code: func (idx *index) lookupKey(offset int64) []byte {
  	data := idx.data[offset:]
  	n, sz := binary.Uvarint(data)
  	if sz == 0 {
  		return nil
  	}
  	return data[sz : sz+int(n)]
  }
docs: // lookupKey returns the key at the given offset in the memory-mapped file.