package com.kaikeba.data.jobspider.util;
import java.util.BitSet;
public class Bloomfilter {
private static final int DEFAULT_SIZE = 2 << 29;//布隆过滤器的比特长度
private static final int[] seeds = {3,5,7, 11, 13, 31, 37, 61};//这里要选取质数，能很好的降低错误率
private BitSet bits = new BitSet(DEFAULT_SIZE);
private SimpleHash[] func = new SimpleHash[seeds.length];
public Bloomfilter()
{
for (int i = 0; i < seeds.length; i++) {
func[i] = new SimpleHash(DEFAULT_SIZE, seeds[i]);
}
}
private void addValue(String value)
{
for(SimpleHash f : func)//将字符串value哈希为8个或多个整数，然后在这些整数的bit上变为1
bits.set(f.hash(value),true);
}
public void add(String value)
{
if(value != null) addValue(value);
}
public boolean contains(String value)
{
if(value == null) return false;
boolean ret = true;
for(SimpleHash f : func)//这里其实没必要全部跑完，只要一次ret==false那么就不包含这个字符串
ret = ret && bits.get(f.hash(value));
return ret;
}
// /**
// *初始化过滤器.
// *
// * @param
// */
// public void init(String file) {
// for (int i = 0; i < seeds.length; i++) {
// func[i] = new SimpleHash(DEFAULT_SIZE, seeds[i]);
// }
//// BufferedReader reader = null;
//// try {
//// reader = new BufferedReader(new FileReader(file));
//// String line = reader.readLine();
//// while (line != null && line.length() > 0) {
//// this.put(line);
//// line = reader.readLine();
//// }
//// } catch (Exception e) {
//// e.printStackTrace();
//// } finally {
//// try {
//// if (reader != null)
//// reader.close();
//// } catch (IOException e) {
//// e.printStackTrace();
//// }
//// }
// }
// public static void main(String[] args) {
// String value = "xkeyideal@gmail.com";
//
// add(value);
// System.out.println(contains(value));
// }
}
class SimpleHash {//这玩意相当于C++中的结构体
private int cap;
private int seed;
public SimpleHash(int cap, int seed) {
this.cap = cap;
this.seed = seed;
}
public int hash(String value) {//字符串哈希，选取好的哈希函数很重要
int result = 0;
int len = value.length();
for (int i = 0; i < len; i++) {
result = seed * result + value.charAt(i);
}
return (cap - 1) & result;
}
}
原文地址：http://www.cnblogs.com/dongxiao-yang/p/3861084.html
