KMP算法

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

int next[32] = {-999};

/* 返回模式串T在母串S中第pos个字符的位置 */
/* 调试小技巧 print x = value 或 set var x = value 可以改变gdb运行时变量的值 */
int index_BM(char *S, char *T, int pos)
{
int i;
int j;

i = pos;
j = 0;

while ( (i < strlen(S)) && (j < strlen(T)) )
{
if (S[i] == T[j])
{
i++;
j++;
}
else
{
i = i - j + 1;
j = 0;
}
}

/* 注意strlen(T)意味着j的取值范围为0 ~ (strlen(T) - 1) */
if (strlen(T) == j)
{
return i - strlen(T);
}
else
{
return -1;
}
}

void get_next(char *T, int *next)
{
int k = -1;
int j = 0;

next[j] = k;

while (j < strlen(T))
{
if ( (k == -1) || (T[j] == T[k]) ) //注意等号是==，而不是=
{
++k; // 注意是先加后使用
++j;
next[j] = k;
}
else
{
k = next[k];
}
}
}

int index_KMP(char *S, char *T, int pos)
{
int i;
int j;

i = pos;
j = 0;

while ( (i < strlen(S)) && (j < strlen(T)) )
{
/* j = -1 表示next[0], 说明失配处在模式串T的第0个字符。所以这里特殊处理，然后令i+1和j+1。*/
if ( (j == -1)  || S[i] == T[j])
{
i++;
j++;
}
else
{
j = next[j];
}
}

if (strlen(T) == j)
{
return i - strlen(T);
}
else
{
return -1;
}
}

void print_next(int next[], int n)
{
int i;

for (i = 0; i < n; i++)
{
printf("next[%d] = %d\n", i, next[i]);
}
}

int main()
{
char  s[30] = "ababcabcacbab";
char t[30] = "abgabm";
int pos = 0;
int index;

printf("================ BM ==============\n");
index = index_BM(s, t, pos);
printf("index = %d\n", index);

printf("================ KMP ==============\n");
get_next(t, next);
print_next(next, strlen(t));

index = index_KMP(s, t, pos);
printf("index = %d\n", index);

return 0;
}