模式匹配KMP

字符串朴素模式匹配算法的2种实现：

//1.朴素的模式匹配算法，用while实现

int StrStr_While(const char* pStr, const char* pSub, int* pos)
{
  int nRet = 0;
  int pStrLen = strlen(pStr);
  int pSubLen = strlen(pSub);
  int i = 0, j = 0;
  int nLen = pStrLen - pSubLen;

  if (pStr == NULL || pSub == NULL)
  {
    nRet = -1;
    printf("param input error!");
    return nRet;
  }

  while (i < pStrLen && j < pSubLen)
  {
    if (*(pStr + i) == *(pSub + j))
    {
      ++i;
      ++j;
    }
    else
    {
      i = i - j + 1;
      j = 0;
    }
  }

  if (j == pSubLen)
    *pos = i - j + 1;

  return nRet;
}

 

//2.朴素的模式匹配，用for循环实现

int StrStr_For(const char* pStr, const char* pSub, int* pos)
{
  int nRet = 0;
  int pStrLen = strlen(pStr);
  int pSubLen = strlen(pSub);
  int i = 0, j = 0;
  int nLen = pStrLen - pSubLen;

  if (pStr == NULL || pSub == NULL)
  {
    nRet = -1;
    printf("param input error!");
    return nRet;
  }

  for (i = 0; i <= nLen; ++i)
  {
    for (j = 0; j < pSubLen; ++j)
    {
      if (*(pStr + i + j) != *(pSub + j))
        break;
    }

    if (j == pSubLen)
    {
      *pos = i + 1;
      break;
    }
  }

  return nRet;
}

 

实现字符串的匹配有高效的算法。那就是KMP算法，实现例如以下：

//取得KMP算法须要用的的next数组

int GetNext(const char* pStr, int nNext[])
{
  int nRet = 0;
  if (pStr == NULL || nNext == NULL)
  {
    nRet = -1;
    printf("param input error!\n");
    return nRet;
  }

  int nLen = strlen(pStr);
  int i = 0, j = -1;
  nNext[i] = j;
  while (i < nLen - 1)
  {
    if (j == -1 || pStr[i] == pStr[j])
    {
      ++i;
      ++j;
      nNext[i] = j;
    }
    else
    {
      j = nNext[j];
    }
  }

  return nRet;
}



//取next数组的改进型算法！剔除多个反复字符的next数组值持续增长。

int GetNextVal(const char* pStr, int nNext[])

{

  int nRet = 0;

  if (pStr == NULL || nNext == NULL)

  {

    nRet = -1;

    printf("param input error!\n");

    return nRet;

  }



  int nLen = strlen(pStr);

  int i = 0, j = -1;

  nNext[i] = j;

  while (i < nLen - 1)

  {

    if (j == -1 || pStr[i] == pStr[j])

    {

      ++i;

      ++j;

      if (pStr[i] != pStr[j])

        nNext[i] = j;

      else

        nNext[i] = nNext[j];

    }

    else

    {

      j = nNext[j];

    }

  }



  return nRet;

}

 

这是针对类似aaaaaaab这种字符串使用上面两个函数取得的next数组值得比較。注意书上的都是字符串从数组的下标为1的位置開始存储的。我改进的算法是还是让字符串从数组下标为0的位置開始存储。

所以上面的next数组值都要相较与原来的值减1。

 

//调用上面的函数实现KMP高效字符串模式匹配算法！

int Index_KMP(const char* pStr, const char* pSub, int* nPos)
{
  int nRet = 0;
  if (pStr == NULL || pSub == NULL || nPos == NULL)
  {
    nRet = -1;
    printf("param input error!\n");
    return nRet;
  }

  int pStrlen = strlen(pStr);
  int pSublen = strlen(pSub);

  int nNext[255] = { 0 };
  //nRet = GetNext(pSub,nNext);
  nRet = GetNextVal(pSub, nNext);
  if (nRet != 0)
  {
    printf("call GetNext() func is error!\n");
    return nRet;
  }

  int i = 0, j = 0;
  while (i < pStrlen && j < pSublen)
  {
    if (j == -1 || *(pStr + i) == *(pSub + j))
    {
      ++i;
      ++j;
    }
    else
    {
      j = nNext[j];
    }
  }

  if (j == pSublen)
    *nPos = i - j + 1;

  return nRet;
}