get_adjacent_post( bool $in_same_term = false, array|string $excluded_terms = '', bool $previous = true, string $taxonomy = 'category' )

function get_adjacent_post( $in_same_term = false, $excluded_terms = '', $previous = true, $taxonomy = 'category' ) {
	global $wpdb;

	if ( ( ! $post = get_post() ) || ! taxonomy_exists( $taxonomy ) )
		return null;

	$current_post_date = $post->post_date;

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

	if ( $in_same_term || ! empty( $excluded_terms ) ) {
		if ( ! empty( $excluded_terms ) && ! is_array( $excluded_terms ) ) {
			// back-compat, $excluded_terms used to be $excluded_terms with IDs separated by " and "
			if ( false !== strpos( $excluded_terms, ' and ' ) ) {
				_deprecated_argument( __FUNCTION__, 'WP-3.3.0', sprintf( __( 'Use commas instead of %s to separate excluded terms.' ), "'and'" ) );
				$excluded_terms = explode( ' and ', $excluded_terms );
			} else {
				$excluded_terms = explode( ',', $excluded_terms );
			}

			$excluded_terms = array_map( 'intval', $excluded_terms );
		}

		if ( $in_same_term ) {
			$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";
			$where .= $wpdb->prepare( "AND tt.taxonomy = %s", $taxonomy );

			if ( ! is_object_in_taxonomy( $post->post_type, $taxonomy ) )
				return '';
			$term_array = wp_get_object_terms( $post->ID, $taxonomy, array( 'fields' => 'ids' ) );

			// Remove any exclusions from the term array to include.
			$term_array = array_diff( $term_array, (array) $excluded_terms );
			$term_array = array_map( 'intval', $term_array );

			if ( ! $term_array || is_wp_error( $term_array ) )
				return '';

			$where .= " AND tt.term_id IN (" . implode( ',', $term_array ) . ")";
		}

		/**
		 * Filters the IDs of terms excluded from adjacent post queries.
		 *
		 * The dynamic portion of the hook name, `$adjacent`, refers to the type
		 * of adjacency, 'next' or 'previous'.
		 *
		 * @since WP-4.4.0
		 *
		 * @param string $excluded_terms Array of excluded term IDs.
		 */
		$excluded_terms = apply_filters( "get_{$adjacent}_post_excluded_terms", $excluded_terms );

		if ( ! empty( $excluded_terms ) ) {
			$where .= " AND p.ID NOT IN ( SELECT tr.object_id FROM $wpdb->term_relationships tr LEFT JOIN $wpdb->term_taxonomy tt ON (tr.term_taxonomy_id = tt.term_taxonomy_id) WHERE tt.term_id IN (" . implode( ',', array_map( 'intval', $excluded_terms ) ) . ') )';
		}
	}

	// 'post_status' clause depends on the current user.
	if ( is_user_logged_in() ) {
		$user_id = get_current_user_id();

		$post_type_object = get_post_type_object( $post->post_type );
		if ( empty( $post_type_object ) ) {
			$post_type_cap    = $post->post_type;
			$read_private_cap = 'read_private_' . $post_type_cap . 's';
		} else {
			$read_private_cap = $post_type_object->cap->read_private_posts;
		}

		/*
		 * Results should include private posts belonging to the current user, or private posts where the
		 * current user has the 'read_private_posts' cap.
		 */
		$private_states = get_post_stati( array( 'private' => true ) );
		$where .= " AND ( p.post_status = 'publish'";
		foreach ( (array) $private_states as $state ) {
			if ( current_user_can( $read_private_cap ) ) {
				$where .= $wpdb->prepare( " OR p.post_status = %s", $state );
			} else {
				$where .= $wpdb->prepare( " OR (p.post_author = %d AND p.post_status = %s)", $user_id, $state );
			}
		}
		$where .= " )";
	} else {
		$where .= " AND p.post_status = 'publish'";
	}

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

	/**
	 * Filters the JOIN clause in the SQL for an adjacent post query.
	 *
	 * The dynamic portion of the hook name, `$adjacent`, refers to the type
	 * of adjacency, 'next' or 'previous'.
	 *
	 * @since WP-2.5.0
	 * @since WP-4.4.0 Added the `$taxonomy` and `$post` parameters.
	 *
	 * @param string  $join           The JOIN clause in the SQL.
	 * @param bool    $in_same_term   Whether post should be in a same taxonomy term.
	 * @param array   $excluded_terms Array of excluded term IDs.
	 * @param string  $taxonomy       Taxonomy. Used to identify the term used when `$in_same_term` is true.
	 * @param WP_Post $post           WP_Post object.
	 */
	$join = apply_filters( "get_{$adjacent}_post_join", $join, $in_same_term, $excluded_terms, $taxonomy, $post );

	/**
	 * Filters the WHERE clause in the SQL for an adjacent post query.
	 *
	 * The dynamic portion of the hook name, `$adjacent`, refers to the type
	 * of adjacency, 'next' or 'previous'.
	 *
	 * @since WP-2.5.0
	 * @since WP-4.4.0 Added the `$taxonomy` and `$post` parameters.
	 *
	 * @param string  $where          The `WHERE` clause in the SQL.
	 * @param bool    $in_same_term   Whether post should be in a same taxonomy term.
	 * @param array   $excluded_terms Array of excluded term IDs.
	 * @param string  $taxonomy       Taxonomy. Used to identify the term used when `$in_same_term` is true.
	 * @param WP_Post $post           WP_Post object.
	 */
	$where = apply_filters( "get_{$adjacent}_post_where", $wpdb->prepare( "WHERE p.post_date $op %s AND p.post_type = %s $where", $current_post_date, $post->post_type ), $in_same_term, $excluded_terms, $taxonomy, $post );

	/**
	 * Filters the ORDER BY clause in the SQL for an adjacent post query.
	 *
	 * The dynamic portion of the hook name, `$adjacent`, refers to the type
	 * of adjacency, 'next' or 'previous'.
	 *
	 * @since WP-2.5.0
	 * @since WP-4.4.0 Added the `$post` parameter.
	 * @since WP-4.9.0 Added the `$order` parameter.
	 *
	 * @param string $order_by The `ORDER BY` clause in the SQL.
	 * @param WP_Post $post    WP_Post object.
	 * @param string  $order   Sort order. 'DESC' for previous post, 'ASC' for next.
	 */
	$sort  = apply_filters( "get_{$adjacent}_post_sort", "ORDER BY p.post_date $order LIMIT 1", $post, $order );

	$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;
}