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string Base64_Encode(const string& plainText)
{
    string cipherText;
    Base64_Encode(cipherText, (unsigned char *) plainText.c_str(), plainText.size());
    return cipherText;
}

// 特别注意，需要考虑解码后的string内部包含取值为0的字符
string Base64_Decode(const string& cipherText)
{
    unsigned char* buf = new unsigned char[cipherText.size()];
    memset(buf, 0, cipherText.size());
    int size = 0;
    Base64_Decode(cipherText, buf, size);

    string plainText;
    plainText.resize(size);
    memcpy(&plainText[0], buf, size);
    delete buf;
    buf = NULL;
    return plainText;
}

void Base64_Encode(string& outStr, const unsigned char* inData,int inSize)
{
    //编码表
    //编码的时候a[0]=A
    const char EncodeTable[]="ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

    unsigned char a1;
    unsigned char a2;
    unsigned char a3;

    int groupSize = inSize/3;

    for(int i=0; i<groupSize; i++)
    {
        a1 = *inData++;
        a2 = *inData++;
        a3 = *inData++;
        outStr+= EncodeTable[a1 >> 2];
        outStr+= EncodeTable[((a1 << 4) | (a2>> 4)) & 0x3F];
        outStr+= EncodeTable[((a2 << 2) | (a3 >> 6)) & 0x3F];
        outStr+= EncodeTable[a3 & 0x3F];
    }
    //对剩余数据进行编码
    int mod=inSize % 3;
    if(mod==1)
    {
        a1 = *inData++;
        outStr+= EncodeTable[(a1 & 0xFC) >> 2];
        outStr+= EncodeTable[((a1 & 0x03) << 4)];
        outStr+= "==";
    }
    else if(mod==2)
    {
        a1 = *inData++;
        a2 = *inData++;
        outStr+= EncodeTable[(a1 & 0xFC) >> 2];
        outStr+= EncodeTable[((a1 & 0x03) << 4) | ((a2 & 0xF0) >> 4)];
        outStr+= EncodeTable[((a2 & 0x0F) << 2)];
        outStr+= "=";
    }
}

void Base64_Decode(const string& inStr, unsigned char*& outData,int& outSize)
{
    unsigned char* inData = (unsigned char*)(inStr.c_str());
    int inSize = inStr.size();
    
    outSize = 0;
    //解码表
    //解码的时候a[A]=0，也就是a[65]=0
    const char DecodeTable[] =
    {
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        62, // '+'
        0, 0, 0,
        63, // '/'
        52, 53, 54, 55, 56, 57, 58, 59, 60, 61, // '0'-'9'
        0, 0, 0, 0, 0, 0, 0,
        0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
        13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, // 'A'-'Z'
        0, 0, 0, 0, 0, 0,
        26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38,
        39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, // 'a'-'z'
    };

    int nValue;
    int i= 0;
    while (i < inSize)
    {
        if (*inData != '\r' && *inData!='\n')
        {
            nValue = DecodeTable[*inData++] << 18;
            nValue += DecodeTable[*inData++] << 12;
            outData[outSize++]=(nValue & 0x00FF0000) >> 16;

            if (*inData != '=')
            {
                nValue += DecodeTable[*inData++] << 6;
                outData[outSize++]=(nValue & 0x0000FF00) >> 8;

                if (*inData != '=')
                {
                    nValue += DecodeTable[*inData++];
                    outData[outSize++]=nValue & 0x000000FF;
                }
                else
                {
                    //printf("Meet 4Th =");
                }
            }
            else
            {
                //printf("Meet 3Th =");
            }
            i += 4;
        }
        else// 回车换行,跳过
        {
            inData++;
            i++;
        }
    }
}

/*
    算法找出增删改的记录，考虑老的集合为a,b,c，新的集合为b,c,d,得出的结果是：删除a,修改b,c,增加d 
*/
void NiuAlgorithm::GetOptVec(vector<Student>& oldVec,vector<Student>& newVec,vector<Student>& optVec)
{
    // 遍历oldVec，newVec找不到是删除，找到了是修改
    for(vector<Student>::iterator iter = oldVec.begin();iter != oldVec.end();++iter)
    {
        vector<Student>::iterator itq = find(newVec.begin(),newVec.end(),*iter);

        if(itq == newVec.end())
        {
            Student st = *iter;
            st._Opt = Opt_Del;
            optVec.push_back(st);
        }
        else
        {
            Student st = *itq;
            st._Opt = Opt_Mod;
            optVec.push_back(st);
        }
    }

    // 遍历newVec，oldVec找不到是增加
    for(vector<Student>::iterator iter = newVec.begin();iter != newVec.end();++iter)
    {
        vector<Student>::iterator itq = find(oldVec.begin(),oldVec.end(),*iter);

        if(itq == oldVec.end())
        {
            Student st = *iter;
            st._Opt = Opt_Add;
            optVec.push_back(st);
        }
    }
}

参见