QuirkPress.php

<?php
/*
 * WHAT DOES THIS CODE DO?
 *
 * When using get_adjacent_post(), there is no way to restrict posts to one or two
 * categories, if the categories in question are not identical to the current post.
 * We get arround that by requesting a list of EVERY CATEGORY EXCEPT the one in
 * question, and then pass that as our list of 'excluded' categories.
 *
 * Take a deep breath.
 * Yes, it's stupid, but it works.
 *
 */
$post_categories = get_categories( array(
    'exclude' => array( 1, 6 ), // Page and chapter categories
    'orderby' => 'id',
) );
$post_categories = wp_list_pluck( $post_categories, 'cat_ID' );

$prev_post   = get_previous_post( FALSE, $post_categories );
$next_post   =     get_next_post( FALSE, $post_categories );

/*
 * For reasons that are not obvious to me,
 * while `get_previous_post` and `get_next_post` return a single
 * $post object ready to rock, `get_boundary_post` returns an
 * ARRAY with a single WordPress post. So that we can use them in
 * similar ways, we have to extract the $post from the returned ARRAY.
 *
 * http://core.trac.wordpress.org/ticket/22950
 */
$first_post  = array_shift( get_boundary_post( FALSE, $page_categories, TRUE ) );
$latest_post = array_shift( get_boundary_post( FALSE, $page_categories, FALSE ) );

if ( ! is_single() || is_attachment() ) {
/*
 * We'll have to impliment our own workaround for WordPress's broken behavior
 *
 * I hate this code.
 * I hate that I have to include it.
 * I hope I can remove it with a future WordPress update.
 *
 * http://core.trac.wordpress.org/ticket/22957
 */
    $first_post  = array_shift( get_posts( array('numberposts' => 1, 'category__not_in' => $page_categories, 'order' => 'ASC',  'update_post_term_cache' => false, 'update_post_meta_cache' => false) ) );
    $latest_post = array_shift( get_posts( array('numberposts' => 1, 'category__not_in' => $page_categories, 'order' => 'DESC', 'update_post_term_cache' => false, 'update_post_meta_cache' => false) ) );
}