code: public RunT getNearestBuild(int n) {
          SortedMap<Integer, ? extends RunT> m = _getRuns().headMap(n - 1); // the map should
                                                                            // include n, so n-1
          if (m.isEmpty())
              return null;
          return m.get(m.lastKey());
      }
docs: Gets the youngest build #m that satisfies {@code n&lt;=m}.
  
  This is useful when you'd like to fetch a build but the exact build might
  be already gone (deleted, rotated, etc.)
  @see LazyBuildMixIn#getNearestBuild