static void ai_gc_scan(S root) {
  ai_gc_clear();
  time "GC Scan" {
    ai_postTriple(root, "has GC distance", "0");
    HashMap<S, Double> distances = lithashmap(root, 0);
    LinkedList<S> queue = litLinkedList(root);
    S word;
    while ping ((word = queue.poll()) != null) {
      double d = distances.get(word);
      for (WebNode node : indexedNodes(word))
        for (WebRelation r : web_forwardRelations(node)) {
          double dist = ai_gc_scan_distance(web_text(r), true);
          if (!isNaN(dist))
            ai_gc_scan_update(distances, queue, r.b, d+dist);
        }
          
        for (WebRelation r : web_backwardRelations(node)) {
          double dist = ai_gc_scan_distance(web_text(r), false);
          if (!isNaN(dist))
            ai_gc_scan_update(distances, queue, r.a, d+dist);
        }
    }
    Collection<S> keys = keys(distances);
    if (ai_inTestMode())
      keys = keysSortedByValueDesc(distances);
    for (S key : keys)
      ai_postTriple(key, "has GC distance", formatDouble(distances.get(key), 1));
      
    print(l(distances) + "/" + l(indexedTerms()) + " terms were reached in GC scan from " + quote(root) + ".");
  }
}

// returns NaN if not a relevant connection
static double ai_gc_scan_distance(S rel, bool forward) {
  if (forward) ret 1;
  if (eqic(rel, "is")) ret 1;
  ret nan();
}

static void ai_gc_scan_update(Map<S, Double> distances, L<S> queue, WebNode n, double d) {
  S text = ai_shortenForIndex(web_text(n));
  Double od = distances.get(text);
  if (od == null || d < od) {
    distances.put(text, d);
    queue.add(text);
  }
}