When registering complex meta values this argument may optionally be an
 *                                         array with 'schema' or 'prepare_callback' keys instead of a boolean.
 *     @type bool       $revisions_enabled Whether to enable revisions support for this meta_key. Can only be used when the
 *                                         object type is 'post'.
 * }
 * @param string|array $deprecated Deprecated. Use `$args` instead.
 * @return bool True if the meta key was successfully registered in the global array, false if not.
 *              Registering a meta key with distinct sanitize and auth callbacks will fire those callbacks,
 *              but will not add to the global registry.
 */
function register_meta( $object_type, $meta_key, $args, $deprecated = null ) {
	global $wp_meta_keys;

	if ( ! is_array( $wp_meta_keys ) ) {
		$wp_meta_keys = array();
	}

	$defaults = array(
		'object_subtype'    => '',
		'type'              => 'string',
		'description'       => '',
		'default'           => '',
		'single'            => false,
		'sanitize_callback' => null,
		'auth_callback'     => null,
		'show_in_rest'      => false,
		'revisions_enabled' => false,
	);

	// There used to be individual args for sanitize and auth callbacks.
	$has_old_sanitize_cb = false;
	$has_old_auth_cb     = false;

	if ( is_callable( $args ) ) {
		$args = array(
			'sanitize_callback' => $args,
		);

		$has_old_sanitize_cb = true;
	} else {
		$args = (array) $args;
	}

	if ( is_callable( $deprecated ) ) {
		$args['auth_callback'] = $deprecated;
		$has_old_auth_cb       = true;
	}

	/**
	 * Filters the registration arguments when registering meta.
	 *
	 * @since 4.6.0
	 *
	 * @param array  $args        Array of meta registration arguments.
	 * @param array  $defaults    Array of default arguments.
	 * @param string $object_type Type of object metadata is for. Accepts 'post', 'comment', 'term', 'user',
	 *                            or any other object type with an associated meta table.
	 * @param string $meta_key    Meta key.
	 */
	$args = apply_filters( 'register_meta_args', $args, $defaults, $object_type, $meta_key );
	unset( $defaults['default'] );
	$args = wp_parse_args( $args, $defaults );

	// Require an item schema when registering array meta.
	if ( false !== $args['show_in_rest'] && 'array' === $args['type'] ) {
		if ( ! is_array( $args['show_in_rest'] ) || ! isset( $args['show_in_rest']['schema']['items'] ) ) {
			_doing_it_wrong( __FUNCTION__, __( 'When registering an "array" meta type to show in the REST API, you must specify the schema for each array item in "show_in_rest.schema.items".' ), '5.3.0' );

			return false;
		}
	}

	$object_subtype = ! empty( $args['object_subtype'] ) ? $args['object_subtype'] : '';
	if ( $args['revisions_enabled'] ) {
		if ( 'post' !== $object_type ) {
			_doing_it_wrong( __FUNCTION__, __( 'Meta keys cannot enable revisions support unless the object type supports revisions.' ), '6.4.0' );

			return false;
		} elseif ( ! empty( $object_subtype ) && ! post_type_supports( $object_subtype, 'revisions' ) ) {
			_doing_it_wrong( __FUNCTION__, __( 'Meta keys cannot enable revisions support unless the object subtype supports revisions.' ), '6.4.0' );

			return false;
		}
	}

	// If `auth_callback` is not provided, fall back to `is_protected_meta()`.
	if ( empty( $args['auth_callback'] ) ) {
		if ( is_protected_meta( $meta_key, $object_type ) ) {
			$args['auth_callback'] = '__return_false';
		} else {
			$args['auth_callback'] = '__return_true';
		}
	}

	// Back-compat: old sanitize and auth callbacks are applied to all of an object type.
	if ( is_callable( $args['sanitize_callback'] ) ) {
		if ( ! empty( $object_subtype ) ) {
			add_filter( "sanitize_{$object_type}_meta_{$meta_key}_for_{$object_subtype}", $args['sanitize_callback'], 10, 4 );
		} else {
			add_filter( "sanitize_{$object_type}_meta_{$meta_key}", $args['sanitize_callback'], 10, 3 );
		}
	}

	if ( is_callable( $args['auth_callback'] ) ) {
		if ( ! empty( $object_subtype ) ) {
			add_filter( "auth_{$object_type}_meta_{$meta_key}_for_{$object_subtype}", $args['auth_callback'], 10, 6 );
		} else {
			add_filter( "auth_{$object_type}_meta_{$meta_key}", $args['auth_callback'], 10, 6 );
		}
	}

	if ( array_key_exists( 'default', $args ) ) {
		$schema = $args;
		if ( is_array( $args['show_in_rest'] ) && isset( $args['show_in_rest']['schema'] ) ) {
			$schema = array_merge( $schema, $args['show_in_rest']['schema'] );
		}

		$check = rest_validate_value_from_schema( $args['default'], $schema );
		if ( is_wp_error( $check ) ) {
			_doing_it_wrong( __FUNCTION__, __( 'When registering a default meta value the data must match the type provided.' ), '5.5.0' );

			return false;
		}

		if ( ! has_filter( "default_{$object_type}_metadata", 'filter_default_metadata' ) ) {
			add_filter( "default_{$object_type}_metadata", 'filter_default_metadata', 10, 5 );
		}
	}

	// Global registry only contains meta keys registered with the array of arguments added in 4.6.0.
	if ( ! $has_old_auth_cb && ! $has_old_sanitize_cb ) {
		unset( $args['object_subtype'] );

		$wp_meta_keys[ $object_type ][ $object_subtype ][ $meta_key ] = $args;

		return true;
	}

	return false;
}

/**
 * Filters into default_{$object_type}_metadata and adds in default value.
 *
 * @since 5.5.0
 *
 * @param mixed  $value     Current value passed to filter.
 * @param int    $object_id ID of the object metadata is for.
 * @param string $meta_key  Metadata key.
 * @param bool   $single    If true, return only the first value of the specified `$meta_key`.
 *                          This parameter has no effect if `$meta_key` is not specified.
 * @param string $meta_type Type of object metadata is for. Accepts 'post', 'comment', 'term', 'user',
 *                          or any other object type with an associated meta table.
 * @return mixed An array of default values if `$single` is false.
 *               The default value of the meta field if `$single` is true.
 */
function filter_default_metadata( $value, $object_id, $meta_key, $single, $meta_type ) {
	global $wp_meta_keys;

	if ( wp_installing() ) {
		return $value;
	}

	if ( ! is_array( $wp_meta_keys ) || ! isset( $wp_meta_keys[ $meta_type ] ) ) {
		return $value;
	}

	$defaults = array();
	foreach ( $wp_meta_keys[ $meta_type ] as $sub_type => $meta_data ) {
		foreach ( $meta_data as $_meta_key => $args ) {
			if ( $_meta_key === $meta_key && array_key_exists( 'default', $args ) ) {
				$defaults[ $sub_type ] = $args;
			}
		}
	}

	if ( ! $defaults ) {
		return $value;
	}

	// If this meta type does not have subtypes, then the default is keyed as an empty string.
	if ( isset( $defaults[''] ) ) {
		$metadata = $defaults[''];
	} else {
		$sub_type = get_object_subtype( $meta_type, $object_id );
		if ( ! isset( $defaults[ $sub_type ] ) ) {
			return $value;
		}
		$metadata = $defaults[ $sub_type ];
	}

	if ( $single ) {
		$value = $metadata['default'];
	} else {
		$value = array( $metadata['default'] );
	}

	return $value;
}

/**
 * Checks if a meta key is registered.
 *
 * @since 4.6.0
 * @since 4.9.8 The `$object_subtype` parameter was added.
 *
 * @param string $object_type    Type of object metadata is for. Accepts 'post', 'comment', 'term', 'user',
 *                               or any other object type with an associated meta table.
 * @param string $meta_key       Metadata key.
 * @param string $object_subtype Optional. The subtype of the object type. Default empty string.
 * @return bool True if the meta key is registered to the object type and, if provided,
 *              the object subtype. False if not.
 */
function registered_meta_key_exists( $object_type, $meta_key, $object_subtype = '' ) {
	$meta_keys = get_registered_meta_keys( $object_type, $object_subtype );

	return isset( $meta_keys[ $meta_key ] );
}

/**
 * Unregisters a meta key from the list of registered keys.
 *
 * @since 4.6.0
 * @since 4.9.8 The `$object_subtype` parameter was added.
 *
 * @param string $object_type    Type of object metadata is for. Accepts 'post', 'comment', 'term', 'user',
 *                               or any other object type with an associated meta table.
 * @param string $meta_key       Metadata key.
 * @param string $object_subtype Optional. The subtype of the object type. Default empty string.
 * @return bool True if successful. False if the meta key was not registered.
 */
function unregister_meta_key( $object_type, $meta_key, $object_subtype = '' ) {
	global $wp_meta_keys;

	if ( ! registered_meta_key_exists( $object_type, $meta_key, $object_subtype ) ) {
		return false;
	}

	$args = $wp_meta_keys[ $object_type ][ $object_subtype ][ $meta_key ];

	if ( isset( $args['sanitize_callback'] ) && is_callable( $args['sanitize_callback'] ) ) {
		if ( ! empty( $object_subtype ) ) {
			remove_filter( "sanitize_{$object_type}_meta_{$meta_key}_for_{$object_subtype}", $args['sanitize_callback'] );
		} else {
			remove_filter( "sanitize_{$object_type}_meta_{$meta_key}", $args['sanitize_callback'] );
		}
	}

	if ( isset( $args['auth_callback'] ) && is_callable( $args['auth_callback'] ) ) {
		if ( ! empty( $object_subtype ) ) {
			remove_filter( "auth_{$object_type}_meta_{$meta_key}_for_{$object_subtype}", $args['auth_callback'] );
		} else {
			remove_filter( "auth_{$object_type}_meta_{$meta_key}", $args['auth_callback'] );
		}
	}

	unset( $wp_meta_keys[ $object_type ][ $object_subtype ][ $meta_key ] );

	// Do some clean up.
	if ( empty( $wp_meta_keys[ $object_type ][ $object_subtype ] ) ) {
		unset( $wp_meta_keys[ $object_type ][ $object_subtype ] );
	}
	if ( empty( $wp_meta_keys[ $object_type ] ) ) {
		unset( $wp_meta_keys[ $object_type ] );
	}

	return true;
}

/**
 * Retrieves a list of registered metadata args for an object type, keyed by their meta keys.
 *
 * @since 4.6.0
 * @since 4.9.8 The `$object_subtype` parameter was added.
 *
 * @param string $object_type    Type of object metadata is for. Accepts 'post', 'comment', 'term', 'user',
 *                               or any other object type with an associated meta table.
 * @param string $object_subtype Optional. The subtype of the object type. Default empty string.
 * @return array[] List of registered metadata args, keyed by their meta keys.
 */
function get_registered_meta_keys( $object_type, $object_subtype = '' ) {
	global $wp_meta_keys;

	if ( ! is_array( $wp_meta_keys ) || ! isset( $wp_meta_keys[ $object_type ] ) || ! isset( $wp_meta_keys[ $object_type ][ $object_subtype ] ) ) {
		return array();
	}

	return $wp_meta_keys[ $object_type ][ $object_subtype ];
}

/**
 * Retrieves registered metadata for a specified object.
 *
 * The results include both meta that is registered specifically for the
 * object's subtype and meta that is registered for the entire object type.
 *
 * @since 4.6.0
 *
 * @param string $object_type Type of object metadata is for. Accepts 'post', 'comment', 'term', 'user',
 *                            or any other object type with an associated meta table.
 * @param int    $object_id   ID of the object the metadata is for.
 * @param string $meta_key    Optional. Registered metadata key. If not specified, retrieve all registered
 *                            metadata for the specified object.
 * @return mixed A single value or array of values for a key if specified. An array of all registered keys
 *               and values for an object ID if not. False if a given $meta_key is not registered.
 */
function get_registered_metadata( $object_type, $object_id, $meta_key = '' ) {
	$object_subtype = get_object_subtype( $object_type, $object_id );

	if ( ! empty( $meta_key ) ) {
		if ( ! empty( $object_subtype ) && ! registered_meta_key_exists( $object_type, $meta_key, $object_subtype ) ) {
			$object_subtype = '';
		}

		if ( ! registered_meta_key_exists( $object_type, $meta_key, $object_subtype ) ) {
			return false;
		}

		$meta_keys     = get_registered_meta_keys( $object_type, $object_subtype );
		$meta_key_data = $meta_keys[ $meta_key ];

		$data = get_metadata( $object_type, $object_id, $meta_key, $meta_key_data['single'] );

		return $data;
	}

	$data = get_metadata( $object_type, $object_id );
	if ( ! $data ) {
		return array();
	}

	$meta_keys = get_registered_meta_keys( $object_type );
	if ( ! empty( $object_subtype ) ) {
		$meta_keys = array_merge( $meta_keys, get_registered_meta_keys( $object_type, $object_subtype ) );
	}

	return array_intersect_key( $data, $meta_keys );
}

/**
 * Filters out `register_meta()` args based on an allowed list.
 *
 * `register_meta()` args may change over time, so requiring the allowed list
 * to be explicitly turned off is a warranty seal of sorts.
 *
 * @access private
 * @since 5.5.0
 *
 * @param array $args         Arguments from `register_meta()`.
 * @param array $default_args Default arguments for `register_meta()`.
 * @return array Filtered arguments.
 */
function _wp_register_meta_args_allowed_list( $args, $default_args ) {
	return array_intersect_key( $args, $default_args );
}

/**
 * Returns the object subtype for a given object ID of a specific type.
 *
 * @since 4.9.8
 *
 * @param string $object_type Type of object metadata is for. Accepts 'post', 'comment', 'term', 'user',
 *                            or any other object type with an associated meta table.
 * @param int    $object_id   ID of the object to retrieve its subtype.
 * @return string The object subtype or an empty string if unspecified subtype.
 */
function get_object_subtype( $object_type, $object_id ) {
	$object_id      = (int) $object_id;
	$object_subtype = '';

	switch ( $object_type ) {
		case 'post':
			$post_type = get_post_type( $object_id );

			if ( ! empty( $post_type ) ) {
				$object_subtype = $post_type;
			}
			break;

		case 'term':
			$term = get_term( $object_id );
			if ( ! $term instanceof WP_Term ) {
				break;
			}

			$object_subtype = $term->taxonomy;
			break;

		case 'comment':
			$comment = get_comment( $object_id );
			if ( ! $comment ) {
				break;
			}

			$object_subtype = 'comment';
			break;

		case 'user':
			$user = get_user_by( 'id', $object_id );
			if ( ! $user ) {
				break;
			}

			$object_subtype = 'user';
			break;
	}

	/**
	 * Filters the object subtype identifier for a non-standard object type.
	 *
	 * The dynamic portion of the hook name, `$object_type`, refers to the meta object type
	 * (post, comment, term, user, or any other type with an associated meta table).
	 *
	 * Possible hook names include:
	 *
	 *  - `get_object_subtype_post`
	 *  - `get_object_subtype_comment`
	 *  - `get_object_subtype_term`
	 *  - `get_object_subtype_user`
	 *
	 * @since 4.9.8
	 *
	 * @param string $object_subtype Empty string to override.
	 * @param int    $object_id      ID of the object to get the subtype for.
	 */
	return apply_filters( "get_object_subtype_{$object_type}", $object_subtype, $object_id );
}