**
 * Deletes a revision.
 *
 * Deletes the row from the posts table corresponding to the specified revision.
 *
 * @since 2.6.0
 *
 * @param int|WP_Post $revision_id Revision ID or revision object.
 * @return array|false|WP_Post|WP_Error|null Null or WP_Error if error, deleted post if success.
 */
function wp_delete_post_revision( $revision_id ) {
	if ( ! $revision = wp_get_post_revision( $revision_id ) ) {
		return $revision;
	}

	$delete = wp_delete_post( $revision->ID );
	if ( $delete ) {
		/**
		 * Fires once a post revision has been deleted.
		 *
		 * @since 2.6.0
		 *
		 * @param int          $revision_id Post revision ID.
		 * @param object|array $revision    Post revision object or array.
		 */
		do_action( 'wp_delete_post_revision', $revision->ID, $revision );
	}

	return $delete;
}

/**
 * Returns all revisions of specified post.
 *
 * @since 2.6.0
 *
 * @see get_children()
 *
 * @param int|WP_Post $post_id Optional. Post ID or WP_Post object. Default is global `$post`.
 * @param array|null  $args    Optional. Arguments for retrieving post revisions. Default null.
 * @return array An array of revisions, or an empty array if none.
 */
function wp_get_post_revisions( $post_id = 0, $args = null ) {
	$post = get_post( $post_id );
	if ( ! $post || empty( $post->ID ) )
		return array();

	$defaults = array( 'order' => 'DESC', 'orderby' => 'date ID', 'check_enabled' => true );
	$args = wp_parse_args( $args, $defaults );

	if ( $args['check_enabled'] && ! wp_revisions_enabled( $post ) )
		return array();

	$args = array_merge( $args, array( 'post_parent' => $post->ID, 'post_type' => 'revision', 'post_status' => 'inherit' ) );

	if ( ! $revisions = get_children( $args ) )
		return array();

	return $revisions;
}

/**
 * Determine if revisions are enabled for a given post.
 *
 * @since 3.6.0
 *
 * @param WP_Post $post The post object.
 * @return bool True if number of revisions to keep isn't zero, false otherwise.
 */
function wp_revisions_enabled( $post ) {
	return wp_revisions_to_keep( $post ) !== 0;
}

/**
 * Determine how many revisions to retain for a given post.
 *
 * By default, an infinite number of revisions are kept.
 *
 * The constant WP_POST_REVISIONS can be set in wp-config to specify the limit
 * of revisions to keep.
 *
 * @since 3.6.0
 *
 * @param WP_Post $post The post object.
 * @return int The number of revisions to keep.
 */
function wp_revisions_to_keep( $post ) {
	$num = WP_POST_REVISIONS;

	if ( true === $num )
		$num = -1;
	else
		$num = intval( $num );

	if ( ! post_type_supports( $post->post_type, 'revisions' ) )
		$num = 0;

	/**
	 * Filters the number of revisions to save for the given post.
	 *
	 * Overrides the value of WP_POST_REVISIONS.
	 *
	 * @since 3.6.0
	 *
	 * @param int     $num  Number of revisions to store.
	 * @param WP_Post $post Post object.
	 */
	return (int) apply_filters( 'wp_revisions_to_keep', $num, $post );
}

/**
 * Sets up the post object for preview based on the post autosave.
 *
 * @since 2.7.0
 * @access private
 *
 * @param WP_Post $post
 * @return WP_Post|false
 */
function _set_preview( $post ) {
	if ( ! is_object( $post ) ) {
		return $post;
	}

	$preview = wp_get_post_autosave( $post->ID );
	if ( ! is_object( $preview ) ) {
		return $post;
	}

	$preview = sanitize_post( $preview );

	$post->post_content = $preview->post_content;
	$post->post_title = $preview->post_title;
	$post->post_excerpt = $preview->post_excerpt;

	add_filter( 'get_the_terms', '_wp_preview_terms_filter', 10, 3 );
	add_filter( 'get_post_metadata', '_wp_preview_post_thumbnail_filter', 10, 3 );

	return $post;
}

/**
 * Filters the latest content for preview from the post autosave.
 *
 * @since 2.7.0
 * @access private
 */
function _show_post_preview() {
	if ( isset($_GET['preview_id']) && isset($_GET['preview_nonce']) ) {
		$id = (int) $_GET['preview_id'];

		if ( false === wp_verify_nonce( $_GET['preview_nonce'], 'post_preview_' . $id ) )
			wp_die( __('Sorry, you are not allowed to preview drafts.') );

		add_filter('the_preview', '_set_preview');
	}
}

/**
 * Filters terms lookup to set the post format.
 *
 * @since 3.6.0
 * @access private
 *
 * @param array  $terms
 * @param int    $post_id
 * @param string $taxonomy
 * @return array
 */
function _wp_preview_terms_filter( $terms, $post_id, $taxonomy ) {
	if ( ! $post = get_post() )
		return $terms;

	if ( empty( $_REQUEST['post_format'] ) || $post->ID != $post_id || 'post_format' != $taxonomy || 'revision' == $post->post_type )
		return $terms;

	if ( 'standard' == $_REQUEST['post_format'] )
		$terms = array();
	elseif ( $term = get_term_by( 'slug', 'post-format-' . sanitize_key( $_REQUEST['post_format'] ), 'post_format' ) )
		$terms = array( $term ); // Can only have one post format

	return $terms;
}

/**
 * Filters post thumbnail lookup to set the post thumbnail.
 *
 * @since 4.6.0
 * @access private
 *
 * @param null|array|string $value    The value to return - a single metadata value, or an array of values.
 * @param int               $post_id  Post ID.
 * @param string            $meta_key Meta key.
 * @return null|array The default return value or the post thumbnail meta array.
 */
function _wp_preview_post_thumbnail_filter( $value, $post_id, $meta_key ) {
	if ( ! $post = get_post() ) {
		return $value;
	}

	if ( empty( $_REQUEST['_thumbnail_id'] ) ||
	     empty( $_REQUEST['preview_id'] ) ||
	     $post->ID != $post_id ||
	     '_thumbnail_id' != $meta_key ||
	     'revision' == $post->post_type ||
	     $post_id != $_REQUEST['preview_id']
	) {
		return $value;
	}

	$thumbnail_id = intval( $_REQUEST['_thumbnail_id'] );
	if ( $thumbnail_id <= 0 ) {
		return '';
	}

	return strval( $thumbnail_id );
}

/**
 * Gets the post revision version.
 *
 * @since 3.6.0
 * @access private
 *
 * @param WP_Post $revision
 * @return int|false
 */
function _wp_get_post_revision_version( $revision ) {
	if ( is_object( $revision ) )
		$revision = get_object_vars( $revision );
	elseif ( !is_array( $revision ) )
		return false;

	if ( preg_match( '/^\d+-(?:autosave|revision)-v(\d+)$/', $revision['post_name'], $matches ) )
		return (int) $matches[1];

	return 0;
}

/**
 * Upgrade the revisions author, add the current post as a revision and set the revisions version to 1
 *
 * @since 3.6.0
 * @access private
 *
 * @global wpdb $wpdb WordPress database abstraction object.
 *
 * @param WP_Post $post      Post object
 * @param array   $revisions Current revisions of the post
 * @return bool true if the revisions were upgraded, false if problems
 */
function _wp_upgrade_revisions_of_post( $post, $revisions ) {
	global $wpdb;

	// Add post option exclusively
	$lock = "revision-upgrade-{$post->ID}";
	$now = time();
	$result = $wpdb->query( $wpdb->prepare( "INSERT IGNORE INTO `$wpdb->options` (`option_name`, `option_value`, `autoload`) VALUES (%s, %s, 'no') /* LOCK */", $lock, $now ) );
	if ( ! $result ) {
		// If we couldn't get a lock, see how old the previous lock is
		$locked = get_option( $lock );
		if ( ! $locked ) {
			// Can't write to the lock, and can't read the lock.
			// Something broken has happened
			return false;
		}

		if ( $locked > $now - 3600 ) {
			// Lock is not too old: some other process may be upgrading this post.  Bail.
			return false;
		}

		// Lock is too old - update it (below) and continue
	}

	// If we could get a lock, re-"add" the option to fire all the correct filters.
	update_option( $lock, $now );

	reset( $revisions );
	$add_last = true;

	do {
		$this_revision = current( $revisions );
		$prev_revision = next( $revisions );

		$this_revision_version = _wp_get_post_revision_version( $this_revision );

		// Something terrible happened
		if ( false === $this_revision_version )
			continue;

		// 1 is the latest revision version, so we're already up to date.
		// No need to add a copy of the post as latest revision.
		if ( 0 < $this_revision_version ) {
			$add_last = false;
			continue;
		}

		// Always update the revision version
		$update = array(
			'post_name' => preg_replace( '/^(\d+-(?:autosave|revision))[\d-]*$/', '$1-v1', $this_revision->post_name ),
		);

		// If this revision is the oldest revision of the post, i.e. no $prev_revision,
		// the correct post_author is probably $post->post_author, but that's only a good guess.
		// Update the revision version only and Leave the author as-is.
		if ( $prev_revision ) {
			$prev_revision_version = _wp_get_post_revision_version( $prev_revision );

			// If the previous revision is already up to date, it no longer has the information we need :(
			if ( $prev_revision_version < 1 )
				$update['post_author'] = $prev_revision->post_author;
		}

		// Upgrade this revision
		$result = $wpdb->update( $wpdb->posts, $update, array( 'ID' => $this_revision->ID ) );

		if ( $result )
			wp_cache_delete( $this_revision->ID, 'posts' );

	} while ( $prev_revision );

	delete_option( $lock );

	// Add a copy of the post as latest revision.
	if ( $add_last )
		wp_save_post_revision( $post->ID );

	return true;
}