KMP的使用——我的总结

/*
    descr:    求KMP的next数组
    param:    p        模式串指针
    param:    nLeng    模式串有效字符长度
    param:    next    存放next值的数组，长度为nLeng+1
*/
void initNextArray(char* p, int nLeng, int* next) 
{    
    next[0] = nLeng;
    
    do {

        if (1== nLeng)
        {
            next[1] = 0;
            break;
        }

        if (2 == nLeng)
        {
            next[1] = 0;
            next[2] = 1;
            break;
                    
        }

        int k = 0;

        next[1] = 0;
        next[2] = 1;

        k = next[3 - 1];

        for (int i = 3; i <= nLeng; )
        {

            if ( p[i-1] ==p[k])
            {
                ++k;                //说明前k个字符与当前i前面的k个字符完全匹配    
                next[i] = k;        //说明当前i前面有k个字符是与最前面的k个字符是相同的
                ++i;                //继续往后比较
            }
            else if (1 == k)
            {
                k = 1;                //已经比较到第一个字符了，故k=1
                next[i] = k;
                ++i;
            }
            else
            {
                k = next[k];
                //此时i不需要动，继续往前找出现k个相同字符的位置
            }

        }


    } while (0);

    for (int i = 1; i <= nLeng; i++)
    {
        std::cout << next[i] << " ";
    }

    std::cout << endl;
    
}

int KMP_Search(char* pMain, char* pSub)
{


    int nIndex = 0;

    int nMainLen = strlen(pMain);

    int nLen = strlen(pSub);


    char* pMain_bk = new char[nMainLen + 2];
    memset(pMain_bk, 0, nMainLen + 2);

    pMain_bk[0] = ' ';
    memcpy(&pMain_bk[1], pMain, nMainLen);


    char* pSub_bk = new char[nLen + 2];
    memset(pSub_bk, 0, nLen + 2);

    pSub_bk[0] = ' ';
    memcpy(&pSub_bk[1], pSub, nLen);
            
    int * next = new int[nLen+1];
    memset(next, 0, nLen+1);

    initNextArray(pSub_bk, nLen, next);

    int i = 1, k = 1;  //从下标1开始匹配
    //下标从1开始
    for (; i <= nMainLen && k<= nLen; )
    {

        if (0 == k)
        {
            //当前字符不匹配，且k=0，说明已经比较到第一个字符了，
            //则需要重新从第一个字符跟当前主串的后一个字符进行比较
            k = 1;
            ++i;
        }
        else if (pMain_bk[i] == pSub_bk[k])
        {
            ++k;
            ++i;        //当前字符匹配，则继续
        }
        else
        {
            k = next[k];    //退回合适的位置，i不变
        }
    }

    if (k > nLen)
    {
        nIndex = i - nLen;
    }

    return nIndex;

}

void    test_KMP()
{
    char szMain[] = "12345Hello789";
    char szSub[] = "Hello";

    int nIndex = KMP_Search(szMain, szSub);

    std::cout << "1, nIndex=" << nIndex << endl;

    char szMain2[] = "12345Hello789";
    char szSub2[] = "456";

    nIndex = KMP_Search(szMain2, szSub2);

    std::cout << "2, nIndex=" << nIndex << endl;

    char szMain3[] = "ababaabca";
    char szSub3[] = "abc";

    nIndex = KMP_Search(szMain3, szSub3);

    std::cout << "3, nIndex=" << nIndex << endl;


}

int main(void)
{
    test_getNext();

    getchar();
}