code: func (idx *index) idByKey(key []byte) (uint64, bool) {
  	hash := hashKey(key)
  	pos := hash & idx.mask
  
  	var dist uint64
  	for {
  		if e := &idx.elems[pos]; e.hash == 0 {
  			return 0, false
  		} else if dist > idx.dist(e.hash, pos) {
  			return 0, false
  		} else if e.hash == hash && bytes.Equal(idx.lookupKey(e.offset), key) {
  			return e.id, true
  		}
  
  		pos = (pos + 1) & idx.mask
  		dist++
  	}
  }
docs: // idByKey returns the ID for a given key, if it exists.