bloom filter简单实现

再流计算中计算UV是个相当麻烦的事情，特别数据量很大的时候，中间存储就大的吓人。最近项目中遇到分类目计算UV，UV量大概在7000W，有20w多个类目。如果使用简单的存储中间结果再去重，如果使用内存内存打不下，使用Hbase的话HBASE的吞吐又不够。于是准备使用bloom近似计算UV。

写了个bloom filter的demo程序，由于uid都为数字在计算hash值时碰撞率比较搞，于是没有直接对uid使用bloom filter而是对uid 的md5值使用bloom filter：

import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.BitSet;
public class  SimpleBloomFilter {
     private static final  int  DEFAULT_SIZE  = 8 << 50000; //2<<60000 ;
//     private static final  int [] seeds =new  int []{5,7, 11 , 13 , 31 , 37 , 61};
     private static final int[] seeds = new int []{13,31, 131, 1313, 13131, 131313};
     private  BitSet bits= new  BitSet(DEFAULT_SIZE);
     private  SimpleHash[]  func=new  SimpleHash[seeds.length];
//     private SimpleHash[]  func = new SimpleHash();
     public static void  main(String[] args) {
//        String value  = "[email protected]" ;
    	 System.out.println(DEFAULT_SIZE);
    	int count=0;
    	SimpleBloomFilter filter=new  SimpleBloomFilter();
    	for(int i = 10000;i<60000;i++){
    		String a = String.valueOf(i);
    		String value = getMD5Str(a);
    		if(!(filter.contains(value))){
    			count++;
    			filter.add(value);
    		}
    		
    	}
    	
    	System.out.println("result is :"+count);
        
    }
     public  SimpleBloomFilter() {
         for( int  i= 0 ; i< seeds.length; i ++ ) {
            func[i]=new  SimpleHash(DEFAULT_SIZE, seeds[i]);
        }
    }
     public void  add(String value) {
         for(SimpleHash f : func) {
        	 System.out.println(f.hash(value));
            bits.set(f.hash(value),  true );
        }
    }
     public boolean  contains(String value) {
         if(value ==null ) {
             return false ;
        }
         boolean  ret  = true ;
         for(SimpleHash f : func) {
            ret=ret&& bits.get(f.hash(value));
        }
         return  ret;
    }
     
     private static String getMD5Str(String str) {  
         MessageDigest messageDigest = null;  
   
         try {  
             messageDigest = MessageDigest.getInstance("MD5");  
   
             messageDigest.reset();  
   
             messageDigest.update(str.getBytes("UTF-8"));  
         } catch (NoSuchAlgorithmException e) {  
             System.out.println("NoSuchAlgorithmException caught!");  
             System.exit(-1);  
         } catch (UnsupportedEncodingException e) {  
             e.printStackTrace();  
         }  
   
         byte[] byteArray = messageDigest.digest();  
   
         StringBuffer md5StrBuff = new StringBuffer();  
   
         for (int i = 0; i < byteArray.length; i++) {              
             if (Integer.toHexString(0xFF & byteArray[i]).length() == 1)  
                 md5StrBuff.append("0").append(Integer.toHexString(0xFF & byteArray[i]));  
             else  
                 md5StrBuff.append(Integer.toHexString(0xFF & byteArray[i]));  
         }  
   
         return md5StrBuff.toString();  
     }  
     
     public static class SimpleHash {
         private int  cap;
         private int  seed;
         public  SimpleHash( int cap, int seed) {
             this.cap= cap;
             this.seed =seed;
        }
         public int hash(String value) {
             int  result=0 ;
             int  len= value.length();
             for  (int i= 0 ; i< len; i ++ ) {
//                result =seed* result + value.charAt(i);
            	 result =seed* result + value.charAt(i);
            }
//             return (cap - 1 ) & (result/20);
             return (cap - 1 ) & result;
//             return result;
        }
    }
}