基数估计：线性概率计数器

    线性概率计数器可以在线性时间你估计数据的基数。

 

# -*- coding: utf-8 -*-
"""
Created on Tue Jan 15 16:17:10 2013

@author: chen ming
基数估计：线性概率计数器
"""
import random
import math
M=120000      #
rna= 20000
N=14
data=[random.random() for i in range(M)]
bitmap=[0]*rna
for it in data:
    ind = ((hash(it))) % (rna-1)
    bitmap[ind]=1
    
rate=float(rna-sum(bitmap))/rna

print 'sum(bitmap)  %d' % sum(bitmap)
print 'size(bitmap) %d' %  rna
print 'num of 0 in bitmap %f' % rate
aa=-rna*math.log(rate)
print 'result: %d ' % aa
ac=aa/M
print 'result//M: %f ' % ac

结果：

sum(bitmap)  19942
size(bitmap) 20000
num of 0 in bitmap 0.002900
result: 116860 
result//M: 0.973841 

准确率为97.3%