consistenthash.go

Documentation: github.com/golang/groupcache/consistenthash

     1  /*
     2  Copyright 2013 Google Inc.
     3  
     4  Licensed under the Apache License, Version 2.0 (the "License");
     5  you may not use this file except in compliance with the License.
     6  You may obtain a copy of the License at
     7  
     8       http://www.apache.org/licenses/LICENSE-2.0
     9  
    10  Unless required by applicable law or agreed to in writing, software
    11  distributed under the License is distributed on an "AS IS" BASIS,
    12  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    13  See the License for the specific language governing permissions and
    14  limitations under the License.
    15  */
    16  
    17  // Package consistenthash provides an implementation of a ring hash.
    18  package consistenthash
    19  
    20  import (
    21  	"hash/crc32"
    22  	"sort"
    23  	"strconv"
    24  )
    25  
    26  type Hash func(data []byte) uint32
    27  
    28  type Map struct {
    29  	hash     Hash
    30  	replicas int
    31  	keys     []int // Sorted
    32  	hashMap  map[int]string
    33  }
    34  
    35  func New(replicas int, fn Hash) *Map {
    36  	m := &Map{
    37  		replicas: replicas,
    38  		hash:     fn,
    39  		hashMap:  make(map[int]string),
    40  	}
    41  	if m.hash == nil {
    42  		m.hash = crc32.ChecksumIEEE
    43  	}
    44  	return m
    45  }
    46  
    47  // IsEmpty returns true if there are no items available.
    48  func (m *Map) IsEmpty() bool {
    49  	return len(m.keys) == 0
    50  }
    51  
    52  // Add adds some keys to the hash.
    53  func (m *Map) Add(keys ...string) {
    54  	for _, key := range keys {
    55  		for i := 0; i < m.replicas; i++ {
    56  			hash := int(m.hash([]byte(strconv.Itoa(i) + key)))
    57  			m.keys = append(m.keys, hash)
    58  			m.hashMap[hash] = key
    59  		}
    60  	}
    61  	sort.Ints(m.keys)
    62  }
    63  
    64  // Get gets the closest item in the hash to the provided key.
    65  func (m *Map) Get(key string) string {
    66  	if m.IsEmpty() {
    67  		return ""
    68  	}
    69  
    70  	hash := int(m.hash([]byte(key)))
    71  
    72  	// Binary search for appropriate replica.
    73  	idx := sort.Search(len(m.keys), func(i int) bool { return m.keys[i] >= hash })
    74  
    75  	// Means we have cycled back to the first replica.
    76  	if idx == len(m.keys) {
    77  		idx = 0
    78  	}
    79  
    80  	return m.hashMap[m.keys[idx]]
    81  }
    82
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