Reversed WordPress Pagination

reversed-pagination.php

<?php
/**
 * Reversed Pagination for WordPress - proof of concept
 *
 * WARNING! Not suitable for use on a live website!
 *
 * Proof of concept for reversed pagination as discussed here
 * http://nerd.vasilis.nl/problem-paginated-archive-pages-blogs/
 * and here (Dutch)
 * http://wasstra.at/035/
 *
 * As of now it lacks the lower and upper limits as discussed in the article.
 * This makes it possible you can only have one post in the first (non paginated) page.
 * With some small adjustments It should be possible to recalculate the posts with the limits.
 *
 * I've made this to see if I could coax WordPress in retrieving the correct posts for paginated pages.
 *
 * Still to do (and probably a whole lot more):
 *     Core pagination functions need be adjusted for the new ordering (or create your own).
 *     Make it work if the default permalink structure is used.
 *     Feeds?
 *     Static front page posts page?
 *     More defensive coding.
 *     Lots more testing.
 *
 * Example of what posts are shown with 10 posts per page.
 *
 * example.com/         - 1 to ? newest posts   - date DESC
 * example.com/page/2   - 10 older posts        - date DESC
 * example.com/page/1   - 10 oldest posts       - date DESC
 */

add_filter( 'redirect_canonical', 'first_paged_canonical', 10, 2 );

/**
 * allows pagination to start at 1 (url/page/1)
 */
function first_paged_canonical( $redirect_url, $requested_url ) {
	global $wp_rewrite;

	// Abort if not using pretty permalinks, is a feed, is a single post or page
	if ( ! $wp_rewrite->using_permalinks() || is_feed() || is_singular() ) {
		return $redirect_url;
	}

	// If paged === 1, apppend pagination back to $redirect_url.
	if ( get_query_var( 'paged' ) === 1 ) {
		$redirect_url .= user_trailingslashit( "page/1", 'paged' );
	}

	return $redirect_url;
}


add_action( 'pre_get_posts', 'my_post_queries' );

/**
 * resets orderby and order for the main query
 */
function my_post_queries( $query ) {

	// not an admin page and is the main query
	if ( !is_admin() && $query->is_main_query() && ! ( is_feed() || is_singular() ) ) {

		// get total posts and set new limits for the query
		add_filter( 'posts_clauses', 'my_post_clauses', 99, 2 );

		// let's order by the 'date'
		$query->set( 'orderby', 'date' );

		$paged = my_get_paged_query_var();

		if ( $paged > 0 ) {

			$query->set( 'order', 'ASC' );
			// reverses the post order back to DESC for the posts on paginated pages
			add_filter( 'the_posts', 'my_reverse_the_posts', 99, 2 );
		} else {

			// first non paginated page
			$query->set( 'order', 'DESC' );
		}
	}
}

/**
 * sets new limits for the 'limits' clause
 */
function my_post_clauses( $pieces , $_this ) {

	global $wpdb;


	extract( $pieces );

	// What page are we on? (starting from 0)
	$paged = my_get_paged_query_var();

	if ( ! empty( $groupby ) ) {
		$groupby = 'GROUP BY ' . $groupby;
	}

	if ( !empty( $orderby ) ) {
		$orderby = 'ORDER BY ' . $orderby;
	}

	// Extra query needed to get the total posts found.
	$query = "SELECT $wpdb->posts.ID FROM $wpdb->posts $join WHERE 1=1 $where $groupby $orderby";
	$total = count( $wpdb->get_results( $query  ) );

	$posts_per_page = $_this->get( 'posts_per_page' );

	if ( $total ) {

		$max_num_pages = ceil( $total / $posts_per_page );

		// Bail for last page posts.
		// Posts are also displayed on the first not paginated page because order by date is now ASC.
		if ( $paged == $max_num_pages ) {
			$_this->set_404();
			return $pieces;
		}

		if ( $paged > 0 ) {
			// set limits on paginated pages starting from 1 (order by date is ASC)
			$pieces['limits'] = 'LIMIT ' . ( ( $posts_per_page * $paged ) - $posts_per_page ) . ',' .  $posts_per_page;
		} else {

			// calculate posts on the first (not paginated) page
			$posts = $total - ( ( $max_num_pages - 1 ) * $posts_per_page );

			// set limits if there is pagination to be done (order by date is DESC)
			$pieces['limits'] = ( $max_num_pages > 1 ) ? "LIMIT 0, $posts" : '';
		}
	}


	return $pieces;
}

/**
 * reverses the posts if $paged > 0
 *
 * pagination starts at 1 instead of the default 2
 */
function my_reverse_the_posts( $posts, $_this ) {
	return array_reverse( $posts );
}

/**
 * $paged query_var will be set to 0 for first (not paginated) page
 *
 * pagination starts at 1 instead of the default 2
 */
function my_get_paged_query_var() {
	$paged = get_query_var( 'paged' );
	if ( !$paged ) {
		$paged = 0;
	}
	return $paged;
}

Proof of concept for reversed WordPress archives pagination as discussed here:
http://nerd.vasilis.nl/problem-paginated-archive-pages-blogs/
And here (Dutch):
http://wasstra.at/035/