字符串匹配之KMP算法

阮一峰的博客

#include<stdio.h>
#include<string.h>
#include<stdlib.h>

void computeLPSArray(char *pat, int M, int *lps);

void KMPSearch(char *pat, char *txt)
{
	int M = strlen(pat);
	int N = strlen(txt);

	// create lps[] that will hold the longest prefix suffix values for pattern
	int *lps = (int *)malloc(sizeof(int)*M);
	int j  = 0;  // index for pat[]

	// Preprocess the pattern (calculate lps[] array)
	computeLPSArray(pat, M, lps);

	int i = 0;  // index for txt[]
	while(i < N)
	{
		if(pat[j] == txt[i])
		{
			j++;
			i++;
		}

		if (j == M)
		{
			printf("Found pattern at index %d \n", i-j);
			j = lps[j-1];
		}

		// mismatch after j matches
		else if(pat[j] != txt[i])
		{
			// Do not match lps[0..lps[j-1]] characters,
			// they will match anyway
			if(j != 0)
				j = lps[j-1];
			else
				i = i+1;
		}
	}
	free(lps); // to avoid memory leak
}

void computeLPSArray(char *pat, int M, int *lps)
{
	int len = 0;  // lenght of the previous longest prefix suffix
	int i;

	lps[0] = 0; // 一个字符的前缀和后缀都为空集，共有元素的长度为0
	i = 1;

	// the loop calculates lps[i] for i = 1 to M-1
	while(i < M)
	{
		if(pat[i] == pat[len])
		{
			len++;
			lps[i] = len;
			i++;
		}
		else // (pat[i] != pat[len])
		{
			if( len != 0 )
			{
				// This is tricky. Consider the example AAACAAAA and i = 7.
				len = lps[len-1];

				// Also, note that we do not increment i here
			}
			else // if (len == 0)
			{
				lps[i] = 0;
				i++;
			}
		}
	}
}

int main()
{
	char *txt = "BBC ABCDAB ABCDABCDABDE";
	char *pat = "ABCDABD";
	KMPSearch(pat, txt);

	getchar();
	return 0;
}