Get adjacent posts from a term of a custom taxonomy

functions.php

<?php
function next_prev_portfolio($post, $current_term){  
	//echo '<pre>';
	$postlist_args = array(
	   'posts_per_page'  => -1,
	   //'orderby'         => 'menu_order title',
	   //'order'           => 'ASC',
	   'post_type'       => 'portfolio',
	   'portfolio_category' => $current_term
	); 
	$postlist = get_posts( $postlist_args );

	$ids = array();
	foreach ($postlist as $thepost) {
	   $ids[] = $thepost->ID;
	}
	$thisindex = array_search($post->ID, $ids);

	$previd = $ids[$thisindex-1];
	$nextid = $ids[$thisindex+1];
	
	if ( !empty($previd) ) {
	   echo '<a class="styled-link next-link" title="' . get_the_title($previd) . '" href="' . get_permalink($previd). '"><span>&nbsp;</span></a>';
	}

	if ( !empty($nextid) ) {
	   echo '<a class="styled-link prev-link" title="' . get_the_title($nextid) . '" href="' . get_permalink($nextid). '"><span>&nbsp;</span></a>';
	}
}