code: func (yfast *YFastTrie) getBucketKey(key uint64) uint64 {
  	i := key/uint64(yfast.bits) + 1
  	return uint64(yfast.bits)*i - 1
  }
docs: // getBucketKey finds the largest possible value in this key's bucket.
  // This is the representative value for the entry in the x-fast trie.