WooCommerce - Show next\prev products from current product category (when viewing a single product) 1. If product is last -> Next product is first 2. If product is first -> Prev product is last forked from https://gist.github.com/2176823 (This question was in http://stackoverflow.com/questions/13597687/woocommerce-get-next-previous-product/13612387)

## content-single-product.php
<?php
// get next and prev products
// Author: Georgy Bunin (bunin.co.il@gmail.com)
// forked from https://gist.github.com/2176823

function ShowLinkToProduct($post_id, $categories_as_array, $label) {
    // get post according post id
    $query_args = array( 'post__in' => array($post_id), 'posts_per_page' => 1, 'post_status' => 'publish', 'post_type' => 'product', 'tax_query' => array(
        array(
            'taxonomy' => 'product_cat',
            'field' => 'id',
            'terms' => $categories_as_array
        )));
    $r_single = new WP_Query($query_args);
    if ($r_single->have_posts()) {
        $r_single->the_post();
        global $product;
    ?>
    <ul class="product_list_widget">
        <li><a href="<?php the_permalink() ?>" title="<?php echo esc_attr(get_the_title() ? get_the_title() : get_the_ID()); ?>">
            <?php if (has_post_thumbnail()) the_post_thumbnail('shop_thumbnail'); else echo '<img src="'. woocommerce_placeholder_img_src() .'" alt="Placeholder" width="'.$woocommerce->get_image_size('shop_thumbnail_image_width').'" height="'.$woocommerce->get_image_size('shop_thumbnail_image_height').'" />'; ?>
            <?php echo $label; ?>
            <?php if ( get_the_title() ) the_title(); else the_ID(); ?>
        </a> <?php echo $product->get_price_html(); ?></li>
    </ul>
    <?php
        wp_reset_query();
    }
}

if ( is_singular('product') ) {
    global $post;

    // get categories
    $terms = wp_get_post_terms( $post->ID, 'product_cat' );
    foreach ( $terms as $term ) $cats_array[] = $term->term_id;

    // get all posts in current categories
    $query_args = array('posts_per_page' => -1, 'post_status' => 'publish', 'post_type' => 'product', 'tax_query' => array(
        array(
            'taxonomy' => 'product_cat',
            'field' => 'id',
            'terms' => $cats_array
        )));
    $r = new WP_Query($query_args);

    // show next and prev only if we have 3 or more
    if ($r->post_count > 2) {

        $prev_product_id = -1;
        $next_product_id = -1;

        $found_product = false;
        $i = 0;

        $current_product_index = $i;
        $current_product_id = get_the_ID();

        if ($r->have_posts()) {
            while ($r->have_posts()) {
                $r->the_post();
                $current_id = get_the_ID();

                if ($current_id == $current_product_id) {
                    $found_product = true;
                    $current_product_index = $i;
                }

                $is_first = ($current_product_index == $first_product_index);

                if ($is_first) {
                    $prev_product_id = get_the_ID(); // if product is first then 'prev' = last product
                } else {
                    if (!$found_product && $current_id != $current_product_id) {
                        $prev_product_id = get_the_ID();
                    }
                }

                if ($i == 0) { // if product is last then 'next' = first product
                    $next_product_id = get_the_ID();
                }

                if ($found_product && $i == $current_product_index + 1) {
                    $next_product_id = get_the_ID();
                }

                $i++;
            }

            if ($prev_product_id != -1) { ShowLinkToProduct($prev_product_id, $cats_array, "next: "); }
            if ($next_product_id != -1) { ShowLinkToProduct($next_product_id, $cats_array, "prev: "); }
        }

        wp_reset_query();
    }
}
?>