linux kernel data struct: KMP算法实现

   KMP算法被三位外国人所发现，其实是利用“欲搜索”字符串内部的特征，避免两个字符串的回溯，从而把字符串模式匹配的时间复杂度从O(m*n)降低到O(m+n)。

    这个算法的核心在于理解和计算next数组。具体实现如下：

 

#include <stdio.h>

/**

return where different.

return 0 means equal.

*/

int is_2str_diff(char *s1, char *s2, int count)

{

    int i;

    printf("is 2str diff: %c & %c count: %d\n", *s1, *s2, count);

    for(i = 0; i < count; i++)

    {

        if(*(s1+i) != *(s2+i)) {printf("return :%d\n", i+1); return i+1;}

    }

   

    {printf("return 0\n"); return 0;}

}

int calc_next(char *s, char *next)

{

    int len = strlen(s);

    int i,j;

    int diff_pos = 0;

   

    //how to calculate next

    *(next + 0) = -1;

    *(next + 1) = 0;

   

    for(i = 2; i < len; i++)

    {

          for(j = i/2; j >= 1; j--)

          {

               printf("compare %c and %c, count is:%d\n",*s,*(s+i-j),j);

               diff_pos = is_2str_diff(s, s+i-j, j);

               if(!diff_pos) break;

          }

         

          *(next+i) = j;     

    }

   

    return 0;

}

 

/*

 return -1: not contain,

 else return the position where match.

*/

int kmp_compare(char *s1, char *s2, int s1_len, int s2_len, char *next)

{

    int i=0 , j=0;

    int diff_pos = 0;

    int from_start = 1;

   

    while(i < s1_len)

    {

        printf("i = %d , j = %d\n", i,j);

       

        if(from_start && s1[i] != s2[0])

        {

            i++;

            j = 0;

        }

        else

        {

          if(-1 == *(next+j) || s1[i] == s2[j])

          {

                from_start = 0;

                i++;

                j++;

          }

          else

          {

                from_start = 1;

                j = *(next+j);

          }

        }

          if(j == s2_len) return i-j;

    }

   

    return -1;

}

int main(void)

{

    char *s1 = "abcdefabcabcabcdefeam";

    char *s2 = "abcabc";

   

    int str1_len = strlen(s1);

    int str2_len = strlen(s2);

   

    char next[str2_len];

   

    int  i;

    int res = 0;

   

    calc_next(s2, next);

    printf("%s\n%s\n calc_next get\n", s1, s2);

    for(i = 0; i < str2_len; i++)

    printf(" %d", next[i]);

    printf("\n");

   

    res = kmp_compare(s1, s2, str1_len, str2_len, next);

    printf("result is: %d\n",res);

   

    system("pause");

}