WordPress custom post type with custom ordered post navigation

gistfile1.php

<?php

	/*
	* I have a custom post type for my Portfolio, using Advanced Custom Fields for extra fields including a field for me to
	* order the portfolio entires how I want. Standard single navigation using previous_post_link and next_post_link don't work as they
	* return the order the posts were added. This fixes things:
	*/
	
	// New query
	$args = array(
		'post_type'	=> 'portfolio',
		'meta_key'	=> 'portfolio_order',
		'orderby'	=> 'meta_value_num',
		'order'		=> 'ASC',
		'posts_per_page'	=> '-1'
	);
	$nav_query = new WP_Query($args);

	// Get an ordered list of the IDs and Title of portfolio entries
	$ids = array();
	while( $nav_query->have_posts() ): $nav_query->the_post();
		$ids[] = array('ID' => $post->ID, 'post_title' => $post->post_title);
	endwhile;
	wp_reset_query();

	// Reindex the list and set previous and next title & IDs
	$thisindex = array_search($post->ID, array_column($ids, 'ID'));
	if ($thisindex > 0) {
		$prev_post_id = $ids[$thisindex-1]['ID'];
		$prev_post_title = $ids[$thisindex-1]['post_title'];
	}

	if ($thisindex < max(array_keys($ids))) {
		$next_post_id = $ids[$thisindex+1]['ID'];
		$next_post_title = $ids[$thisindex+1]['post_title'];
	}
?>


<div class="article-nav">

	<?php if ( !empty($prev_post_id) ) { ?>
		<div class="nav-left"><a href="<?php echo get_permalink($prev_post_id); ?>" rel="prev"><div class="article-prev"><span><?php echo $prev_post_title; ?></span></div></a></div>
	<?php } ?>

	<?php if ( !empty($next_post_id) ) { ?>
		<div class="nav-right"><a href="<?php echo get_permalink($next_post_id); ?>" rel="next"><div class="article-next"><span><?php echo $next_post_title; ?></span></div></a></div>
	<?php } ?>

</div>