hash算法

 

 

单调性Monotonicity: 当缓存增加或者减少时,尽量保护原来被映射的内容不会被映射到新的缓存去

分散性:不同的客户端可能看到的缓存不一样多(主要对于分布式缓存中得hash算法把),于是相同的内容在不同的客户端被映射到不同的缓存。尽量要避免这种情况。

平衡性:尽量均衡把(一致性hash,虚拟节点)

 

/*
Copyright 2013 Google Inc.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

     http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

// Package consistenthash provides an implementation of a ring hash.
package consistenthash

import (
	"fmt"
	"hash/crc32"
	"sort"
	"strconv"
)

type Hash func(data []byte) uint32

type Map struct {
	hash     Hash
	replicas int
	keys     []int // Sorted
	hashMap  map[int]string
}

func New(replicas int, fn Hash) *Map {
	m := &Map{
		replicas: replicas,
		hash:     fn,
		hashMap:  make(map[int]string),
	}
	if m.hash == nil {
		m.hash = crc32.ChecksumIEEE
	}
	return m
}

// Returns true if there are no items available.
func (m *Map) IsEmpty() bool {
	return len(m.keys) == 0
}

// Adds some keys to the hash.
func (m *Map) Add(keys ...string) {
	for _, key := range keys {
		for i := 0; i < m.replicas; i++ {
			hash := int(m.hash([]byte(strconv.Itoa(i) + key)))
			idx := sort.Search(len(m.keys), func(i int) bool { return m.keys[i] >= hash })
			if idx < len(m.keys) && m.keys[idx] == hash {
				// key in closest
				continue
			} else {
				m.keys = append(m.keys, hash)
				m.hashMap[hash] = key
			}
		}
	}
	sort.Ints(m.keys)
}

func (m *Map) Remove(keys ...string) {
	for _, key := range keys {
		for i := 0; i < m.replicas; i++ {
			hash := int(m.hash([]byte(strconv.Itoa(i) + key)))
			idx := sort.Search(len(m.keys), func(i int) bool { return m.keys[i] >= hash })
			if idx < len(m.keys) && m.keys[idx] == hash {
				m.keys = append(m.keys[:idx], m.keys[idx+1:]...)
				delete(m.hashMap, hash)
			}
		}
	}
}

// Gets the closest item in the hash to the provided key.
func (m *Map) Get(key string) string {
	if m.IsEmpty() {
		return ""
	}

	hash := int(m.hash([]byte(key)))
	// Binary search for appropriate replica.
	idx := sort.Search(len(m.keys), func(i int) bool { return m.keys[i] >= hash })

	// Means we have cycled back to the first replica.
	if idx == len(m.keys) {
		idx = 0
	}
	return m.hashMap[m.keys[idx]]
}

func (m *Map) Show() string {
	var str string
	l := len(m.keys)
	for i := 0; i < l; i++ {
		k := m.keys[i]
		v := m.hashMap[k]
		s := fmt.Sprintf("%s %d\n", v, k)
		str += s
	}

	return str
}

  

posted on 2015-04-16 11:23  不忘初衷,方能致远  阅读(277)  评论(0)    收藏  举报

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