code: private long allocateRun(int normCapacity) {
          int d = maxOrder - (log2(normCapacity) - pageShifts);
          int id = allocateNode(d);
          if (id < 0) {
              return id;
          }
          freeBytes -= runLength(id);
          return id;
      }
docs: Allocate a run of pages (>=1)
  
  @param normCapacity normalized capacity
  @return index in memoryMap