=> 'title',
			'metadesc'                       => 'description',
			'bctitle'                        => 'breadcrumb_title',
			'opengraph-title'                => 'open_graph_title',
			'opengraph-image'                => 'open_graph_image',
			'opengraph-image-id'             => 'open_graph_image_id',
			'opengraph-description'          => 'open_graph_description',
			'twitter-title'                  => 'twitter_title',
			'twitter-image'                  => 'twitter_image',
			'twitter-image-id'               => 'twitter_image_id',
			'twitter-description'            => 'twitter_description',
			'estimated-reading-time-minutes' => 'estimated_reading_time_minutes',
		];
	}

	/**
	 * Retrieves the current value for the meta field.
	 *
	 * @param int    $post_id  The post ID to use.
	 * @param string $meta_key Meta key to fetch.
	 *
	 * @return mixed The value of the indexable entry to use.
	 */
	protected function get_meta_value( $post_id, $meta_key ) {
		$value = WPSEO_Meta::get_value( $meta_key, $post_id );
		if ( \is_string( $value ) && $value === '' ) {
			return null;
		}

		return $value;
	}

	/**
	 * Finds an alternative image for the social image.
	 *
	 * @param Indexable $indexable The indexable.
	 *
	 * @return array|bool False when not found, array with data when found.
	 */
	protected function find_alternative_image( Indexable $indexable ) {
		if (
			$indexable->object_sub_type === 'attachment'
			&& $this->image->is_valid_attachment( $indexable->object_id )
		) {
			return [
				'image_id' => $indexable->object_id,
				'source'   => 'attachment-image',
			];
		}

		$featured_image_id = $this->image->get_featured_image_id( $indexable->object_id );
		if ( $featured_image_id ) {
			return [
				'image_id' => $featured_image_id,
				'source'   => 'featured-image',
			];
		}

		$gallery_image = $this->image->get_gallery_image( $indexable->object_id );
		if ( $gallery_image ) {
			return [
				'image'  => $gallery_image,
				'source' => 'gallery-image',
			];
		}

		$content_image = $this->image->get_post_content_image( $indexable->object_id );
		if ( $content_image ) {
			return [
				'image'  => $content_image,
				'source' => 'first-content-image',
			];
		}

		return false;
	}

	/**
	 * Gets the number of pages for a post.
	 *
	 * @param object $post The post object.
	 *
	 * @return int|null The number of pages or null if the post isn't paginated.
	 */
	protected function get_number_of_pages_for_post( $post ) {
		$number_of_pages = ( \substr_count( $post->post_content, '<!--nextpage-->' ) + 1 );

		if ( $number_of_pages <= 1 ) {
			return null;
		}

		return $number_of_pages;
	}

	/**
	 * Checks whether an indexable should be built for this post.
	 *
	 * @param \WP_Post $post The post for which an indexable should be built.
	 *
	 * @return bool `true` if the post should be excluded from building, `false` if not.
	 */
	protected function should_exclude_post( $post ) {
		return $this->post_type_helper->is_excluded( $post->post_type );
	}
}