搜索相关度排序

向量空间模型

http://hi.baidu.com/zhumzhu/blog/item/fc49ef3d19b0a4c09f3d62a3.html

lucene的相关度计算方式，向量空间模型

W(t,d)=tf(t,d)*log(n/df(t))

W(t,d):the weight of term t in document d.

tf(t,d):the frequency of term t in document d

n:total number of document

df(t):the number of document that contains term t.