Ajax pagination for posts linked via ACF Relationships

pagination.php

<?php 

function ajax_pagination_enqueue() {

	// Register and enqueue our javascript file
	wp_register_script(  'pagination',
		get_template_directory_uri() . '/library/js/pagination.js', // or wherever you put the file in your theme directory
		array('jquery'));

	wp_enqueue_script('pagination');

	// Connect to WordPress' built-in Ajax file
	wp_localize_script( 'pagination', 'ajaxpagination', array(
		'ajaxurl' => admin_url( 'admin-ajax.php' )
	));

}

add_action( 'wp_enqueue_scripts', 'ajax_pagination_enqueue' );

// Hook this function to WordPress' Ajax actions
add_action( 'wp_ajax_nopriv_dl_bike_features', 'dl_bike_features' );
add_action( 'wp_ajax_dl_bike_features', 'dl_bike_features' );

function dl_bike_features() {
	global $post;
	
	if ($post->ID) {
		// On initial page load, just grab the post ID...
		$bike_id = $post->ID;
	} else {
		// ... but once you're using Ajax, need to get the ID via Ajax
		$bike_id = $_POST['bikeID'];
	}

	if (isset($_POST['page'])) {
		// Set $page to data from Ajax, if available
		$page = $_POST['page'];
	} else {
		// ... if not, set default to 1 (for initial page load)
		$page = 1;
	}

	// Set up $current_page to calculate page for previous / next buttons
	if (!empty($page)) {
		$current_page = intval($page);
	}

	// $page is coming from the href, so need to extract the page from the url
	$tens = substr($page, -3, 1);
	$hundreds = substr($page, -4, 1);
	
	/* 
	** If the character 3 or 4 spaces back from the end is not a number, 
	** intval will convert it to 0, which will test as false in the if statements below
	*/
	$tens = intval($tens);
	$hundreds = intval($hundreds);
	
	if ($hundreds) {
		$page = intval(substr($page, -4, 3));
	} elseif ($tens) {
		$page = intval(substr($page, -3, 2));
	} else { // single digit 
		$page = intval(substr($page, -2, 1));
	}

	// Get all posts that are linked to the current post via ACF Relationship
	$features = get_posts(array(
		// post type of the posts linked to current post
		'post_type' => 'bike-features', 
		// fetch all of them--we'll divide them into separate pages in a moment
		'posts_per_page' => -1, 
		'meta_query' => array(
			array(
				// name of ACF Relationship field
				'key' => 'bikes_with_this_feature', 
				// ID of current post
				'value' => $bike_id, 
				'compare' => 'LIKE'
			)
		)
	));

	// Pagination variables
	$feature_count      = 0;
	$features_per_page  = 16; // How many features to display on each page
	$total              = count( $features );
	$pages              = ceil( $total / $features_per_page );
	$min                = ( ( $page * $features_per_page ) - $features_per_page ) + 1;
	$max                = ( $min + $features_per_page ) - 1;

	if( $features ) { 
		// Iterator for rows
		$i = 1;
		?>
		<div id="bike-features-container">
			<div id="bike-features-inner">
				<div class="bike-features-content">
					<!-- Note: using Bootstrap grid -->
					<div class="row">
						<?php foreach( $features as $feature ) { 

							$feature_count++;

					    // Ignore this feature if $feature_count is lower than $min
					    if($feature_count < $min) { continue; }

					    // Stop loop completely if $feature_count is higher than $max
					    if($feature_count > $max) { break; } 
					    ?>

							<div class="col-xs-6 col-sm-3">
								<article id="post-<?php echo $feature->ID; ?>" <?php post_class('clearfix bike-features'); ?> role="article">
									<?php echo $feature->post_title; ?>
								</article> <!-- end article -->
							</div>

							<?php
							// Start a new row after every 4 posts
							if ($i % 4 == 0) {
								echo '</div><!-- .row --><div class="row">';
							}
							$i++;
						}; 
						?>
					</div><!-- .row -->
				</div><!-- .bike-features-content -->


				<div id="pagination">
					<?php

					// Ajax will grab the post ID from here.
					echo '<span id="bike-id" data-bike-id="' . $bike_id . '"></span>';

					// Pagination
					$prev = ($current_page - 1);
					$next = ($current_page + 1);

					$pagination = paginate_links( array(
					  'base' => get_permalink() . '%#%' . '/',
					  'format' => '?page=%#%',
					  'current' => $page,
					  'total' => $pages,
				  	'prev_text'          => '&lsaquo;',
						'next_text'          => '&rsaquo;',
						'type'               => 'plain',
					) );

					echo $pagination;
					?>
				</div> <!-- #pagination -->
			</div> <!-- #bike-features-inner -->
		</div> <!-- #bike-features-container -->

	<?php } // end if ($features)

	wp_reset_postdata();

}