KMP模式匹配

好文推荐：The Knuth-Morris-Pratt Algorithm
注意：此处有一个坑：C++中，string.length()返回的是无符号数，有符号跟无符号数一起运算时全部当成无符号数处理，负数最高位是符号位，是1，于是出现负数比正数大的情况，从而跳出循环，因此，我们计算字符串长度时需强制转换成int，统一变成有符号数进行处理。

//kmp算法改进  PrintfData(nextval, (int)pattern.length(),1);
void GetNextval(string pattern,int nextval[]){
    int i = 1;
    int j = 0;
    nextval[1] = 0;
    while (i < (int)pattern.length()) {
        if (j == 0 || pattern[i] == pattern[j]) {
            i++;
            j++;
            if (pattern[i] == pattern[j]) {
                nextval[i] = nextval[j];
            }else{
                nextval[i] = j;
            }
        }else{
            j = nextval[j];
        }
    }
    //PrintfData(nextval, (int)pattern.length(),1,"nextval ");
}

//计算next数组    PrintfData(next, (int)pattern.length(),1);
void GetNext(string pattern,int next[]){
    int i = 1;
    int j = 0;
    next[1] = 0;
    while (i < (int)pattern.length()) {
        if (j == 0 || pattern[i] == pattern[j]) {
            i++;
            j++;
            next[i] = j;
        }else{
            j = next[j];
        }
    }
    //PrintfData(next, (int)pattern.length(),1,"next ");
}

//进行模式匹配
int Kmp(string data,string pattern,int next[]){
    int i = 0;
    int j = 0;
    while (i < (int)data.length() && j < (int)pattern.length()) {
        if (j == -1 || data[i] == pattern[j]) {
            i++;
            j++;
        }else{
            j = next[j];
        }
    }
    if (j == (int)pattern.length()) {
        return i - j;
    }else{
        return -1;
    }
}

int main(int argc, const char * argv[]) {

    string data  = "xxyxxxyxxxxyxyx";
    string pattern = "cddcdececdea";

    int patLength = (int)pattern.length();
    int next[patLength];
    
    GetNext(pattern , next);

    int ret = Kmp(data, pattern, next);
    if (ret != -1) {
        cout<<"Success:Index = "<<ret<<endl;
    }else{
        cout<<"fail"<<endl;
    }
    return 0;
}