code: private int getChunkId(String chromosome, int chunk64k) {
          int chunkId, pos = chunk64k * chunkSize64k + 1;
          String key = buildChunkMapKey(chromosome, pos);
          if (chunkIdMap.containsKey(key)) {
              chunkId = chunkIdMap.get(key);
          } else {
              throw new InternalError("Coverage chunk " + chromosome
                      + ":" + pos + "-, not found in database (64-chunk : " + chunk64k + ")");
          }
          return chunkId;
      }
docs: Get the chunk ID from the map of chunks.
  
  @param chromosome    Chromosome of the chunk target
  @param chunk64k         Number of 64k chunk
  @return              Chunk ID in the database