) {
					$name = $replacement_variable->get_variable();

					return [
						'name'   => $name,
						'value'  => '',
						'label'  => $replacement_variable->get_label(),
						'hidden' => in_array( $name, $hidden, true ),
					];
				},
				array_merge( $replacement_variables, $custom_variables )
			)
		);
	}

	/**
	 * Generates a list of replacement variables based on the help texts.
	 *
	 * @return array List of replace vars.
	 */
	public function get_replacement_variables_list() {
		self::setup_statics_once();

		$replacement_variables = array_merge(
			$this->get_replacement_variables(),
			WPSEO_Custom_Fields::get_custom_fields(),
			WPSEO_Custom_Taxonomies::get_custom_taxonomies()
		);

		return array_map( [ $this, 'format_replacement_variable' ], $replacement_variables );
	}

	/**
	 * Creates a merged associative array of both the basic and advanced help texts.
	 *
	 * @return array Array with the replacement variables.
	 */
	private function get_replacement_variables() {
		$help_texts = array_merge( self::$help_texts['basic'], self::$help_texts['advanced'] );

		return array_filter( array_keys( $help_texts ), [ $this, 'is_not_prefixed' ] );
	}

	/**
	 * Checks whether the replacement variable contains a `ct_` or `cf_` prefix, because they follow different logic.
	 *
	 * @param string $replacement_variable The replacement variable.
	 *
	 * @return bool True when the replacement variable is not prefixed.
	 */
	private function is_not_prefixed( $replacement_variable ) {
		$prefixes = [ 'cf_', 'ct_' ];
		$prefix   = $this->get_prefix( $replacement_variable );

		return ! in_array( $prefix, $prefixes, true );
	}

	/**
	 * Strip the prefix from a replacement variable name.
	 *
	 * @param string $replacement_variable The replacement variable.
	 *
	 * @return string The replacement variable name without the prefix.
	 */
	private function strip_prefix( $replacement_variable ) {
		return substr( $replacement_variable, 3 );
	}

	/**
	 * Gets the prefix from a replacement variable name.
	 *
	 * @param string $replacement_variable The replacement variable.
	 *
	 * @return string The prefix of the replacement variable.
	 */
	private function get_prefix( $replacement_variable ) {
		return substr( $replacement_variable, 0, 3 );
	}

	/**
	 * Strips 'desc_' if present, and appends ' description' at the end.
	 *
	 * @param string $label The replacement variable.
	 *
	 * @return string The altered replacement variable name.
	 */
	private function handle_description( $label ) {
		if ( strpos( $label, 'desc_' ) === 0 ) {
			return substr( $label, 5 ) . ' description';
		}

		return $label;
	}

	/**
	 * Creates a label for prefixed replacement variables that matches the format in the editors.
	 *
	 * @param string $replacement_variable The replacement variable.
	 *
	 * @return string The replacement variable label.
	 */
	private function get_label( $replacement_variable ) {
		$prefix = $this->get_prefix( $replacement_variable );
		if ( $prefix === 'cf_' ) {
			return $this->strip_prefix( $replacement_variable ) . ' (custom field)';
		}

		if ( $prefix === 'ct_' ) {
			$label = $this->strip_prefix( $replacement_variable );
			$label = $this->handle_description( $label );
			return ucfirst( $label . ' (custom taxonomy)' );
		}

		if ( $prefix === 'pt_' ) {
			if ( $replacement_variable === 'pt_single' ) {
				return 'Post type (singular)';
			}

			return 'Post type (plural)';
		}

		return '';
	}

	/**
	 * Formats the replacement variables.
	 *
	 * @param string $replacement_variable The replacement variable to format.
	 *
	 * @return array The formatted replacement variable.
	 */
	private function format_replacement_variable( $replacement_variable ) {
		return [
			'name'  => $replacement_variable,
			'value' => '',
			'label' => $this->get_label( $replacement_variable ),
		];
	}

	/**
	 * Set/translate the help texts for the WPSEO standard basic variables.
	 */
	private static function set_basic_help_texts() {
		/* translators: %s: wp_title() function. */
		$separator_description = __( 'The separator defined in your theme\'s %s tag.', 'wordpress-seo' );
		$separator_description = sprintf(
			$separator_description,
			// '<code>wp_title()</code>'
			'wp_title()'
		);

		$replacement_variables = [
			new WPSEO_Replacement_Variable( 'date', __( 'Date', 'wordpress-seo' ), __( 'Replaced with the date of the post/page', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'title', __( 'Title', 'wordpress-seo' ), __( 'Replaced with the title of the post/page', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'parent_title', __( 'Parent title', 'wordpress-seo' ), __( 'Replaced with the title of the parent page of the current page', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'archive_title', __( 'Archive title', 'wordpress-seo' ), __( 'Replaced with the normal title for an archive generated by WordPress', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'sitename', __( 'Site title', 'wordpress-seo' ), __( 'The site\'s name', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'sitedesc', __( 'Tagline', 'wordpress-seo' ), __( 'The site\'s tagline', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'excerpt', __( 'Excerpt', 'wordpress-seo' ), __( 'Replaced with the post/page excerpt (or auto-generated if it does not exist)', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'excerpt_only', __( 'Excerpt only', 'wordpress-seo' ), __( 'Replaced with the post/page excerpt (without auto-generation)', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'tag', __( 'Tag', 'wordpress-seo' ), __( 'Replaced with the current tag/tags', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'category', __( 'Category', 'wordpress-seo' ), __( 'Replaced with the post categories (comma separated)', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'primary_category', __( 'Primary category', 'wordpress-seo' ), __( 'Replaced with the primary category of the post/page', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'category_description', __( 'Category description', 'wordpress-seo' ), __( 'Replaced with the category description', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'tag_description', __( 'Tag description', 'wordpress-seo' ), __( 'Replaced with the tag description', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'term_description', __( 'Term description', 'wordpress-seo' ), __( 'Replaced with the term description', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'term_title', __( 'Term title', 'wordpress-seo' ), __( 'Replaced with the term name', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'searchphrase', __( 'Search phrase', 'wordpress-seo' ), __( 'Replaced with the current search phrase', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'term_hierarchy', __( 'Term hierarchy', 'wordpress-seo' ), __( 'Replaced with the term ancestors hierarchy', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'sep', __( 'Separator', 'wordpress-seo' ), $separator_description ),
			new WPSEO_Replacement_Variable( 'currentdate', __( 'Current date', 'wordpress-seo' ), __( 'Replaced with the current date', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'currentyear', __( 'Current year', 'wordpress-seo' ), __( 'Replaced with the current year', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'currentmonth', __( 'Current month', 'wordpress-seo' ), __( 'Replaced with the current month', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'currentday', __( 'Current day', 'wordpress-seo' ), __( 'Replaced with the current day', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'post_year', __( 'Post year', 'wordpress-seo' ), __( 'Replaced with the year the post was published', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'post_month', __( 'Post month', 'wordpress-seo' ), __( 'Replaced with the month the post was published', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'post_day', __( 'Post day', 'wordpress-seo' ), __( 'Replaced with the day the post was published', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'author_first_name', __( 'Author first name', 'wordpress-seo' ), __( 'Replaced with the first name of the author', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'author_last_name', __( 'Author last name', 'wordpress-seo' ), __( 'Replaced with the last name of the author', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'permalink', __( 'Permalink', 'wordpress-seo' ), __( 'Replaced with the permalink', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'post_content', __( 'Post Content', 'wordpress-seo' ), __( 'Replaced with the post content', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'category_title', __( 'Category Title', 'wordpress-seo' ), __( 'Current or first category title', 'wordpress-seo' ) ),
		];

		foreach ( $replacement_variables as $replacement_variable ) {
			self::register_help_text( 'basic', $replacement_variable );
		}
	}

	/**
	 * Set/translate the help texts for the WPSEO standard advanced variables.
	 */
	private static function set_advanced_help_texts() {
		$replacement_variables = [
			new WPSEO_Replacement_Variable( 'pt_single', __( 'Post type (singular)', 'wordpress-seo' ), __( 'Replaced with the content type single label', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'pt_plural', __( 'Post type (plural)', 'wordpress-seo' ), __( 'Replaced with the content type plural label', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'modified', __( 'Modified', 'wordpress-seo' ), __( 'Replaced with the post/page modified time', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'id', __( 'ID', 'wordpress-seo' ), __( 'Replaced with the post/page ID', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'name', __( 'Name', 'wordpress-seo' ), __( 'Replaced with the post/page author\'s \'nicename\'', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'user_description', __( 'User description', 'wordpress-seo' ), __( 'Replaced with the post/page author\'s \'Biographical Info\'', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'page', __( 'Page', 'wordpress-seo' ), __( 'Replaced with the current page number with context (i.e. page 2 of 4)', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'pagetotal', __( 'Pagetotal', 'wordpress-seo' ), __( 'Replaced with the current page total', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'pagenumber', __( 'Pagenumber', 'wordpress-seo' ), __( 'Replaced with the current page number', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'caption', __( 'Caption', 'wordpress-seo' ), __( 'Attachment caption', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'focuskw', __( 'Focus keyword', 'wordpress-seo' ), __( 'Replaced with the posts focus keyphrase', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'term404', __( 'Term404', 'wordpress-seo' ), __( 'Replaced with the slug which caused the 404', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'cf_<custom-field-name>', '<custom-field-name> ' . __( '(custom field)', 'wordpress-seo' ), __( 'Replaced with a posts custom field value', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'ct_<custom-tax-name>', '<custom-tax-name> ' . __( '(custom taxonomy)', 'wordpress-seo' ), __( 'Replaced with a posts custom taxonomies, comma separated.', 'wordpress-seo' ) ),
			new WPSEO_Replacement_Variable( 'ct_desc_<custom-tax-name>', '<custom-tax-name> ' . __( 'description (custom taxonomy)', 'wordpress-seo' ), __( 'Replaced with a custom taxonomies description', 'wordpress-seo' ) ),
		];

		foreach ( $replacement_variables as $replacement_variable ) {
			self::register_help_text( 'advanced', $replacement_variable );
		}
	}

	/* *********************** GENERAL HELPER METHODS ************************** */

	/**
	 * Remove the '%%' delimiters from a variable string.
	 *
	 * @param string $text Variable string to be cleaned.
	 *
	 * @return string
	 */
	private static function remove_var_delimiter( $text ) {
		return trim( $text, '%' );
	}

	/**
	 * Add the '%%' delimiters to a variable string.
	 *
	 * @param string $text Variable string to be delimited.
	 *
	 * @return string
	 */
	private static function add_var_delimiter( $text ) {
		return '%%' . $text . '%%';
	}

	/**
	 * Retrieve a post's terms, comma delimited.
	 *
	 * @param int    $id            ID of the post to get the terms for.
	 * @param string $taxonomy      The taxonomy to get the terms for this post from.
	 * @param bool   $return_single If true, return the first term.
	 *
	 * @return string Either a single term or a comma delimited string of terms.
	 */
	public function get_terms( $id, $taxonomy, $return_single = false ) {
		$output = '';

		// If we're on a specific tag, category or taxonomy page, use that.
		if ( ! empty( $this->args->term_id ) ) {
			$output = $this->args->name;
		}
		elseif ( ! empty( $id ) && ! empty( $taxonomy ) ) {
			$terms = get_the_terms( $id, $taxonomy );
			if ( is_array( $terms ) && $terms !== [] ) {
				foreach ( $terms as $term ) {
					if ( $return_single ) {
						$output = $term->name;
						break;
					}
					else {
						$output .= $term->name . ', ';
					}
				}
				$output = rtrim( trim( $output ), ',' );
			}
		}
		unset( $terms, $term );

		/**
		 * Allows filtering of the terms list used to replace %%category%%, %%tag%%
		 * and %%ct_<custom-tax-name>%% variables.
		 *
		 * @api    string    $output    Comma-delimited string containing the terms.
		 * @api    string    $taxonomy  The taxonomy of the terms.
		 */
		return apply_filters( 'wpseo_terms', $output, $taxonomy );
	}

	/**
	 * Gets a taxonomy term hierarchy including the term to get the parents for.
	 *
	 * @return string
	 */
	private function get_term_hierarchy() {
		if ( ! is_taxonomy_hierarchical( $this->args->taxonomy ) ) {
			return '';
		}

		$separator = ' ' . $this->retrieve_sep() . ' ';

		$args = [
			'format'    => 'name',
			'separator' => $separator,
			'link'      => false,
			'inclusive' => true,
		];

		return rtrim(
			get_term_parents_list( $this->args->term_id, $this->args->taxonomy, $args ),
			$separator
		);
	}

	/**
	 * Retrieves the term ancestors hierarchy.
	 *
	 * @return string|null The term ancestors hierarchy.
	 */
	private function retrieve_term_hierarchy() {
		$replacement = null;

		if ( ! empty( $this->args->term_id ) && ! empty( $this->args->taxonomy ) ) {
			$hierarchy = $this->get_term_hierarchy();

			if ( $hierarchy !== '' ) {
				$replacement = esc_html( $hierarchy );
			}
		}

		return $replacement;
	}
}