串匹配算法之BM算法

参考资料：

http://blog.csdn.net/eric491179912/article/details/6210009

 

http://blog.163.com/pengfeicui@yeah/blog/static/106403250201092681158650/

 

 

这里，我们仅仅用了坏字符规则

定义一个滑动距离函数：

dist（C）=  1）  m – j                          j =  max{j| tj = c && 1<= j <= m - 1}

                  2)    m                              若C不出现在模式中或tm = c

比如 T = pattern    dist（a）= 7 – 2  ；  dist（t） = 7 – 4 （算最后出现的）  ； dist（e） =  7 – 5 ； dist（r） = 7 - 6；

        最后一个dist（n） =  7 （m ，模式串T的总长度） ，其他的未出现的字符， dist（ch） =  7；

程序如下：

   1:  void  BMDist(string str,int dist[]) 

   2:  {

   3:      int i;

   4:      for (i = 0; i < 256; i++)

   5:      {

   6:          dist[i] = str.length();

   7:      }

   8:      for (i = 0; i < str.length() - 1; i++)//最后一个字符取不到

   9:      {

  10:          dist[str[i]] = str.length() - i - 1;

  11:      }

  12:      dist[str[str.length()-1]] = str.length();

  13:   

  14:  }

  15:   

  16:  int  BM(string s, string t,int dist[]) 

  17:  {

  18:       int i = t.length();

  19:       while(i <= s.length())

  20:       {

  21:        int j = t.length();

  22:        while(j>0 && s[i-1]==t[j-1])

  23:        {

  24:            j--;

  25:            i--;

  26:        }

  27:        if (j == 0)

  28:        {

  29:            return i + 1;

  30:        }

  31:        else

  32:        { 

  33:            i = i + dist[s[i-1]];

  34:        }

  35:       }

  36:       return -1;

  37:  }

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测试代码：

   1:  int main()

   2:   

   3:  {

   4:      

   5:      while(1)

   6:  {

   7:      clock_t start,stop;

   8:      string Str,Tsr;

   9:      

  10:      int flag1,flag2,flag3;

  11:      int dist[256];

  12:   

  13:      int next[1000]={0,};

  14:      cout <<"请输入S串与T串：" <<endl;

  15:      cin >> Str >> Tsr;

  16:      cout << endl;

  17:   

  18:      start = clock();

  19:      for (int i=0; i< 1000; i++)

  20:      {

  21:         flag1 = BF(Str,Tsr);

  22:      }

  23:      stop = clock();

  24:      cout << "BF算法的执行时间是："<< stop - start << endl;

  25:   

  26:     

  27:      start = clock();

  28:      for (int i=0; i< 1000; i++)

  29:      {

  30:          kmpNext(Tsr,next);

  31:          flag2 =  kmp(Str,Tsr,next);

  32:      }

  33:      stop = clock();

  34:      cout << "KMP算法的执行时间是："<< stop - start << endl;

  35:   

  36:      

  37:      start = clock();

  38:      for (int i=0; i< 1000; i++)

  39:      {

  40:          BMDist(Tsr,dist);

  41:          flag3 = BM(Str,Tsr,dist);

  42:      }

  43:      stop = clock();

  44:      cout << "BM算法的执行时间是："<< stop - start << endl;

  45:   

  46:      if (flag1 == -1)

  47:      {

  48:          cout << "没有找到子串"<<endl;

  49:      }

  50:      else

  51:      {

  52:          cout << "找到子串的位置为"<< flag1 <<endl;

  53:      }

  54:      if (flag2 == -1)

  55:      {

  56:          cout << "没有找到子串"<<endl;

  57:      }

  58:      else

  59:      {

  60:          cout << "找到子串的位置为"<< flag2 <<endl;

  61:      }

  62:      if (flag3 == -1)

  63:      {

  64:          cout << "没有找到子串"<<endl;

  65:      }

  66:      else

  67:      {

  68:          cout << "找到子串的位置为"<< flag3 <<endl;

  69:      }

  70:  }

  71:   

  72:      return 0;

  73:  }

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