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[mediawiki.git] / includes / pager / IndexPager.php

<?php
/**
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 */

namespace MediaWiki\Pager;

use HtmlArmor;
use MediaWiki\Context\ContextSource;
use MediaWiki\Context\IContextSource;
use MediaWiki\Html\Html;
use MediaWiki\Linker\LinkRenderer;
use MediaWiki\MediaWikiServices;
use MediaWiki\Navigation\PagerNavigationBuilder;
use MediaWiki\Request\WebRequest;
use stdClass;
use Wikimedia\Rdbms\IReadableDatabase;
use Wikimedia\Rdbms\IResultWrapper;

/**
 * Efficient paging for SQL queries that use a (roughly unique) index.
 *
 * This is for paging through data sets stored in tables with a unique
 * index, instead of a naive "LIMIT offset,limit" clause.
 *
 * In MySQL, such a limit/offset clause requires counting through the
 * specified number of offset rows to find the desired data, which can be
 * expensive for large offsets.
 *
 * ReverseChronologicalPager is a child class of the abstract IndexPager, and
 * contains  some formatting and display code which is specific to the use of
 * timestamps as  indexes. Here is a synopsis of its operation:
 *
 *    * The query is specified by the offset, limit and direction (dir)
 *      parameters, in addition to any subclass-specific parameters.
 *    * The offset is the non-inclusive start of the DB query. A row with an
 *      index value equal to the offset will never be shown.
 *    * The query may either be done backwards, where the rows are returned by
 *      the database in the opposite order to which they are displayed to the
 *      user, or forwards. This is specified by the "dir" parameter, dir=prev
 *      means backwards, anything else means forwards. The offset value
 *      specifies the start of the database result set, which may be either
 *      the start or end of the displayed data set. This allows "previous"
 *      links to be implemented without knowledge of the index value at the
 *      start of the previous page.
 *    * An additional row beyond the user-specified limit is always requested.
 *      This allows us to tell whether we should display a "next" link in the
 *      case of forwards mode, or a "previous" link in the case of backwards
 *      mode. Determining whether to display the other link (the one for the
 *      page before the start of the database result set) can be done
 *      heuristically by examining the offset.
 *
 *    * An empty offset indicates that the offset condition should be omitted
 *      from the query. This naturally produces either the first page or the
 *      last page depending on the dir parameter.
 *
 * Subclassing the pager to implement concrete functionality should be fairly
 * simple, please see the examples in HistoryAction.php and
 * SpecialBlockList.php. You just need to override formatRow(),
 * getQueryInfo() and getIndexField(). Don't forget to call the parent
 * constructor if you override it.
 *
 * @stable to extend
 * @ingroup Pager
 */
abstract class IndexPager extends ContextSource implements Pager {

        /** Backwards-compatible constant for $mDefaultDirection field (do not change) */
        public const DIR_ASCENDING = false;
        /** Backwards-compatible constant for $mDefaultDirection field (do not change) */
        public const DIR_DESCENDING = true;

        /** Backwards-compatible constant for reallyDoQuery() (do not change) */
        public const QUERY_ASCENDING = true;
        /** Backwards-compatible constant for reallyDoQuery() (do not change) */
        public const QUERY_DESCENDING = false;

        /** @var WebRequest */
        public $mRequest;
        /** @var int[] List of default entry limit options to be presented to clients */
        public $mLimitsShown = [ 20, 50, 100, 250, 500 ];
        /** @var int The default entry limit choosen for clients */
        public $mDefaultLimit = 50;
        /** @var mixed The starting point to enumerate entries */
        public $mOffset;
        /** @var int The maximum number of entries to show */
        public $mLimit;
        /** @var bool Whether the listing query completed */
        public $mQueryDone = false;
        /** @var IReadableDatabase */
        public $mDb;
        /** @var stdClass|bool|null Extra row fetched at the end to see if the end was reached */
        public $mPastTheEndRow;

        /**
         * The index to actually be used for ordering. This can be a single column,
         * an array of single columns, or an array of arrays of columns. See getIndexField
         * for more details.
         * @var string|string[]
         */
        protected $mIndexField;
        /**
         * An array of secondary columns to order by. These fields are not part of the offset.
         * This is a column list for one ordering, even if multiple orderings are supported.
         * @var string[]
         */
        protected $mExtraSortFields;
        /** For pages that support multiple types of ordering, which one to use.
         * @var string|null
         */
        protected $mOrderType;
        /**
         * $mDefaultDirection gives the direction to use when sorting results:
         * DIR_ASCENDING or DIR_DESCENDING. If $mIsBackwards is set, we start from
         * the opposite end, but we still sort the page itself according to
         * $mDefaultDirection. For example, if $mDefaultDirection is DIR_ASCENDING
         * but we're going backwards, we'll display the last page of results, but
         * the last result will be at the bottom, not the top.
         *
         * Like $mIndexField, $mDefaultDirection will be a single value even if the
         * class supports multiple default directions for different order types.
         * @var bool|null
         */
        public $mDefaultDirection;
        /** @var bool */
        public $mIsBackwards;

        /** @var bool True if the current result set is the first one */
        public $mIsFirst;
        /** @var bool */
        public $mIsLast;

        /** @var array */
        protected $mLastShown;
        /** @var array */
        protected $mFirstShown;
        /** @var array */
        protected $mPastTheEndIndex;
        /** @var array|null */
        protected $mDefaultQuery;
        /** @var string|null */
        protected $mNavigationBar;

        /**
         * Whether to include the offset in the query
         * @var bool
         */
        protected $mIncludeOffset = false;

        /**
         * Result object for the query. Warning: seek before use.
         *
         * @var IResultWrapper
         */
        public $mResult;

        /** @var LinkRenderer */
        private $linkRenderer;

        /**
         * @stable to call
         *
         * @param IContextSource|null $context
         * @param LinkRenderer|null $linkRenderer
         */
        public function __construct( ?IContextSource $context = null, ?LinkRenderer $linkRenderer = null ) {
                if ( $context ) {
                        $this->setContext( $context );
                }

                $this->mRequest = $this->getRequest();

                # NB: the offset is quoted, not validated. It is treated as an
                # arbitrary string to support the widest variety of index types. Be
                # careful outputting it into HTML!
                $this->mOffset = $this->mRequest->getText( 'offset' );

                # Use consistent behavior for the limit options
                $this->mDefaultLimit = MediaWikiServices::getInstance()
                        ->getUserOptionsLookup()
                        ->getIntOption( $this->getUser(), 'rclimit' );
                if ( !$this->mLimit ) {
                        // Don't override if a subclass calls $this->setLimit() in its constructor.
                        [ $this->mLimit, /* $offset */ ] = $this->mRequest
                                ->getLimitOffsetForUser( $this->getUser() );
                }

                $this->mIsBackwards = ( $this->mRequest->getVal( 'dir' ) == 'prev' );
                // Let the subclass set the DB here; otherwise use a replica DB for the current wiki
                if ( !$this->mDb ) {
                        $this->mDb = MediaWikiServices::getInstance()->getConnectionProvider()->getReplicaDatabase();
                }

                $index = $this->getIndexField(); // column to sort on
                $extraSort = $this->getExtraSortFields(); // extra columns to sort on for query planning
                $order = $this->mRequest->getVal( 'order' );

                if ( is_array( $index ) && isset( $index[$order] ) ) {
                        $this->mOrderType = $order;
                        $this->mIndexField = $index[$order];
                        $this->mExtraSortFields = isset( $extraSort[$order] )
                                ? (array)$extraSort[$order]
                                : [];
                } elseif ( is_array( $index ) ) {
                        # First element is the default
                        $this->mIndexField = reset( $index );
                        $this->mOrderType = key( $index );
                        $this->mExtraSortFields = isset( $extraSort[$this->mOrderType] )
                                ? (array)$extraSort[$this->mOrderType]
                                : [];
                } else {
                        # $index is not an array
                        $this->mOrderType = null;
                        $this->mIndexField = $index;
                        $isSortAssociative = array_values( $extraSort ) !== $extraSort;
                        if ( $isSortAssociative ) {
                                $this->mExtraSortFields = isset( $extraSort[$index] )
                                        ? (array)$extraSort[$index]
                                        : [];
                        } else {
                                $this->mExtraSortFields = (array)$extraSort;
                        }
                }

                if ( $this->mDefaultDirection === null ) {
                        $dir = $this->getDefaultDirections();
                        $this->mDefaultDirection = is_array( $dir )
                                ? $dir[$this->mOrderType]
                                : $dir;
                }
                $this->linkRenderer = $linkRenderer;
        }

        /**
         * Get the Database object in use
         *
         * @since 1.20
         *
         * @return IReadableDatabase
         */
        public function getDatabase() {
                return $this->mDb;
        }

        /**
         * Do the query, using information from the object context. This function
         * has been kept minimal to make it overridable if necessary, to allow for
         * result sets formed from multiple DB queries.
         *
         * @stable to override
         */
        public function doQuery() {
                $defaultOrder = ( $this->mDefaultDirection === self::DIR_ASCENDING )
                        ? self::QUERY_ASCENDING
                        : self::QUERY_DESCENDING;
                $order = $this->mIsBackwards ? self::oppositeOrder( $defaultOrder ) : $defaultOrder;

                # Plus an extra row so that we can tell the "next" link should be shown
                $queryLimit = $this->mLimit + 1;
                if ( $this->mOffset == '' ) {
                        $isFirst = true;
                } else {
                        // If there's an offset, we may or may not be at the first entry.
                        // The only way to tell is to run the query in the opposite
                        // direction see if we get a row.
                        $oldIncludeOffset = $this->mIncludeOffset;
                        $this->mIncludeOffset = !$this->mIncludeOffset;
                        $oppositeOrder = self::oppositeOrder( $order );
                        $isFirst = !$this->reallyDoQuery( $this->mOffset, 1, $oppositeOrder )->numRows();
                        $this->mIncludeOffset = $oldIncludeOffset;
                }

                $this->mResult = $this->reallyDoQuery(
                        $this->mOffset,
                        $queryLimit,
                        $order
                );

                $this->extractResultInfo( $isFirst, $queryLimit, $this->mResult );
                $this->mQueryDone = true;

                $this->preprocessResults( $this->mResult );
                $this->mResult->rewind(); // Paranoia
        }

        /**
         * @param bool $order One of the IndexPager::QUERY_* class constants
         * @return bool The opposite query order as an IndexPager::QUERY_ constant
         */
        final protected static function oppositeOrder( $order ) {
                return ( $order === self::QUERY_ASCENDING )
                        ? self::QUERY_DESCENDING
                        : self::QUERY_ASCENDING;
        }

        /**
         * @return IResultWrapper The result wrapper.
         */
        public function getResult() {
                return $this->mResult;
        }

        /**
         * @return int The current offset into the result. Valid during formatRow().
         */
        public function getResultOffset() {
                return $this->mResult->key();
        }

        /**
         * Set the offset from an other source than the request
         *
         * @param int|string $offset
         */
        public function setOffset( $offset ) {
                $this->mOffset = $offset;
        }

        /**
         * Set the limit from an other source than the request
         *
         * Verifies limit is between 1 and 5000
         *
         * @stable to override
         *
         * @param int|string $limit
         */
        public function setLimit( $limit ) {
                $limit = (int)$limit;
                // WebRequest::getLimitOffsetForUser() puts a cap of 5000, so do same here.
                if ( $limit > 5000 ) {
                        $limit = 5000;
                }
                if ( $limit > 0 ) {
                        $this->mLimit = $limit;
                }
        }

        /**
         * Get the current limit
         *
         * @return int
         */
        public function getLimit() {
                return $this->mLimit;
        }

        /**
         * Set whether a row matching exactly the offset should be also included
         * in the result or not. By default this is not the case, but when the
         * offset is user-supplied this might be wanted.
         *
         * @param bool $include
         */
        public function setIncludeOffset( $include ) {
                $this->mIncludeOffset = $include;
        }

        /**
         * Extract some useful data from the result object for use by
         * the navigation bar, put it into $this
         *
         * @stable to override
         *
         * @param bool $isFirst False if there are rows before those fetched (i.e.
         *     if a "previous" link would make sense)
         * @param int $limit Exact query limit
         * @param IResultWrapper $res
         */
        protected function extractResultInfo( $isFirst, $limit, IResultWrapper $res ) {
                $numRows = $res->numRows();

                $firstIndex = [];
                $lastIndex = [];
                $this->mPastTheEndIndex = [];
                $this->mPastTheEndRow = null;

                if ( $numRows ) {
                        $indexColumns = array_map( static function ( $v ) {
                                // Remove any table prefix from index field
                                $parts = explode( '.', $v );
                                return end( $parts );
                        }, (array)$this->mIndexField );

                        $row = $res->fetchRow();
                        foreach ( $indexColumns as $indexColumn ) {
                                $firstIndex[] = $row[$indexColumn];
                        }

                        # Discard the extra result row if there is one
                        if ( $numRows > $this->mLimit && $numRows > 1 ) {
                                $res->seek( $numRows - 1 );
                                $this->mPastTheEndRow = $res->fetchObject();
                                foreach ( $indexColumns as $indexColumn ) {
                                        $this->mPastTheEndIndex[] = $this->mPastTheEndRow->$indexColumn;
                                }
                                $res->seek( $numRows - 2 );
                                $row = $res->fetchRow();
                                foreach ( $indexColumns as $indexColumn ) {
                                        $lastIndex[] = $row[$indexColumn];
                                }
                        } else {
                                $this->mPastTheEndRow = null;
                                $res->seek( $numRows - 1 );
                                $row = $res->fetchRow();
                                foreach ( $indexColumns as $indexColumn ) {
                                        $lastIndex[] = $row[$indexColumn];
                                }
                        }
                }

                if ( $this->mIsBackwards ) {
                        $this->mIsFirst = ( $numRows < $limit );
                        $this->mIsLast = $isFirst;
                        $this->mLastShown = $firstIndex;
                        $this->mFirstShown = $lastIndex;
                } else {
                        $this->mIsFirst = $isFirst;
                        $this->mIsLast = ( $numRows < $limit );
                        $this->mLastShown = $lastIndex;
                        $this->mFirstShown = $firstIndex;
                }
        }

        /**
         * Get some text to go in brackets in the "function name" part of the SQL comment
         *
         * @stable to override
         *
         * @return string
         */
        protected function getSqlComment() {
                return static::class;
        }

        /**
         * Do a query with specified parameters, rather than using the object context
         *
         * @note For b/c, query direction is true for ascending and false for descending
         *
         * @stable to override
         *
         * @param string $offset Index offset, inclusive
         * @param int $limit Exact query limit
         * @param bool $order IndexPager::QUERY_ASCENDING or IndexPager::QUERY_DESCENDING
         * @return IResultWrapper
         */
        public function reallyDoQuery( $offset, $limit, $order ) {
                [ $tables, $fields, $conds, $fname, $options, $join_conds ] =
                        $this->buildQueryInfo( $offset, $limit, $order );

                return $this->mDb->newSelectQueryBuilder()
                        ->rawTables( $tables )
                        ->fields( $fields )
                        ->conds( $conds )
                        ->caller( $fname )
                        ->options( $options )
                        ->joinConds( $join_conds )
                        ->fetchResultSet();
        }

        /**
         * Build variables to use by the database wrapper.
         *
         * @note For b/c, query direction is true for ascending and false for descending
         *
         * @stable to override
         *
         * @param int|string|null $offset Index offset, inclusive
         * @param int $limit Exact query limit
         * @param bool $order IndexPager::QUERY_ASCENDING or IndexPager::QUERY_DESCENDING
         * @return array
         */
        protected function buildQueryInfo( $offset, $limit, $order ) {
                $fname = __METHOD__ . ' (' . $this->getSqlComment() . ')';
                $info = $this->getQueryInfo();
                $tables = $info['tables'];
                $fields = $info['fields'];
                $conds = $info['conds'] ?? [];
                $options = $info['options'] ?? [];
                $join_conds = $info['join_conds'] ?? [];
                $indexColumns = (array)$this->mIndexField;
                $sortColumns = array_merge( $indexColumns, $this->mExtraSortFields );

                if ( $order === self::QUERY_ASCENDING ) {
                        $options['ORDER BY'] = $sortColumns;
                        $operator = $this->mIncludeOffset ? '>=' : '>';
                } else {
                        $orderBy = [];
                        foreach ( $sortColumns as $col ) {
                                $orderBy[] = $col . ' DESC';
                        }
                        $options['ORDER BY'] = $orderBy;
                        $operator = $this->mIncludeOffset ? '<=' : '<';
                }
                if ( $offset ) {
                        $offsets = explode( '|', $offset, /* Limit to max of indices */ count( $indexColumns ) );

                        $conds[] = $this->buildOffsetConds(
                                $offsets,
                                $indexColumns,
                                $operator
                        );
                }
                $options['LIMIT'] = intval( $limit );
                return [ $tables, $fields, $conds, $fname, $options, $join_conds ];
        }

        /**
         * Build the conditions for the offset, given that we may be paginating on a
         * single column or multiple columns. Where we paginate on multiple columns,
         * the sort order is defined by the order of the columns in $mIndexField.
         *
         * @param string[] $offsets The offset for each index field
         * @param string[] $columns The name of each index field
         * @param string $operator Operator for the final part of each inner
         *  condition. This will be '>' if the query order is ascending, or '<' if
         *  the query order is descending. If the offset should be included, it will
         *  also have '=' appended.
         * @return string The conditions for getting results from the offset
         */
        private function buildOffsetConds( $offsets, $columns, $operator ) {
                // $offsets may be shorter than $columns, in which case the remaining columns should be ignored
                // (T318080)
                $columns = array_slice( $columns, 0, count( $offsets ) );
                $conds = array_combine( $columns, $offsets );
                return $this->mDb->buildComparison( $operator, $conds );
        }

        /**
         * Pre-process results; useful for performing batch existence checks, etc.
         *
         * @stable to override
         *
         * @param IResultWrapper $result
         */
        protected function preprocessResults( $result ) {
        }

        /**
         * Get the HTML of a pager row.
         *
         * @stable to override
         * @since 1.38
         * @param stdClass $row
         * @return string
         */
        protected function getRow( $row ): string {
                return $this->formatRow( $row );
        }

        /**
         * Get the formatted result list. Calls getStartBody(), formatRow() and
         * getEndBody(), concatenates the results and returns them.
         *
         * @stable to override
         *
         * @return string
         */
        public function getBody() {
                $this->getOutput()->addModuleStyles( $this->getModuleStyles() );
                if ( !$this->mQueryDone ) {
                        $this->doQuery();
                }

                if ( $this->mResult->numRows() ) {
                        # Do any special query batches before display
                        $this->doBatchLookups();
                }

                # Don't use any extra rows returned by the query
                $numRows = min( $this->mResult->numRows(), $this->mLimit );

                $s = $this->getStartBody();
                if ( $numRows ) {
                        if ( $this->mIsBackwards ) {
                                for ( $i = $numRows - 1; $i >= 0; $i-- ) {
                                        $this->mResult->seek( $i );
                                        $row = $this->mResult->fetchObject();
                                        $s .= $this->getRow( $row );
                                }
                        } else {
                                $this->mResult->seek( 0 );
                                for ( $i = 0; $i < $numRows; $i++ ) {
                                        $row = $this->mResult->fetchObject();
                                        $s .= $this->getRow( $row );
                                }
                        }
                        $s .= $this->getFooter();
                } else {
                        $s .= $this->getEmptyBody();
                }
                $s .= $this->getEndBody();
                return $s;
        }

        /**
         * ResourceLoader modules that must be loaded to provide correct styling for this pager
         *
         * @stable to override
         * @since 1.38
         * @return string[]
         */
        public function getModuleStyles() {
                return [ 'mediawiki.pager.styles' ];
        }

        /**
         * Classes can extend to output a footer at the bottom of the pager list.
         *
         * @since 1.38
         * @return string
         */
        protected function getFooter(): string {
                return '';
        }

        /**
         * Make a self-link
         *
         * @stable to call (since 1.39)
         *
         * @param string $text Text displayed on the link
         * @param array|null $query Associative array of parameter to be in the query string.
         *  If null, no link is generated.
         * @param string|null $type Link type used to create additional attributes, like "rel", "class" or
         *  "title". Valid values (non-exhaustive list): 'first', 'last', 'prev', 'next', 'asc', 'desc'.
         * @return string HTML fragment
         */
        protected function makeLink( $text, ?array $query = null, $type = null ) {
                $attrs = [];
                if ( $query !== null && in_array( $type, [ 'prev', 'next' ] ) ) {
                        $attrs['rel'] = $type;
                }

                if ( in_array( $type, [ 'asc', 'desc' ] ) ) {
                        $attrs['title'] = $this->msg( $type == 'asc' ? 'sort-ascending' : 'sort-descending' )->text();
                }

                if ( $type ) {
                        $attrs['class'] = "mw-{$type}link";
                }

                if ( $query !== null ) {
                        return $this->getLinkRenderer()->makeKnownLink(
                                $this->getTitle(),
                                new HtmlArmor( $text ),
                                $attrs,
                                $query + $this->getDefaultQuery()
                        );
                } else {
                        return Html::rawElement( 'span', $attrs, $text );
                }
        }

        /**
         * Called from getBody(), before getStartBody() is called and
         * after doQuery() was called. This will be called only if there
         * are rows in the result set.
         *
         * @stable to override
         *
         * @return void
         */
        protected function doBatchLookups() {
        }

        /**
         * Hook into getBody(), allows text to be inserted at the start. This
         * will be called even if there are no rows in the result set.
         *
         * @return string
         */
        protected function getStartBody() {
                return '';
        }

        /**
         * Hook into getBody() for the end of the list
         *
         * @stable to override
         *
         * @return string
         */
        protected function getEndBody() {
                return '';
        }

        /**
         * Hook into getBody(), for the bit between the start and the
         * end when there are no rows
         *
         * @stable to override
         *
         * @return string
         */
        protected function getEmptyBody() {
                return '';
        }

        /**
         * Get an array of query parameters that should be put into self-links.
         * By default, all parameters passed in the URL are used, apart from a
         * few exceptions.
         *
         * @stable to override
         *
         * @return array Associative array
         */
        public function getDefaultQuery() {
                if ( $this->mDefaultQuery === null ) {
                        $this->mDefaultQuery = $this->getRequest()->getQueryValues();
                        unset( $this->mDefaultQuery['title'] );
                        unset( $this->mDefaultQuery['dir'] );
                        unset( $this->mDefaultQuery['offset'] );
                        unset( $this->mDefaultQuery['limit'] );
                        unset( $this->mDefaultQuery['order'] );
                        unset( $this->mDefaultQuery['month'] );
                        unset( $this->mDefaultQuery['year'] );
                }
                return $this->mDefaultQuery;
        }

        /**
         * Get the number of rows in the result set
         *
         * @return int
         */
        public function getNumRows() {
                if ( !$this->mQueryDone ) {
                        $this->doQuery();
                }
                return $this->mResult->numRows();
        }

        /**
         * Get a URL query array for the prev, next, first and last links.
         *
         * @stable to override
         *
         * @return array
         */
        public function getPagingQueries() {
                if ( !$this->mQueryDone ) {
                        $this->doQuery();
                }

                # Don't announce the limit everywhere if it's the default
                $urlLimit = $this->mLimit == $this->mDefaultLimit ? null : $this->mLimit;

                if ( $this->mIsFirst ) {
                        $prev = false;
                        $first = false;
                } else {
                        $prev = [
                                'dir' => 'prev',
                                'offset' => implode( '|', (array)$this->mFirstShown ),
                                'limit' => $urlLimit
                        ];
                        $first = [ 'offset' => null, 'limit' => $urlLimit ];
                }
                if ( $this->mIsLast ) {
                        $next = false;
                        $last = false;
                } else {
                        $next = [ 'offset' => implode( '|', (array)$this->mLastShown ), 'limit' => $urlLimit ];
                        $last = [ 'dir' => 'prev', 'offset' => null, 'limit' => $urlLimit ];
                }

                return [
                        'prev' => $prev,
                        'next' => $next,
                        'first' => $first,
                        'last' => $last
                ];
        }

        /**
         * Get the current offset for the URL query parameter.
         *
         * @stable to override
         * @since 1.39
         * @return string
         */
        public function getOffsetQuery() {
                if ( $this->mIsBackwards ) {
                        return implode( '|', (array)$this->mPastTheEndIndex );
                } else {
                        return $this->mOffset;
                }
        }

        /**
         * @stable to override
         * @since 1.39
         * @return PagerNavigationBuilder
         */
        public function getNavigationBuilder(): PagerNavigationBuilder {
                $pagingQueries = $this->getPagingQueries();
                $baseQuery = array_merge( $this->getDefaultQuery(), [
                        // These query parameters are all defined here, even though some are null,
                        // to ensure consistent order of parameters when they're used.
                        'dir' => null,
                        'offset' => $this->getOffsetQuery(),
                        'limit' => null,
                ] );

                $navBuilder = new PagerNavigationBuilder( $this->getContext() );
                $navBuilder
                        ->setPage( $this->getTitle() )
                        ->setLinkQuery( $baseQuery )
                        ->setLimits( $this->mLimitsShown )
                        ->setLimitLinkQueryParam( 'limit' )
                        ->setCurrentLimit( $this->mLimit )
                        ->setPrevLinkQuery( $pagingQueries['prev'] ?: null )
                        ->setNextLinkQuery( $pagingQueries['next'] ?: null )
                        ->setFirstLinkQuery( $pagingQueries['first'] ?: null )
                        ->setLastLinkQuery( $pagingQueries['last'] ?: null );

                return $navBuilder;
        }

        /**
         * Returns whether to show the "navigation bar"
         * @stable to override
         *
         * @return bool
         */
        protected function isNavigationBarShown() {
                if ( !$this->mQueryDone ) {
                        $this->doQuery();
                }
                // Hide navigation by default if there is nothing to page
                return !( $this->mIsFirst && $this->mIsLast );
        }

        /**
         * Returns an HTML string representing the result row $row.
         * Rows will be concatenated and returned by getBody()
         *
         * @param array|stdClass $row Database row
         * @return string
         */
        abstract public function formatRow( $row );

        /**
         * Provides all parameters needed for the main paged query. It returns
         * an associative array with the following elements:
         *    tables => Table(s) for passing to Database::select()
         *    fields => Field(s) for passing to Database::select(), may be *
         *    conds => WHERE conditions
         *    options => option array
         *    join_conds => JOIN conditions
         *
         * @return array
         */
        abstract public function getQueryInfo();

        /**
         * Returns the name of the index field.  If the pager supports multiple
         * orders, it may return an array of 'querykey' => 'indexfield' pairs,
         * so that a request with &order=querykey will use indexfield to sort.
         * In this case, the first returned key is the default.
         *
         * Needless to say, it's really not a good idea to use a non-unique index
         * for this!  That won't page right.
         *
         * The pager may paginate on multiple fields in combination. If paginating
         * on multiple fields, they should be unique in combination (e.g. when
         * paginating on user and timestamp, rows may have the same user, rows may
         * have the same timestamp, but rows should all have a different combination
         * of user and timestamp).
         *
         * Examples:
         * - Always paginate on the user field:
         *   'user'
         * - Paginate on either the user or the timestamp field (default to user):
         *   [
         *       'name' => 'user',
         *       'time' => 'timestamp',
         *   ]
         * - Always paginate on the combination of user and timestamp:
         *   [
         *       [ 'user', 'timestamp' ]
         *   ]
         * - Paginate on the user then timestamp, or the timestamp then user:
         *   [
         *       'nametime' => [ 'user', 'timestamp' ],
         *       'timename' => [ 'timestamp', 'user' ],
         *   ]
         *
         *
         * @return string|string[]|array[]
         */
        abstract public function getIndexField();

        /**
         * Returns the names of secondary columns to order by in addition to the
         * column in getIndexField(). These fields will not be used in the pager
         * offset or in any links for users.
         *
         * If getIndexField() returns an array of 'querykey' => 'indexfield' pairs then
         * this must return a corresponding array of 'querykey' => [ fields... ] pairs
         * in order for a request with &order=querykey to use [ fields... ] to sort.
         *
         * If getIndexField() returns a string with the field to sort by, this must either:
         * 1 - return an associative array like above, but only the elements for the current
         *   field will be used.
         * 2 - return a non-associative array, for secondary keys to use always.
         *
         * This is useful for pagers that GROUP BY a unique column (say page_id)
         * and ORDER BY another (say page_len). Using GROUP BY and ORDER BY both on
         * page_len,page_id avoids temp tables (given a page_len index). This would
         * also work if page_id was non-unique but we had a page_len,page_id index.
         *
         * @stable to override
         *
         * @return string[]|array[]
         */
        protected function getExtraSortFields() {
                return [];
        }

        /**
         * Return the default sorting direction: DIR_ASCENDING or DIR_DESCENDING.
         * You can also have an associative array of ordertype => dir,
         * if multiple order types are supported.  In this case getIndexField()
         * must return an array, and the keys of that must exactly match the keys
         * of this.
         *
         * For backward compatibility, this method's return value will be ignored
         * if $this->mDefaultDirection is already set when the constructor is
         * called, for instance if it's statically initialized.  In that case the
         * value of that variable (which must be a boolean) will be used.
         *
         * Note that despite its name, this does not return the value of the
         * $this->mDefaultDirection member variable.  That's the default for this
         * particular instantiation, which is a single value.  This is the set of
         * all defaults for the class.
         *
         * @stable to override
         *
         * @return bool
         */
        protected function getDefaultDirections() {
                return self::DIR_ASCENDING;
        }

        /**
         * @since 1.34
         * @return LinkRenderer
         */
        protected function getLinkRenderer() {
                if ( $this->linkRenderer === null ) {
                        $this->linkRenderer = MediaWikiServices::getInstance()->getLinkRenderer();
                }
                return $this->linkRenderer;
        }
}

/** @deprecated class alias since 1.41 */
class_alias( IndexPager::class, 'IndexPager' );