Display WooCommerce Single Products in same category.

functions.php

function astra_next_post_link_product($format='%link »', $link='%title', $in_same_cat = false, $excluded_categories = '') {
    adjacent_post_link_product($format, $link, $in_same_cat, $excluded_categories, false);
}

function astra_previous_post_link_product($format='« %link', $link='%title', $in_same_cat = false, $excluded_categories = '') {
    adjacent_post_link_product($format, $link, $in_same_cat, $excluded_categories, true);
}

function adjacent_post_link_product( $format, $link, $in_same_cat = false, $excluded_categories = '', $previous = true ) {
    if ( $previous && is_attachment() )
        $post = get_post( get_post()->post_parent );
    else
        $post = get_adjacent_post_product( $in_same_cat, $excluded_categories, $previous );

    if ( ! $post ) {
        $output = '';
    } else {
        $title = $post->post_title;

        if ( empty( $post->post_title ) )
            $title = $previous ? __( 'Previous Post' ) : __( 'Next Post' );

        $title = apply_filters( 'the_title', $title, $post->ID );
        $date = mysql2date( get_option( 'date_format' ), $post->post_date );
        $rel = $previous ? 'prev' : 'next';

        $string = '<a href="' . get_permalink( $post ) . '" rel="'.$rel.'">';
        $inlink = str_replace( '%title', $title, $link );
        $inlink = str_replace( '%date', $date, $inlink );
        $inlink = $string . $inlink . '</a>';

        $output = str_replace( '%link', $inlink, $format );
    }

    $adjacent = $previous ? 'previous' : 'next';

    echo apply_filters( "{$adjacent}_post_link", $output, $format, $link, $post );
}

function get_adjacent_post_product( $in_same_cat = false, $excluded_categories = '', $previous = true ) {
    global $wpdb;

    if ( ! $post = get_post() )
        return null;

    $current_post_date = $post->post_date;

    $join = '';
    $posts_in_ex_cats_sql = '';
    if ( $in_same_cat || ! empty( $excluded_categories ) ) {
        $join = " INNER JOIN $wpdb->term_relationships AS tr ON p.ID = tr.object_id INNER JOIN $wpdb->term_taxonomy tt ON tr.term_taxonomy_id = tt.term_taxonomy_id";

        if ( $in_same_cat ) {
            if ( ! is_object_in_taxonomy( $post->post_type, 'product_cat' ) )
                return '';
            $cat_array = wp_get_object_terms($post->ID, 'product_cat', array('fields' => 'ids'));
            if ( ! $cat_array || is_wp_error( $cat_array ) )
                return '';
            $join .= " AND tt.taxonomy = 'product_cat' AND tt.term_id IN (" . implode(',', $cat_array) . ")";
        }

        $posts_in_ex_cats_sql = "AND tt.taxonomy = 'product_cat'";
        if ( ! empty( $excluded_categories ) ) {
            if ( ! is_array( $excluded_categories ) ) {
                // back-compat, $excluded_categories used to be IDs separated by " and "
                if ( strpos( $excluded_categories, ' and ' ) !== false ) {
                    _deprecated_argument( __FUNCTION__, '3.3', sprintf( __( 'Use commas instead of %s to separate excluded categories.' ), "'and'" ) );
                    $excluded_categories = explode( ' and ', $excluded_categories );
                } else {
                    $excluded_categories = explode( ',', $excluded_categories );
                }
            }

            $excluded_categories = array_map( 'intval', $excluded_categories );

            if ( ! empty( $cat_array ) ) {
                $excluded_categories = array_diff($excluded_categories, $cat_array);
                $posts_in_ex_cats_sql = '';
            }

            if ( !empty($excluded_categories) ) {
                $posts_in_ex_cats_sql = " AND tt.taxonomy = 'product_cat' AND tt.term_id NOT IN (" . implode($excluded_categories, ',') . ')';
            }
        }
    }

    $adjacent = $previous ? 'previous' : 'next';
    $op = $previous ? '<' : '>';
    $order = $previous ? 'DESC' : 'ASC';

    $join  = apply_filters( "get_{$adjacent}_post_join", $join, $in_same_cat, $excluded_categories );
    $where = apply_filters( "get_{$adjacent}_post_where", $wpdb->prepare("WHERE p.post_date $op %s AND p.post_type = %s AND p.post_status = 'publish' $posts_in_ex_cats_sql", $current_post_date, $post->post_type), $in_same_cat, $excluded_categories );
    $sort  = apply_filters( "get_{$adjacent}_post_sort", "ORDER BY p.post_date $order LIMIT 1" );

    $query = "SELECT p.id FROM $wpdb->posts AS p $join $where $sort";
    $query_key = 'adjacent_post_' . md5($query);
    $result = wp_cache_get($query_key, 'counts');
    if ( false !== $result ) {
        if ( $result )
            $result = get_post( $result );
        return $result;
    }

    $result = $wpdb->get_var( $query );
    if ( null === $result )
        $result = '';

    wp_cache_set($query_key, $result, 'counts');

    if ( $result )
        $result = get_post( $result );

    return $result;
}

add_action('wp' , 'remove_prev_next' , 50 );
function remove_prev_next(){

	remove_action( 'woocommerce_single_product_summary', array( ASTRA_Ext_WooCommerce_Markup::get_instance(), 'next_previous_links' ), 1, 0 );
}

add_action('wp' , 'add_prev_next' , 100 );
function add_prev_next(){

	if ( 'disable' != astra_get_option( 'single-product-nav-style' ) ) {
		add_action( 'woocommerce_single_product_summary', 'next_previous_links_custom' , 1, 0 );

		function next_previous_links_custom(){
			if ( ! is_product() ) {
				return;
			}
			?>
			<div class="product-links">
				<?php astra_previous_post_link_product('%link', '<i class="ast-icon-previous"></i>', true); ?>
				<?php astra_next_post_link_product('%link', '<i class="ast-icon-next"></i>', true); ?>
			</div>
			<?php
		}
	}
}