倒排索引是搜索引擎中常用的算法，主要用来实现full text searching，建立关键词和所在文档的映射关系，很多强大的功能都建立在此基础之上，关于Inverted Index的详尽描述可以看Wikipedia。下面按照自己的想法实现之，只是为了体会这个数据结构的运作。
package mythought.invertedindex;
import java.io.BufferedReader;
import java.io.FileReader;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;
public class InvertedIndex {
// key word <----> doc file
private Map<String, Set<String>> indexs = new HashMap<String, Set<String>>();
// 这里假设都是小文件
public void addFile(String fileName, String content) {
String[] words = content.split(" ");
for (int i = 0; i < words.length; i++) {
String word = words[i];
// only record first appeared position
Set<String> wordIndex = indexs.get(word);
if (wordIndex == null) {
wordIndex = new HashSet<String>();
indexs.put(word, wordIndex);
}
wordIndex.add("("+fileName + "," + i+")");
}
}
public void addFile(String fileName) throws Exception{
BufferedReader br = new BufferedReader(new FileReader(fileName));
try {
StringBuilder sb = new StringBuilder();
String line = br.readLine();
while (line != null) {
sb.append(line);
sb.append(" ");// 每行直接串接
line = br.readLine();
}
this.addFile(fileName, sb.toString());
} catch(Exception e){
e.printStackTrace();
}finally {
br.close();
}
}
public Set<String> search(String keyword) {
Set<String> results = indexs.get(keyword);
return new HashSet<String>(results);
}
public static void main(String[] args) throws Exception{
InvertedIndex test = new InvertedIndex();
test.addFile("file1", "hello fuck world todotodo");
test.addFile("file2", "go to get it if you want it");
test.addFile("C:/data/hello.txt");
System.out.println(test.search("it"));
System.out.println(test.search("you"));
System.out.println(test.search("vonzhou"));
}
}
