C/C++ 轻量级搜索引擎实战：正/倒排索引构建

（3）说明

已提供外部接口：由 ID 返回文档内容、由关键词返回文档 ID。接下来进行索引关系建立。

（4）建立索引

原理：读取清洗后的 data_clean.txt 文件，按行获取单个文档内容，同时建立正排和倒排内容。

bool BuildIndex(const std::string &input) {
    std::ifstream in(input, std::ios::in | std::ios::binary);
    if (!in.is_open()) {
        std::cerr << "BuildIndex error" << std::endl;
        return false;
    }
    std::string line;
    while (std::getline(in, line)) {
        Forwardindex *doc = Build_Forward_Index(line);
        printf("正在建立索引:%lld\ntitle:%s\nchain:%s\n", doc->doc_id, doc->title.c_str(), doc->chain.c_str());
        if (doc == nullptr) continue;
        Build_Inverted_Index(*doc);
    }
    return true;
}

（5）建立正排索引

思路：解析 line 变量（格式：标题 | 内容 | URL），填入 vector，ID 为 vector 下标。

Forwardindex* Build_Forward_Index(const std::string& line) {
    Forwardindex* index = new Forwardindex();
    size_t set_pos1 = line.find('\3');
    if (set_pos1 == std::string::npos) {
        delete index;
        return nullptr;
    }
    index->title = line.substr(0, set_pos1);
    if (index->title.empty()) index->title = "空";

    size_t set_pos2 = line.find('\3', set_pos1 + 1);
    if (set_pos2 == std::string::npos) {
        delete index;
        return nullptr;
    }
    index->source = line.substr(set_pos1 + 1, set_pos2 - (set_pos1 + 1));
    if (index->source.empty()) index->source = "空";

    index->chain = line.substr(set_pos2 + 1);
    if (index->chain.empty()) index->chain = "空";

    index->doc_id = Forward.size();
    Forward.push_back(*index);
    return index;
}

（6）建立倒排索引

bool Build_Inverted_Index(const Forwardindex& doc) {
    JiebaUtil jieba;
    std::vector<std::string> S;
    
    // 对标题统计
    S = jieba.Tokenize(doc.title);
    struct calculate {
        int title_size = 0;
        int source_size = 0;
    };
    std::unordered_map<std::string, calculate> V;
    for (auto e : S) {
        (V[e].title_size)++;
    }

    // 统计内容出现次数
    S.clear();
    S = jieba.Tokenize(doc.source);
    for (auto e : S) {
        (V[e].source_size)++;
    }

    // 遍历结果写入到 Inverted
    for (auto it : V) {
        Invertedindex index_t;
        index_t.word = it.first;
        index_t.doc_id = doc.doc_id;
        index_t.weight = ((it.second.title_size) * 2 + (it.second.source_size) * 1);
        Inverted[it.first].push_back(std::move(index_t));
    }
    return true;
}

【四】单例模式

单例模式：只允许创建一个 Index 对象，后续通过指针调用。

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
class Index {
public:
    typedef std::vector<Invertedindex> Stock_Inverted;
    Index(const Index&) = delete;
    Index& operator=(const Index&) = delete;

    static Index* handle() {
        if (instance == nullptr) {
            pthread_mutex_lock(&mutex);
            if (instance == nullptr) {
                instance = new Index;
            }
            pthread_mutex_unlock(&mutex);
        }
        return instance;
    }
private:
    static Index* instance;
};

Index* Index::instance = nullptr;