摩尔机在 C++ 中的实现

什么是摩尔机

摩尔机是一种有限状态自动机，用于解析一些复杂的字符串。它通过维护一个状态机来分析输入的字符串，然后根据特定的模式来执行相应的操作。

摩尔机的实现

在 C++ 中，可以使用结构体来实现摩尔机。以下是一个简单的结构体定义，用于实现一个查找一个字符串中是否出现特定子串的摩尔机：

struct MooreMachine {
   int states[100][128];
   int failure[100];
   int output[100];
   int num_states;
   int num_outputs;

   void build_machine(const std::vector<std::string>& keywords) {
      std::memset(states, 0, sizeof(states));
      std::memset(failure, 0, sizeof(failure));
      std::memset(output, 0, sizeof(output));
      num_states = 1;
      num_outputs = 0;

      for (const auto& keyword : keywords) {
         int current_state = 0;
         for (const auto& c : keyword) {
            if (states[current_state][c] == 0) {
               states[current_state][c] = num_states++;
            }
            current_state = states[current_state][c];
         }
         output[current_state] = ++num_outputs;
      }

      std::queue<int> q;
      for (int i = 0; i < 128; ++i) {
         if (states[0][i] != 0) {
            q.push(states[0][i]);
         }
      }

      while (!q.empty()) {
         int state = q.front();
         q.pop();
         for (int i = 0; i < 128; ++i) {
            int next_state = states[state][i];
            if (next_state != 0) {
               q.push(next_state);
               int failure_state = failure[state];
               while (states[failure_state][i] == 0) {
                  failure_state = failure[failure_state];
               }
               failure[next_state] = states[failure_state][i];
               output[next_state] |= output[failure[next_state]];
            }
         }
      }
   }

   std::vector<int> search(const std::string& text) {
      std::vector<int> results(num_outputs, 0);
      int current_state = 0;
      for (size_t i = 0; i < text.size(); ++i) {
         while (states[current_state][text[i]] == 0) {
            current_state = failure[current_state];
         }
         current_state = states[current_state][text[i]];
         if (output[current_state] != 0) {
            results[output[current_state] - 1] = 1;
         }
      }
      return results;
   }
};

摩尔机的使用

我们可以使用以下代码来使用上面实现的摩尔机：

int main() {
   std::vector<std::string> keywords = {"he", "she", "his", "hers"};
   MooreMachine mm;
   mm.build_machine(keywords);
   std::string text = "ushershershehershisshehissherhissherhe";
   auto results = mm.search(text);
   for (size_t i = 0; i < results.size(); ++i) {
      if (results[i]) {
         std::cout << keywords[i] << " found\n";
      }
   }
   return 0;
}

上面的代码将查找特定子串 "he", "she", "his", "hers" 是否出现在字符串 text 中，并输出找到的子串。