<?php
/**
 * @package WPSEO\Internals
 * @since      1.5.0
 */

/**
 * This class implements defaults and value validation for all WPSEO Post Meta values.
 *
 * Some guidelines:
 * - To update a meta value, you can just use update_post_meta() with the full (prefixed) meta key
 *        or the convenience method WPSEO_Meta::set_value() with the internal key.
 *        All updates will be automatically validated.
 *        Meta values will only be saved to the database if they are *not* the same as the default to
 *        keep database load low.
 * - To retrieve a WPSEO meta value, you **must** use WPSEO_Meta::get_value() which will always return a
 *        string value, either the saved value or the default.
 *        This method can also retrieve a complete set of WPSEO meta values for one specific post, see
 *        the method documentation for the parameters.
 *
 * @internal   Unfortunately there isn't a filter available to hook into before returning the results
 * for get_post_meta(), get_post_custom() and the likes. That would have been the preferred solution.
 *
 * @internal   all WP native get_meta() results get cached internally, so no need to cache locally.
 * @internal   use $key when the key is the WPSEO internal name (without prefix), $meta_key when it
 *             includes the prefix
 */
class WPSEO_Meta {

	/**
	 * @var    string    Prefix for all WPSEO meta values in the database
	 * @static
	 *
	 * @internal if at any point this would change, quite apart from an upgrade routine, this also will need to
	 * be changed in the wpml-config.xml file.
	 */
	public static $meta_prefix = '_yoast_wpseo_';


	/**
	 * @var    string   Prefix for all WPSEO meta value form field names and ids
	 * @static
	 */
	public static $form_prefix = 'yoast_wpseo_';


	/**
	 * @var    int       Allowed length of the meta description.
	 * @static
	 */
	public static $meta_length = 156;


	/**
	 * @var    string   Reason the meta description is not the default length.
	 * @static
	 */
	public static $meta_length_reason = '';


	/**
	 * @var    array  $meta_fields Meta box field definitions for the meta box form
	 *                Array format:
	 *                (required)        'type'            => (string) field type. i.e. text / textarea / checkbox /
	 *                                                    radio / select / multiselect / upload / snippetpreview etc
	 *                (required)        'title'            => (string) table row title
	 *                (recommended)    'default_value' => (string|array) default value for the field
	 *                                                    IMPORTANT:
	 *                                                    - if the field has options, the default has to be the
	 *                                                      key of one of the options
	 *                                                    - if the field is a text field, the default **has** to be
	 *                                                      an empty string as otherwise the user can't save
	 *                                                      an empty value/delete the meta value
	 *                                                    - if the field is a checkbox, the only valid values
	 *                                                      are 'on' or 'off'
	 *                (semi-required)    'options'        => (array) options for used with (multi-)select and radio
	 *                                                    fields, required if that's the field type
	 *                                                    key = (string) value which will be saved to db
	 *                                                    value = (string) text label for the option
	 *                (optional)        'autocomplete'    => (bool) whether autocomplete is on for text fields,
	 *                                                    defaults to true
	 *                (optional)        'class'            => (string) classname(s) to add to the actual <input> tag
	 *                (optional)        'description'    => (string) description to show underneath the field
	 *                (optional)        'expl'            => (string) label for a checkbox
	 *                (optional)        'help'            => (string) help text to show on mouse over ? image
	 *                (optional)        'rows'            => (int) number of rows for a textarea, defaults to 3
	 *
	 *                (optional)        'placeholder'    => (string) Currently only used by add-on plugins
	 *                (optional)        'serialized'    => (bool) whether the value is expected to be serialized,
	 *                                                     i.e. an array or object, defaults to false
	 *                                                     Currently only used by add-on plugins
	 *
	 * @static
	 *
	 * @internal
	 * - Titles, help texts, description text and option labels are added via a translate_meta_boxes() method
	 *     in the relevant child classes (WPSEO_Metabox and WPSEO_Social_admin) as they are only needed there.
	 * - Beware: even though the meta keys are divided into subsets, they still have to be uniquely named!
	 */
	public static $meta_fields = array(
		'general'  => array(
			'snippetpreview' => array(
				'type'         => 'snippetpreview',
				'title'        => '', // Translation added later.
				'help'         => '', // Translation added later.
				'help-button'  => '', // Translation added later.
			),
			'focuskw_text_input' => array(
				'type'          => 'focuskeyword',
				'title'         => '', // Translation added later.
				'default_value' => '',
				'autocomplete'  => false,
				'help'          => '', // Translation added later.
				'description'   => '<div id="focuskwresults"></div>',
				'help-button'   => '', // Translation added later.
			),
			'focuskw' => array(
				'type'  => 'hidden',
				'title' => '',
			),
			'title'          => array(
				'type'          => 'hidden',
				'title'         => '', // Translation added later.
				'default_value' => '',
				'description'   => '', // Translation added later.
				'help'          => '', // Translation added later.
			),
			'metadesc'       => array(
				'type'          => 'hidden',
				'title'         => '', // Translation added later.
				'default_value' => '',
				'class'         => 'metadesc',
				'rows'          => 2,
				'description'   => '', // Translation added later.
				'help'          => '', // Translation added later.
			),
			'linkdex'        => array(
				'type'          => 'hidden',
				'title'         => 'linkdex',
				'default_value' => '0',
				'description'   => '',
			),
			'content_score'  => array(
				'type'          => 'hidden',
				'title'         => 'content_score',
				'default_value' => '0',
				'description'   => '',
			),
			'metakeywords'   => array(
				'type'          => 'metakeywords',
				'title'         => '', // Translation added later.
				'default_value' => '',
				'class'         => 'metakeywords',
				'description'   => '', // Translation added later.
			),
			'pageanalysis'   => array(
				'type'         => 'pageanalysis',
				'title'        => '', // Translation added later.
				'help'         => '', // Translation added later.
				'help-button'  => '', // Translation added later.
			),
		),
		'advanced' => array(
			'meta-robots-noindex'  => array(
				'type'          => 'select',
				'title'         => '', // Translation added later.
				'default_value' => '0', // = post-type default.
				'options'       => array(
					'0' => '', // Post type default - translation added later.
					'2' => '', // Index - translation added later.
					'1' => '', // No-index - translation added later.
				),
			),
			'meta-robots-nofollow' => array(
				'type'          => 'radio',
				'title'         => '', // Translation added later.
				'default_value' => '0', // = follow.
				'options'       => array(
					'0' => '', // Follow - translation added later.
					'1' => '', // No-follow - translation added later.
				),
			),
			'meta-robots-adv'      => array(
				'type'          => 'multiselect',
				'title'         => '', // Translation added later.
				'default_value' => '-', // = site-wide default.
				'description'   => '', // Translation added later.
				'options'       => array(
					'-'            => '', // Site-wide default - translation added later.
					'none'         => '', // Translation added later.
					'noodp'        => '', // Translation added later.
					'noimageindex' => '', // Translation added later.
					'noarchive'    => '', // Translation added later.
					'nosnippet'    => '', // Translation added later.
				),
			),
			'bctitle'              => array(
				'type'          => 'text',
				'title'         => '', // Translation added later.
				'default_value' => '',
				'description'   => '', // Translation added later.
			),
			'canonical'            => array(
				'type'          => 'text',
				'title'         => '', // Translation added later.
				'default_value' => '',
				'description'   => '', // Translation added later.
			),
			'redirect'             => array(
				'type'          => 'text',
				'title'         => '', // Translation added later.
				'default_value' => '',
				'description'   => '', // Translation added later.
			),
		),
		'social'   => array(),
		/* Fields we should validate & save, but not show on any form */
		'non_form' => array(
			'linkdex' => array(
				'type'          => null,
				'default_value' => '0',
			),
		),
	);


	/**
	 * @var    array    Helper property - reverse index of the definition array
	 *                  Format: [full meta key including prefix]    => array
	 *                          ['subset']    => (string) primary index
	 *                          ['key']       => (string) internal key
	 * @static
	 */
	public static $fields_index = array();


	/**
	 * @var    array    Helper property - array containing only the defaults in the format:
	 *                  [full meta key including prefix]    => (string) default value
	 * @static
	 */
	public static $defaults = array();

	/**
	 * @var    array    Helper property to define the social network meta field definitions - networks
	 * @static
	 */
	private static $social_networks = array(
		'opengraph'  => 'opengraph',
		'twitter'    => 'twitter',
	);

	/**
	 * @var    array    Helper property to define the social network meta field definitions - fields and their type
	 * @static
	 */
	private static $social_fields = array(
		'title'       => 'text',
		'description' => 'textarea',
		'image'       => 'upload',
	);


	/**
	 * Register our actions and filters
	 *
	 * @static
	 * @return void
	 */
	public static function init() {

		$options = WPSEO_Options::get_option( 'wpseo_social' );
		foreach ( self::$social_networks as $option => $network ) {
			if ( true === $options[ $option ] ) {
				foreach ( self::$social_fields as $box => $type ) {
					self::$meta_fields['social'][ $network . '-' . $box ] = array(
						'type'          => $type,
						'title'         => '', // Translation added later.
						'default_value' => '',
						'description'   => '', // Translation added later.
					);
				}
			}
		}
		unset( $options, $option, $network, $box, $type );

		/**
		 * Allow add-on plugins to register their meta fields for management by this class
		 * add_filter() calls must be made before plugins_loaded prio 14
		 */
		$extra_fields = apply_filters( 'add_extra_wpseo_meta_fields', array() );
		if ( is_array( $extra_fields ) ) {
			self::$meta_fields = self::array_merge_recursive_distinct( $extra_fields, self::$meta_fields );
		}
		unset( $extra_fields );

		$register = function_exists( 'register_meta' );

		foreach ( self::$meta_fields as $subset => $field_group ) {
			foreach ( $field_group as $key => $field_def ) {
				if ( $field_def['type'] !== 'snippetpreview' ) {
					/**
					 * Function register_meta() is undocumented and not used by WP internally, wrapped in
					 * function_exists as a precaution in case they remove it.
					 */
					if ( $register === true ) {
						register_meta( 'post', self::$meta_prefix . $key, array( __CLASS__, 'sanitize_post_meta' ) );
					}
					else {
						add_filter( 'sanitize_post_meta_' . self::$meta_prefix . $key, array( __CLASS__, 'sanitize_post_meta' ), 10, 2 );
					}

					// Set the $fields_index property for efficiency.
					self::$fields_index[ self::$meta_prefix . $key ] = array(
						'subset' => $subset,
						'key'    => $key,
					);

					// Set the $defaults property for efficiency.
					if ( isset( $field_def['default_value'] ) ) {
						self::$defaults[ self::$meta_prefix . $key ] = $field_def['default_value'];
					}
					else {
						// Meta will always be a string, so let's make the meta meta default also a string.
						self::$defaults[ self::$meta_prefix . $key ] = '';
					}
				}
			}
		}
		unset( $subset, $field_group, $key, $field_def, $register );

		add_filter( 'update_post_metadata', array( __CLASS__, 'remove_meta_if_default' ), 10, 5 );
		add_filter( 'add_post_metadata', array( __CLASS__, 'dont_save_meta_if_default' ), 10, 4 );
	}


	/**
	 * Retrieve the meta box form field definitions for the given tab and post type.
	 *
	 * @static
	 *
	 * @param  string $tab       Tab for which to retrieve the field definitions.
	 * @param  string $post_type Post type of the current post.
	 *
	 * @return array             Array containing the meta box field definitions
	 */
	public static function get_meta_field_defs( $tab, $post_type = 'post' ) {
		if ( ! isset( self::$meta_fields[ $tab ] ) ) {
			return array();
		}

		$field_defs = self::$meta_fields[ $tab ];

		switch ( $tab ) {
			case 'non-form':
				// Prevent non-form fields from being passed to forms.
				$field_defs = array();
				break;


			case 'general':
				$options = WPSEO_Options::get_option( 'wpseo_titles' );
				if ( $options['usemetakeywords'] === true ) {
					/* Adjust the link in the keywords description text string based on the post type */
					$field_defs['metakeywords']['description'] = sprintf( $field_defs['metakeywords']['description'], '<a target="_blank" href="' . esc_url( admin_url( 'admin.php?page=wpseo_titles#top#post_types' ) ) . '">', '</a>' );
				}
				else {
					/* Don't show the keywords field if keywords aren't enabled */
					unset( $field_defs['metakeywords'] );
				}
				/**
				 * Filter the WPSEO metabox form field definitions for the general tab, backward compatibility
				 *
				 * @deprecated 1.5.0
				 * @deprecated use the 'wpseo_metabox_entries_general' filter instead
				 * @see        WPSEO_Meta::get_meta_field_defs()
				 *
				 * @param      array $field_defs Metabox orm definitions.
				 *
				 * @return     array
				 */
				$field_defs = apply_filters( 'wpseo_metabox_entries', $field_defs );
				break;


			case 'advanced':
				global $post;

				$options = WPSEO_Options::get_options( array( 'wpseo', 'wpseo_titles', 'wpseo_internallinks' ) );

				if ( ! current_user_can( 'manage_options' ) && $options['disableadvanced_meta'] ) {
					return array();
				}

				$post_type = '';
				if ( isset( $post->post_type ) ) {
					$post_type = $post->post_type;
				}
				elseif ( ! isset( $post->post_type ) && isset( $_GET['post_type'] ) ) {
					$post_type = sanitize_text_field( $_GET['post_type'] );
				}

				/* Adjust the no-index 'default for post type' text string based on the post type */
				$field_defs['meta-robots-noindex']['options']['0'] = sprintf( $field_defs['meta-robots-noindex']['options']['0'], ( ( isset( $options[ 'noindex-' . $post_type ] ) && $options[ 'noindex-' . $post_type ] === true ) ? 'noindex' : 'index' ) );

				/* Adjust the robots advanced 'site-wide default' text string based on those settings */
				if ( $options['noodp'] !== false ) {
					$robots_adv = array();
					if ( $options['noodp'] === true ) {
						// Use translation from field def options - mind that $options and $field_def['options'] keys should be the same!
						$robots_adv[] = $field_defs['meta-robots-adv']['options']['noodp'];
					}
					$robots_adv = implode( ', ', $robots_adv );
				}
				else {
					$robots_adv = __( 'None', 'wordpress-seo' );
				}
				$field_defs['meta-robots-adv']['options']['-'] = sprintf( $field_defs['meta-robots-adv']['options']['-'], $robots_adv );
				unset( $robots_adv );


				/* Don't show the breadcrumb title field if breadcrumbs aren't enabled */
				if ( $options['breadcrumbs-enable'] !== true && ! current_theme_supports( 'yoast-seo-breadcrumbs' ) ) {
					unset( $field_defs['bctitle'] );
				}

				global $post;

				if ( empty( $post->ID ) || ( ! empty( $post->ID ) && self::get_value( 'redirect', $post->ID ) === '' ) ) {
					unset( $field_defs['redirect'] );
				}
				break;
		}

		/**
		 * Filter the WPSEO metabox form field definitions for a tab
		 * {tab} can be 'general', 'advanced' or 'social'
		 *
		 * @param  array  $field_defs Metabox form definitions.
		 * @param  string $post_type  Post type of the post the metabox is for, defaults to 'post'.
		 *
		 * @return array
		 */

		return apply_filters( 'wpseo_metabox_entries_' . $tab, $field_defs, $post_type );
	}


	/**
	 * Validate the post meta values
	 *
	 * @static
	 *
	 * @param  mixed  $meta_value The new value.
	 * @param  string $meta_key   The full meta key (including prefix).
	 *
	 * @return string             Validated meta value
	 */
	public static function sanitize_post_meta( $meta_value, $meta_key ) {
		$field_def = self::$meta_fields[ self::$fields_index[ $meta_key ]['subset'] ][ self::$fields_index[ $meta_key ]['key'] ];
		$clean     = self::$defaults[ $meta_key ];

		switch ( true ) {
			case ( $meta_key === self::$meta_prefix . 'linkdex' ):
				$int = WPSEO_Utils::validate_int( $meta_value );
				if ( $int !== false && $int >= 0 ) {
					$clean = strval( $int ); // Convert to string to make sure default check works.
				}
				break;

			case ( $field_def['type'] === 'checkbox' ):
				// Only allow value if it's one of the predefined options.
				if ( in_array( $meta_value, array( 'on', 'off' ), true ) ) {
					$clean = $meta_value;
				}
				break;


			case ( $field_def['type'] === 'select' || $field_def['type'] === 'radio' ):
				// Only allow value if it's one of the predefined options.
				if ( isset( $field_def['options'][ $meta_value ] ) ) {
					$clean = $meta_value;
				}
				break;


			case ( $field_def['type'] === 'multiselect' && $meta_key === self::$meta_prefix . 'meta-robots-adv' ):
				$clean = self::validate_meta_robots_adv( $meta_value );
				break;


			case ( $field_def['type'] === 'text' && $meta_key === self::$meta_prefix . 'canonical' ):
			case ( $field_def['type'] === 'text' && $meta_key === self::$meta_prefix . 'redirect' ):
				// Validate as url(-part).
				$url = WPSEO_Utils::sanitize_url( $meta_value );
				if ( $url !== '' ) {
					$clean = $url;
				}
				break;


			case ( $field_def['type'] === 'upload' && $meta_key === self::$meta_prefix . 'opengraph-image' ):
				// Validate as url.
				$url = WPSEO_Utils::sanitize_url( $meta_value, array( 'http', 'https', 'ftp', 'ftps' ) );
				if ( $url !== '' ) {
					$clean = $url;
				}
				break;


			case ( $field_def['type'] === 'textarea' ):
				if ( is_string( $meta_value ) ) {
					// Remove line breaks and tabs.
					// @todo [JRF => Yoast] verify that line breaks and the likes aren't allowed/recommended in meta header fields.
					$meta_value = str_replace( array( "\n", "\r", "\t", '  ' ), ' ', $meta_value );
					$clean      = WPSEO_Utils::sanitize_text_field( trim( $meta_value ) );
				}
				break;

			case ( 'multiselect' === $field_def['type'] ):
				$clean = $meta_value;
				break;


			case ( $field_def['type'] === 'text' ):
			default:
				if ( is_string( $meta_value ) ) {
					$clean = WPSEO_Utils::sanitize_text_field( trim( $meta_value ) );
				}

				if ( $meta_key === self::$meta_prefix . 'focuskw' ) {
					$clean = str_replace( array(
						'&lt;',
						'&gt;',
						'&quot',
						'&#96',
						'<',
						'>',
						'"',
						'`',
					), '', $clean );
				}
				break;
		}

		$clean = apply_filters( 'wpseo_sanitize_post_meta_' . $meta_key, $clean, $meta_value, $field_def, $meta_key );

		return $clean;
	}


	/**
	 * Validate a meta-robots-adv meta value
	 *
	 * @todo [JRF => Yoast] Verify that this logic for the prioritisation is correct
	 *
	 * @static
	 *
	 * @param  array|string $meta_value The value to validate.
	 *
	 * @return string       Clean value
	 */
	public static function validate_meta_robots_adv( $meta_value ) {
		$clean   = self::$meta_fields['advanced']['meta-robots-adv']['default_value'];
		$options = self::$meta_fields['advanced']['meta-robots-adv']['options'];

		if ( is_string( $meta_value ) ) {
			$meta_value = explode( ',', $meta_value );
		}

		if ( is_array( $meta_value ) && $meta_value !== array() ) {
			$meta_value = array_map( 'trim', $meta_value );

			if ( in_array( 'none', $meta_value, true ) ) {
				// None is one of the selected values, takes priority over everything else.
				$clean = 'none';
			}
			elseif ( in_array( '-', $meta_value, true ) ) {
				// Site-wide defaults is one of the selected values, takes priority over individual selected entries.
				$clean = '-';
			}
			else {
				// Individual selected entries.
				$cleaning = array();
				foreach ( $meta_value as $value ) {
					if ( isset( $options[ $value ] ) ) {
						$cleaning[] = $value;
					}
				}

				if ( $cleaning !== array() ) {
					$clean = implode( ',', $cleaning );
				}
				unset( $cleaning, $value );
			}
		}

		return $clean;
	}


	/**
	 * Prevent saving of default values and remove potential old value from the database if replaced by a default
	 *
	 * @static
	 *
	 * @param  null   $null       Old, disregard.
	 * @param  int    $object_id  ID of the current object for which the meta is being updated.
	 * @param  string $meta_key   The full meta key (including prefix).
	 * @param  string $meta_value New meta value.
	 * @param  string $prev_value The old meta value.
	 *
	 * @return null|bool          true = stop saving, null = continue saving
	 */
	public static function remove_meta_if_default( $null, $object_id, $meta_key, $meta_value, $prev_value = '' ) {
		/* If it's one of our meta fields, check against default */
		if ( isset( self::$fields_index[ $meta_key ] ) && self::meta_value_is_default( $meta_key, $meta_value ) === true ) {
			if ( $prev_value !== '' ) {
				delete_post_meta( $object_id, $meta_key, $prev_value );
			}
			else {
				delete_post_meta( $object_id, $meta_key );
			}

			return true; // Stop saving the value.
		}

		return null; // Go on with the normal execution (update) in meta.php.
	}


	/**
	 * Prevent adding of default values to the database
	 *
	 * @static
	 *
	 * @param  null   $null       Old, disregard.
	 * @param  int    $object_id  ID of the current object for which the meta is being added.
	 * @param  string $meta_key   The full meta key (including prefix).
	 * @param  string $meta_value New meta value.
	 *
	 * @return null|bool          true = stop saving, null = continue saving
	 */
	public static function dont_save_meta_if_default( $null, $object_id, $meta_key, $meta_value ) {
		/* If it's one of our meta fields, check against default */
		if ( isset( self::$fields_index[ $meta_key ] ) && self::meta_value_is_default( $meta_key, $meta_value ) === true ) {
			return true; // Stop saving the value.
		}

		return null; // Go on with the normal execution (add) in meta.php.
	}


	/**
	 * Is the given meta value the same as the default value ?
	 *
	 * @static
	 *
	 * @param  string $meta_key   The full meta key (including prefix).
	 * @param  mixed  $meta_value The value to check.
	 *
	 * @return bool
	 */
	public static function meta_value_is_default( $meta_key, $meta_value ) {
		return ( isset( self::$defaults[ $meta_key ] ) && $meta_value === self::$defaults[ $meta_key ] );
	}


	/**
	 * Get a custom post meta value
	 * Returns the default value if the meta value has not been set
	 *
	 * @internal Unfortunately there isn't a filter available to hook into before returning the results
	 * for get_post_meta(), get_post_custom() and the likes. That would have been the preferred solution.
	 *
	 * @static
	 *
	 * @param  string $key    Internal key of the value to get (without prefix).
	 * @param  int    $postid Post ID of the post to get the value for.
	 *
	 * @return string         All 'normal' values returned from get_post_meta() are strings.
	 *                        Objects and arrays are possible, but not used by this plugin
	 *                        and therefore discarted (except when the special 'serialized' field def
	 *                        value is set to true - only used by add-on plugins for now).
	 *                        Will return the default value if no value was found..
	 *                        Will return empty string if no default was found (not one of our keys) or
	 *                        if the post does not exist.
	 */
	public static function get_value( $key, $postid = 0 ) {
		global $post;

		$postid = absint( $postid );
		if ( $postid === 0 ) {
			if ( ( isset( $post ) && is_object( $post ) ) && ( isset( $post->post_status ) && $post->post_status !== 'auto-draft' ) ) {
				$postid = $post->ID;
			}
			else {
				return '';
			}
		}

		$custom = get_post_custom( $postid ); // Array of strings or empty array.

		if ( isset( $custom[ self::$meta_prefix . $key ][0] ) ) {
			$unserialized = maybe_unserialize( $custom[ self::$meta_prefix . $key ][0] );
			if ( $custom[ self::$meta_prefix . $key ][0] === $unserialized ) {
				return $custom[ self::$meta_prefix . $key ][0];
			}
			else {
				$field_def = self::$meta_fields[ self::$fields_index[ self::$meta_prefix . $key ]['subset'] ][ self::$fields_index[ self::$meta_prefix . $key ]['key'] ];
				if ( isset( $field_def['serialized'] ) && $field_def['serialized'] === true ) {
					// Ok, serialize value expected/allowed.
					return $unserialized;
				}
			}
		}

		// Meta was either not found or found, but object/array while not allowed to be.
		if ( isset( self::$defaults[ self::$meta_prefix . $key ] ) ) {
			return self::$defaults[ self::$meta_prefix . $key ];
		}
		else {
			/*
			Shouldn't ever happen, means not one of our keys as there will always be a default available
			   for all our keys
			*/
			return '';
		}
	}


	/**
	 * Update a meta value for a post
	 *
	 * @static
	 *
	 * @param  string $key        The internal key of the meta value to change (without prefix).
	 * @param  mixed  $meta_value The value to set the meta to.
	 * @param  int    $post_id    The ID of the post to change the meta for.
	 *
	 * @return bool   whether the value was changed
	 */
	public static function set_value( $key, $meta_value, $post_id ) {
		return update_post_meta( $post_id, self::$meta_prefix . $key, $meta_value );
	}

	/**
	 * Deletes a meta value for a post
	 *
	 * @static
	 *
	 * @param string $key The internal key of the meta value to change (without prefix).
	 * @param int    $post_id The ID of the post to change the meta for.
	 *
	 * @return bool Whether the value was changed
	 */
	public static function delete( $key, $post_id ) {
		return delete_post_meta( $post_id, self::$meta_prefix . $key );
	}

	/**
	 * Used for imports, this functions imports the value of $old_metakey into $new_metakey for those post
	 * where no WPSEO meta data has been set.
	 * Optionally deletes the $old_metakey values.
	 *
	 * @static
	 *
	 * @param  string $old_metakey The old key of the meta value.
	 * @param  string $new_metakey The new key, usually the WPSEO meta key (including prefix).
	 * @param  bool   $delete_old  Whether to delete the old meta key/value-sets.
	 *
	 * @return void
	 */
	public static function replace_meta( $old_metakey, $new_metakey, $delete_old = false ) {
		global $wpdb;

		/*
		Get only those rows where no wpseo meta values exist for the same post
		   (with the exception of linkdex as that will be set independently of whether the post has been edited)
		   @internal Query is pretty well optimized this way
		*/
		$query  = $wpdb->prepare(
			"
				SELECT `a`.*
				FROM {$wpdb->postmeta} AS a
				WHERE `a`.`meta_key` = %s
					AND NOT	EXISTS (
						SELECT DISTINCT `post_id` , count( `meta_id` ) AS count
						FROM {$wpdb->postmeta} AS b
						WHERE `a`.`post_id` = `b`.`post_id`
							AND `meta_key` LIKE %s
							AND `meta_key` <> %s
						GROUP BY `post_id`
					)
				;",
			$old_metakey,
			$wpdb->esc_like( self::$meta_prefix . '%' ),
			self::$meta_prefix . 'linkdex'
		);
		$oldies = $wpdb->get_results( $query );

		if ( is_array( $oldies ) && $oldies !== array() ) {
			foreach ( $oldies as $old ) {
				update_post_meta( $old->post_id, $new_metakey, $old->meta_value );
			}
		}

		// Delete old keys.
		if ( $delete_old === true ) {
			delete_post_meta_by_key( $old_metakey );
		}
	}


	/**
	 * General clean-up of the saved meta values
	 * - Remove potentially lingering old meta keys
	 * - Remove all default and invalid values
	 *
	 * @static
	 * @return void
	 */
	public static function clean_up() {
		global $wpdb;

		/**
		 * Clean up '_yoast_wpseo_meta-robots'
		 *
		 * Retrieve all '_yoast_wpseo_meta-robots' meta values and convert if no new values found
		 *
		 * @internal Query is pretty well optimized this way
		 *
		 * @todo [JRF => Yoast] find out all possible values which the old '_yoast_wpseo_meta-robots' could contain
		 * to convert the data correctly
		 */
		$query  = $wpdb->prepare(
			"
				SELECT `a`.*
				FROM {$wpdb->postmeta} AS a
				WHERE `a`.`meta_key` = %s
					AND NOT	EXISTS (
						SELECT DISTINCT `post_id` , count( `meta_id` ) AS count
						FROM {$wpdb->postmeta} AS b
						WHERE `a`.`post_id` = `b`.`post_id`
							AND ( `meta_key` = %s
							OR `meta_key` = %s )
						GROUP BY `post_id`
					)
				;",
			self::$meta_prefix . 'meta-robots',
			self::$meta_prefix . 'meta-robots-noindex',
			self::$meta_prefix . 'meta-robots-nofollow'
		);
		$oldies = $wpdb->get_results( $query );

		if ( is_array( $oldies ) && $oldies !== array() ) {
			foreach ( $oldies as $old ) {
				$old_values = explode( ',', $old->meta_value );
				foreach ( $old_values as $value ) {
					if ( $value === 'noindex' ) {
						update_post_meta( $old->post_id, self::$meta_prefix . 'meta-robots-noindex', 1 );
					}
					elseif ( $value === 'nofollow' ) {
						update_post_meta( $old->post_id, self::$meta_prefix . 'meta-robots-nofollow', 1 );
					}
				}
			}
		}
		unset( $query, $oldies, $old, $old_values, $value );

		// Delete old keys.
		delete_post_meta_by_key( self::$meta_prefix . 'meta-robots' );


		/**
		 * Remove all default values and (most) invalid option values
		 * Invalid option values for the multiselect (meta-robots-adv) field will be dealt with seperately
		 *
		 * @internal some of the defaults have changed in v1.5, but as the defaults will be removed and
		 * new defaults will now automatically be passed when no data found, this update is automatic
		 * (as long as we remove the old values which we do in the below routine)
		 *
		 * @internal unfortunately we can't use the normal delete_meta() with key/value combination as ''
		 * (empty string) values will be ignored and would result in all metas with that key being deleted,
		 * not just the empty fields.
		 * Still, the below implementation is largely based on the delete_meta() function
		 */
		$query = array();

		foreach ( self::$meta_fields as $subset => $field_group ) {
			foreach ( $field_group as $key => $field_def ) {
				if ( $field_def['type'] === 'snippetpreview' || ! isset( $field_def['default_value'] ) ) {
					continue;
				}

				if ( $key === 'meta-robots-adv' ) {
					$query[] = $wpdb->prepare(
						"( meta_key = %s AND ( meta_value = 'none' OR meta_value = '-' ) )",
						self::$meta_prefix . $key
					);
				}
				elseif ( isset( $field_def['options'] ) && is_array( $field_def['options'] ) && $field_def['options'] !== array() ) {
					$valid = $field_def['options'];
					// Remove the default value from the valid options.
					unset( $valid[ $field_def['default_value'] ] );
					$valid = array_keys( $valid );

					$query[] = $wpdb->prepare(
						"( meta_key = %s AND meta_value NOT IN ( '" . implode( "','", esc_sql( $valid ) ) . "' ) )",
						self::$meta_prefix . $key
					);
					unset( $valid );
				}
				elseif ( is_string( $field_def['default_value'] ) && $field_def['default_value'] !== '' ) {
					$query[] = $wpdb->prepare(
						'( meta_key = %s AND meta_value = %s )',
						self::$meta_prefix . $key,
						$field_def['default_value']
					);
				}
				else {
					$query[] = $wpdb->prepare(
						"( meta_key = %s AND meta_value = '' )",
						self::$meta_prefix . $key
					);
				}
			}
		}
		unset( $subset, $field_group, $key, $field_def );

		$query    = "SELECT meta_id FROM {$wpdb->postmeta} WHERE " . implode( ' OR ', $query ) . ';';
		$meta_ids = $wpdb->get_col( $query );

		if ( is_array( $meta_ids ) && $meta_ids !== array() ) {
			// WP native action.
			do_action( 'delete_post_meta', $meta_ids, null, null, null );

			$query = "DELETE FROM {$wpdb->postmeta} WHERE meta_id IN( " . implode( ',', $meta_ids ) . ' )';
			$count = $wpdb->query( $query );

			if ( $count ) {
				foreach ( $meta_ids as $object_id ) {
					wp_cache_delete( $object_id, 'post_meta' );
				}

				// WP native action.
				do_action( 'deleted_post_meta', $meta_ids, null, null, null );
			}
		}
		unset( $query, $meta_ids, $count, $object_id );


		/**
		 * Deal with the multiselect (meta-robots-adv) field
		 *
		 * Removes invalid option combinations, such as 'none,noarchive'
		 *
		 * Default values have already been removed, so we should have a small result set and
		 * (hopefully) even smaller set of invalid results.
		 */
		$query  = $wpdb->prepare(
			"SELECT meta_id, meta_value FROM {$wpdb->postmeta} WHERE meta_key = %s",
			self::$meta_prefix . 'meta-robots-adv'
		);
		$oldies = $wpdb->get_results( $query );

		if ( is_array( $oldies ) && $oldies !== array() ) {
			foreach ( $oldies as $old ) {
				$clean = self::validate_meta_robots_adv( $old->meta_value );

				if ( $clean !== $old->meta_value ) {
					if ( $clean !== self::$meta_fields['advanced']['meta-robots-adv']['default_value'] ) {
						update_metadata_by_mid( 'post', $old->meta_id, $clean );
					}
					else {
						delete_metadata_by_mid( 'post', $old->meta_id );
					}
				}
			}
		}
		unset( $query, $oldies, $old, $clean );

		do_action( 'wpseo_meta_clean_up' );
	}


	/**
	 * Recursively merge a variable number of arrays, using the left array as base,
	 * giving priority to the right array.
	 *
	 * Difference with native array_merge_recursive():
	 * array_merge_recursive converts values with duplicate keys to arrays rather than
	 * overwriting the value in the first array with the duplicate value in the second array.
	 *
	 * array_merge_recursive_distinct does not change the data types of the values in the arrays.
	 * Matching keys' values in the second array overwrite those in the first array, as is the
	 * case with array_merge.
	 *
	 * Freely based on information found on http://www.php.net/manual/en/function.array-merge-recursive.php
	 *
	 * @internal Should be moved to a general utility class
	 *
	 * @return array
	 */
	public static function array_merge_recursive_distinct() {

		$arrays = func_get_args();
		if ( count( $arrays ) < 2 ) {
			if ( $arrays === array() ) {
				return array();
			}
			else {
				return $arrays[0];
			}
		}

		$merged = array_shift( $arrays );

		foreach ( $arrays as $array ) {
			foreach ( $array as $key => $value ) {
				if ( is_array( $value ) && ( isset( $merged[ $key ] ) && is_array( $merged[ $key ] ) ) ) {
					$merged[ $key ] = self::array_merge_recursive_distinct( $merged[ $key ], $value );
				}
				else {
					$merged[ $key ] = $value;
				}
			}
			unset( $key, $value );
		}

		return $merged;
	}

	/**
	 * Get a value from $_POST for a given key
	 * Returns the $_POST value if exists, returns an empty string if key does not exist
	 *
	 * @static
	 *
	 * @param  string $key Key of the value to get from $_POST.
	 *
	 * @return string      Returns $_POST value, which will be a string the majority of the time
	 *                     Will return empty string if key does not exists in $_POST
	 */
	public static function get_post_value( $key ) {
		return ( array_key_exists( $key, $_POST ) ) ? $_POST[ $key ] : '';
	}

	/**
	 * Counts the total of all the keywords being used for posts except the given one
	 *
	 * @param string  $keyword The keyword to be counted.
	 * @param integer $post_id The is of the post to which the keyword belongs.
	 *
	 * @return array
	 */
	public static function keyword_usage( $keyword, $post_id ) {

		if ( empty( $keyword ) ) {
			return array();
		}

		$get_posts = new WP_Query(
			array(
				'meta_key'       => '_yoast_wpseo_focuskw',
				'meta_value'     => $keyword,
				'post__not_in'   => array( $post_id ),
				'fields'         => 'ids',
				'post_type'      => 'any',

				/*
				 * We only need to return zero, one or two results:
				 * - Zero: keyword hasn't been used before
				 * - One: Keyword has been used once before
				 * - Two or more: Keyword has been used twice before
				 */
				'posts_per_page' => 2,
			)
		);

		return $get_posts->posts;
	}
} /* End of class */