简易数据库实现 UNIX环境高级编程(APUE)第二十章 A Database Library

将课程的源代码使用C++写了一部分

LINUX WINDOW均可运行

#ifndef MYDB_H
#define MYDB_H

#include <iostream>
#include <fstream>
#include <string>

#define NHASH_DEF   137    /* default hash table size */
#define FREE_LINK  NHASH_DEF
#define PTR_SZ      8
#define MAGIC_WORDS "MyDB_Start:\n"
#define SEPARATOE    "#"



class MyDB{
public:
    MyDB(const char* pathName) : hashOffset(0),
    dataOffset(0),writeDataLength(0),idxOffset(0),writeIdxLength(0) 
    {
        DBidxName = pathName;
        DBidxName += ".idx";
        DBdataName = pathName;
        DBdataName += ".dat";
    }
    void CloseDB(){
            idxFile.close();
            datFile.close();
        }
    ~MyDB(){ CloseDB();}

    bool Init();
    unsigned long HashFunc(const char *key);
    bool WriteData(const char* data);
    bool WriteIndex(const char* key);
    size_t GetIndexOffsetBykey(const char* key);
    long ReadPtr(std::fstream& fs, size_t offset);
    bool Writeptr(std::fstream& fs, size_t offset, const std::string& writeString);
    void TestFunc(const char* searchKey);
    size_t SearchLinkForKey(size_t offset, const char* key);
    std::string Query(const char* key);
private:

    bool DoWriteIndex(const char* key, const size_t nextOffset);
    bool OpenFileOrExit(std::fstream& fs, const std::string& fileName);
    size_t hashOffset;
    size_t dataOffset;
    size_t writeDataLength;
    unsigned long idxOffset;
    size_t writeIdxLength;
    MyDB& operator=(const MyDB&);
    MyDB(const MyDB&);
    std::string DBidxName;
    std::string DBdataName;
    std::fstream idxFile;
    std::fstream datFile;
};

#endif // MYDB_H

mydb.h

#include <string>
#include "mydb.h"

void MyDB::TestFunc(const char* key)
{


}

std::string MyDB::Query(const char* key)
{
    size_t offset = GetIndexOffsetBykey(key);
    if (0 == offset)
        return "";

    size_t nextPtr = ReadPtr(idxFile, offset);
    size_t indexLength = ReadPtr(idxFile, offset + PTR_SZ);

    char* indexstr = new char[indexLength + 1];
    idxFile.read(indexstr, indexLength);
    indexstr[indexLength] = '\0';
    std::string indexString(indexstr);
    delete[] indexstr;

    std::string keyString = indexString.substr(0, indexString.find_first_of(SEPARATOE));

    return keyString;
}

bool MyDB::OpenFileOrExit(std::fstream& fs, const std::string& fileName) {
    fs.open(fileName, std::ios::in | std::ios::out | std::ios::binary);
    if (!fs.is_open()) {
        fs.open(fileName, std::ios::in | std::ios::out | std::ios::binary | std::ios::app);
        fs.close();
        fs.open(fileName, std::ios::in | std::ios::out | std::ios::binary);
        if (!fs.is_open()) {
            std::cerr << "open idxFile error.exit!" << std::endl;
            exit(-1);
        }
    }
    return true;
}

bool MyDB::Init() {
    OpenFileOrExit(idxFile,DBidxName);
    OpenFileOrExit(datFile, DBdataName);

    idxFile.seekg(0, std::ios_base::end);
    size_t len = idxFile.tellg();
    hashOffset = 0;

    if (len != 0)
        return true;

    //进行初始化
    idxFile.seekp(0, std::ios::beg);

    std::string ptrstr(PTR_SZ, ' ');
    ptrstr[PTR_SZ - 1] = '0';
    //std::cout << ptrstr << std::endl;
    for (int i = 0; i < NHASH_DEF + 1; i++)
        idxFile << ptrstr;
    idxFile << '\n';
    idxFile.flush();
    return true;
}

unsigned long
MyDB::HashFunc(const char *key)
{
    unsigned long   hval = 0;
    char        c;
    int            i;

    for (i = 1; (c = *key++) != 0; i++)
        hval += c * i;
    return(hval % NHASH_DEF);
}

bool MyDB::Writeptr(std::fstream& fs,size_t offset,const std::string& writeString) {
    fs.seekp(offset, std::ios::beg);
    fs << writeString;
    return true;
}

long MyDB::ReadPtr(std::fstream& fs,size_t offset) {
    long ret = 0;

    char ptrstr[PTR_SZ + 1];
    fs.seekg(offset, std::ios::beg);
    fs.read(ptrstr, PTR_SZ);
    ptrstr[PTR_SZ] = '\0';

    ret = atol(ptrstr);
    return ret;
}

size_t MyDB::SearchLinkForKey(size_t offset, const char* key)
{
    size_t currentPtr = offset;
    
    do {
        size_t nextPtr = ReadPtr(idxFile, currentPtr);
        size_t indexLength = ReadPtr(idxFile, currentPtr + PTR_SZ);

        char* indexstr = new char[indexLength + 1];
        idxFile.read(indexstr, indexLength);
        indexstr[indexLength] = '\0';
        std::string indexString(indexstr);
        delete[] indexstr;

        std::string keyString = indexString.substr(0, indexString.find_first_of(SEPARATOE));
        if (keyString == std::string(key))
            break;
        currentPtr = nextPtr;
    } while (0 != currentPtr);

    return currentPtr;

}


size_t MyDB::GetIndexOffsetBykey(const char* key) {
    size_t indexOff = 0;
    unsigned long hashKeyValue = HashFunc(key);

    size_t linkOffset = hashKeyValue*PTR_SZ + hashOffset;
    size_t idxOffset = ReadPtr(idxFile,linkOffset);
    if (0 == idxOffset)
        return 0;
    //有链表  便历链表 查看是否已有关键字
    size_t offset = SearchLinkForKey(idxOffset,key);

    return offset;
}

bool MyDB::DoWriteIndex(const char* key,const size_t nextOffset) {
    idxFile.seekp(0, std::ios::end);
    idxOffset = idxFile.tellp();

    std::string s = std::to_string(nextOffset);
    if (s.size() > PTR_SZ) {
        return false;
    }

    std::string writeString(PTR_SZ - s.size(), ' ');
    writeString += s;
    
    char ptrstr[PTR_SZ + 1];
    sprintf(ptrstr, "%*d", PTR_SZ,0);

    std::string idxString(writeString);
    idxString += ptrstr;
    idxString += key;
    idxString += SEPARATOE;
    idxString += std::to_string(dataOffset);
    idxString += SEPARATOE;
    idxString += std::to_string(writeDataLength);
    idxString += "\n";

    unsigned long size = idxString.size() - strlen(ptrstr) * 2;
    sprintf(ptrstr, "%*d", PTR_SZ, size);
    idxString.replace(PTR_SZ, PTR_SZ, ptrstr);
    //std::cout << idxString << std::endl;
    idxFile << idxString;
    //idxFile.flush();

    return true;
}

bool MyDB::WriteIndex(const char* key)
{
    unsigned long hashKeyValue = HashFunc(key);
    size_t linkOffset = hashKeyValue*PTR_SZ + hashOffset;
    size_t idxInLinkHeadoffset = ReadPtr(idxFile, linkOffset);

    if (0 == idxInLinkHeadoffset) {
        DoWriteIndex(key, 0);
        std::string s = std::to_string(idxOffset);
        if (s.size() <= PTR_SZ) {
            std::string writeString(PTR_SZ - s.size(), ' ');
            writeString += s;
            Writeptr(idxFile, linkOffset, writeString);
            return true;
        }
        else {
            return false;
        }
    }
    else {
        //在链表中寻找
        size_t offset = GetIndexOffsetBykey(key);
        if (0 != offset) {
            //寻找到则直接返回
            return  true;
        }
        DoWriteIndex(key, idxInLinkHeadoffset);
        std::string s = std::to_string(idxOffset);
        if (s.size() <= PTR_SZ) {
            std::string writeString(PTR_SZ - s.size(), ' ');
            writeString += s;
            Writeptr(idxFile, linkOffset, writeString);
            return true;
        }
        else {
            return false;
        }

    }

    return true;
}

bool MyDB::WriteData(const char* data)
{
    try {
        writeDataLength = strlen(data);
        datFile.seekp(0, std::ios::end);
        dataOffset = datFile.tellp();
        datFile << data;
    }
    catch (std::exception& e) {
        std::cerr << e.what() << std::endl;
        exit(1);
    }

    return true;
}

mydb.cpp

测试代码

// DB0_4.cpp : 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include <assert.h>
#include "mydb.h"



int main()
{
    MyDB db("123");
    db.Init();
    for (int i = 0; i < 500; ++i) {
        std::string s = std::to_string(i);
        std::string writeIdxString("indexWrite");
        std::string writeDatString("dataWrite");
        writeIdxString += s;
        writeDatString += s;
        db.WriteData(writeDatString.c_str());
        db.WriteIndex(writeIdxString.c_str());
    }
    std::string queryKeyString1("indexWrite0");
    std::string queryKeyString2("indexWrite1");
    std::string queryKeyString3("indexWrite10");
    std::string queryKeyString4("indexWrite100");
    std::string queryKeyString5("indexWrite_");
    std::string queryKeyString6("indexWritex");
    std::string queryKeyString7("ShouldNotFind");
    assert(db.Query(queryKeyString1.c_str())  == queryKeyString1);
    assert(db.Query(queryKeyString2.c_str()) == queryKeyString2);
    assert(db.Query(queryKeyString3.c_str()) == queryKeyString3);
    assert(db.Query(queryKeyString4.c_str()) == queryKeyString4);


    assert(db.Query(queryKeyString5.c_str()) != queryKeyString5);
    assert(db.Query(queryKeyString6.c_str()) != queryKeyString6);
    assert(db.Query(queryKeyString7.c_str()) != queryKeyString7);

    std::cout << "fine" << std::endl;
    return 0;
}

main.cpp

代码运行后文件效果

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View Code

大体思路如下：

使用两个文件分开存储 key 和value

value存储在data文件中中规中矩的存储不多描述

key值存储在index文件中使用素数(137)个桶来存储key值

0　　0　　0　　0　　0　　0.........0　　0 <-----137 个桶

当输入一个 kv 值时，比如输入 ''key" "value"

经过哈希运算 key的哈希值为2 则在第3个桶(计数规则从0开始，0,1,2)记录key写入index文件的偏移值

则index文件内容如下

0　　0　　1137　　0　　0　　0.......0　　0

0　8key#0#5

表示在第3个桶内有内容由于仅有一个记录所以INDEX的一开始为0 若有其他记录则记录桶内下个元素在文件里的偏移后面的8 表示接下来的字节加'\n'共长8个字节

后面的内容则是key值与data在data文件里的起始位置和长度

posted on 2017-07-25 11:26 itdef 阅读(880) 评论(0) 编辑收藏举报

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简易数据库实现 UNIX环境高级编程(APUE)第二十章 A Database Library

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