SpecialRandompage.php

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<?php
class RandomPage extends SpecialPage {
        private $namespaces;  // namespaces to select pages from
        protected $isRedir = false; // should the result be a redirect?
        protected $extra = array(); // Extra SQL statements

        public function __construct( $name = 'Randompage' ){
                $this->namespaces = MWNamespace::getContentNamespaces();
                parent::__construct( $name );
        }

        public function getNamespaces() {
                return $this->namespaces;
        }

        public function setNamespace ( $ns ) {
                if( !$ns || $ns < NS_MAIN ) {
                        $ns = NS_MAIN;
                }
                $this->namespaces = array( $ns );
        }

        // select redirects instead of normal pages?
        public function isRedirect(){
                return $this->isRedir;
        }

        public function execute( $par ) {
                global $wgContLang;

                if ( $par ) {
                        $this->setNamespace( $wgContLang->getNsIndex( $par ) );
                }

                $title = $this->getRandomTitle();

                if( is_null( $title ) ) {
                        $this->setHeaders();
                        $this->getOutput()->addWikiMsg( strtolower( $this->getName() ) . '-nopages',
                                $this->getNsList(), count( $this->namespaces ) );
                        return;
                }

                $redirectParam = $this->isRedirect() ? array( 'redirect' => 'no' ) : array();
                $query = array_merge( $this->getRequest()->getValues(), $redirectParam );
                unset( $query['title'] );
                $this->getOutput()->redirect( $title->getFullUrl( $query ) );
        }

        private function getNsList() {
                global $wgContLang;
                $nsNames = array();
                foreach( $this->namespaces as $n ) {
                        if( $n === NS_MAIN ) {
                                $nsNames[] = wfMsgNoTrans( 'blanknamespace' );
                        } else {
                                $nsNames[] = $wgContLang->getNsText( $n );
                        }
                }
                return $wgContLang->commaList( $nsNames );
        }

        public function getRandomTitle() {
                $randstr = wfRandom();
                $title = null;
                if ( !wfRunHooks( 'SpecialRandomGetRandomTitle', array( &$randstr, &$this->isRedir, &$this->namespaces,
                        &$this->extra, &$title ) ) ) {
                        return $title;
                }
                $row = $this->selectRandomPageFromDB( $randstr );

                /* If we picked a value that was higher than any in
                 * the DB, wrap around and select the page with the
                 * lowest value instead!  One might think this would
                 * skew the distribution, but in fact it won't cause
                 * any more bias than what the page_random scheme
                 * causes anyway.  Trust me, I'm a mathematician. :)
                 */
                if( !$row ) {
                        $row = $this->selectRandomPageFromDB( "0" );
                }

                if( $row ) {
                        return Title::makeTitleSafe( $row->page_namespace, $row->page_title );
                } else {
                        return null;
                }
        }

        protected function getQueryInfo( $randstr ) {
                $redirect = $this->isRedirect() ? 1 : 0;

                return array(
                        'tables' => array( 'page' ),
                        'fields' => array( 'page_title', 'page_namespace' ),
                        'conds' => array_merge( array(
                                'page_namespace' => $this->namespaces,
                                'page_is_redirect' => $redirect,
                                'page_random >= ' . $randstr
                        ), $this->extra ),
                        'options' => array(
                                'ORDER BY' => 'page_random',
                                'USE INDEX' => 'page_random',
                                'LIMIT' => 1,
                        ),
                        'join_conds' => array()
                );
        }

        private function selectRandomPageFromDB( $randstr, $fname = __METHOD__ ) {
                $dbr = wfGetDB( DB_SLAVE );

                $query = $this->getQueryInfo( $randstr );
                $res = $dbr->select(
                        $query['tables'],
                        $query['fields'],
                        $query['conds'],
                        $fname,
                        $query['options'],
                        $query['join_conds']
                );

                return $dbr->fetchObject( $res );
        }
}