static int md5OfFile_throughString_cached_interval = 10*60000; // forget after 10 minutes

static Map<File, Pair<Pair<Long>, S>> md5OfFile_throughString_cached_map = syncExpiringMap(md5OfFile_throughString_cached_interval); // pair(pair(lastModified, size), md5)

sS md5OfFile_throughString_cached(File f) {
  if (f == null) null;
  if (!f.exists()) ret md5OfFile_throughString(f);
  Pair<Pair<Long>, S> p = md5OfFile_throughString_cached_map.get(f);
  Pair<Long> modInfo = pair(lastModified(f), fileSize(f));
  if (!eq(pairA(p), modInfo))
    md5OfFile_throughString_cached_map.put(f, p = pair(modInfo, md5OfFile_throughString(f)));
  ret p.b;
}