String 匹配算法（2）第32章 分类： 算法导论 20110303 10:55 163人阅读 评论(0) 收藏

3.KMP算法（Knuth-Morris-Pratt ）

该算法的与Rabin-Karp一脉相承。也就是先排除可能不是的，在可能中逐一比较。

I.π函数的得到：

COMPUTE-PREFIX-FUNCTION(P)
 1 m ← length[P]
 2 π[1] ← 0
 3 k ← 0
 4 for q ← 2 to m
 5      do while k > 0 and P[k + 1] ≠ P[q]
 6             do k ← π[k]
 7         if P[k + 1] = P[q]
 8            then k ← k + 1
 9         π[q] ← k
10 return π

II.伪代码算法:



KMP-MATCHER(T, P)
 1 n ← length[T]
 2 m ← length[P]
 3 π ← COMPUTE-PREFIX-FUNCTION(P)
 4 q ← 0                          ▹Number of characters matched.
 5 for i ← 1 to n                 ▹Scan the text from left to right.
 6      do while q > 0 and P[q + 1] ≠ T[i]
 7             do q ← π[q]    ▹Next character does not match.
 8         if P[q + 1] = T[i]
 9            then q ← q + 1      ▹Next character matches.
10         if q = m                    ▹Is all of P matched?
11            then print "Pattern occurs with shift" i - m
12                 q ← π[q]    ▹Look for the next match.

C语言算法实现：

void COMPUTE_PREFIX_FUNCTION(char *P, char *PI)
{
    /*---------------------------------------------------------------*/
    /* Local Variables                                               */
    /*---------------------------------------------------------------*/
        int m = 0;
	int k =0,q =0;
	DWORD begin,end;
    /*----------------------------------------------------------------*/
    /* Code Body                                                      */
    /*----------------------------------------------------------------*/   
	begin = GetTickCount();
	assert(P);
	assert(PI);
	
	m = strlen(P);
	PI[0] = 0;
	for(q=1;q<m;q++)
	{
		while((k>0 )&& (P[k]!=P[q]))
		{
			k = PI[k];
		}
		if(P[k]==P[q])
		{
			k = k+1;		
		}
		PI[q] = k; 
	}
	end = GetTickCount();
	printf("the time is %d ms/n",end-begin);
}

/**********************************************************************
*[in]	T string
*[in]	P key
*
***********************************************************************/


void mmi_KMP_string_match(char* T, char* P)
{
    /*----------------------------------------------------------------*/
    /* Local Variables                                                */
    /*-------------------------------------------------------------*/
	int m,n = 0;
	int k =0,q,i =0;
	char PI[20] = {0};
	DWORD begin,end;
    /*----------------------------------------------------------------*/
    /* Code Body                                                      */
   /*----------------------------------------------------------------*/    
	begin = GetTickCount();
	n = strlen(T);
	m = strlen(P);
	COMPUTE_PREFIX_FUNCTION(P,PI);
	q =0;
	
	for(i =0;i<n;i++)
	{
		while((q>0)&&(P[q]!=T[i]))
		{
			q = PI[q];
		}
		if(P[q] ==T[i])
		{
			q =q+1;
		}
		if(q==m)
		{
			printf("Pattern occurs here, %d!/n",i-m+1);
			q = PI[q];
		}
		
	}
	end = GetTickCount();
	DWORD count = end - begin;
	printf("the time is %d ms in KMP/n",begin);
	printf("the time is %d ms in KMP/n",end);
	printf("the time is %d ms in KMP/n",count);
}

 

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