// the map is: invalidated id -> invalidators

static DependentCache<MultiMap<GlobalID>> ai_invalidatedWebs_cache = new DependentCache(f ai_invalidatedWebs_calc, f cachedNodeIndex /*_changeCount*/);

static Set<GlobalID> ai_invalidatedWebs() {
  ret keys(ai_invalidatedWebs_cache!);
}

static MultiMap<GlobalID> ai_invalidatedWebs_calc() {
  //print("ai_invalidatedWebs_calc");
  ai_onNewOrRemovedWeb(f ai_invalidatedWebs_onNewWeb, f ai_invalidatedWebs_onRemovedWeb);
  ai_invalidatedWebs_cache.set(new MultiMap);
  
  // old method
  //Collection<Web> webs = allWebsFromCachedNodeIndex_uncached();
  //print("Have " + nWebs(webs));
  //for (Web web : webs)
  //  ai_invalidatedWebs_onNewWeb(web);
  
  // new method
  new Matches m;
  for (WebNode n : indexedNodes_withInvalids("invalid"))
    for (WebRelation r : web_backwardRelations(n))
      if (web_hasLabelIC(r, "is")
        && web_match("Web *", r.a, m) && isGlobalID($1))
        ai_invalidatedWebs_cache->put(asGlobalID($1), r.web.globalIDObj());

  print("ai_invalidatedWebs calc done with " + n2(ai_invalidatedWebs_cache->keysSize(), "invalidated web", "invalidated webs"));
  ret ai_invalidatedWebs_cache!;
}

svoid ai_invalidatedWebs_onNewWeb(Web web) {
  if (web.unverified) ret;
  lock ai_invalidatedWebs_cache.lock;
  new Matches m;
  for (WebNode n : web_search_dollarX(webFromTriple("$X", "is", "invalid"), web))
    if (web_match("Web *", n, m) && isGlobalID($1))
      ai_invalidatedWebs_cache->put(asGlobalID($1), web.globalIDObj());
}

svoid ai_invalidatedWebs_onRemovedWeb(Web web) {
  if (web.unverified) ret;
  lock ai_invalidatedWebs_cache.lock;
  new Matches m;
  for (WebNode n : web_search_dollarX(webFromTriple("$X", "is", "invalid"), web))
    if (web_match("Web *", n, m) && isGlobalID($1))
      ai_invalidatedWebs_cache->remove(asGlobalID($1), web.globalIDObj());
}

svoid ai_invalidatedWebs_cache_put(GlobalID a, GlobalID b) {
  ai_invalidatedWebs_cache->put(a, b);
}

Began life as a copy of #1011311