串的模式匹配算法 ------ KMP算法

//KMP串的模式匹配算法
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int* get_next(char t[], int length)
{
    int i = 0, j = -1;
    int* next = (int *)malloc(length * sizeof(int));
    next[0] = -1;
    while (i < length)
    {
        if (j == -1 || t[i] == t[j])
        {
            i++;
            j++;
            next[i] = j;
        }
        else
        {
            j = next[j];
        }
    }

    return next;
}

int  Index_KMP(char s[], char t[], int sl, int tl, int next[])
{
    int j, i;
    j = 0;
    i = 0;
    while (j < tl && i < sl)
    {
        if (j == -1 || s[i] == t[j])
        {
            i++;
            j++;
        }
        else
        {
            j = next[j];
        }
    }
    if (j == tl)
    {
        return i - tl;
    }
    else
    {
        return 0;
    }
}


//计算next函数修正值的算法
int *get_nextval(char t[], int length)
{
    int i = 0, j = -1;
    int* nextval = (int*)malloc(length*sizeof(int));
    nextval[0] = -1;
    while (i < length)
    {
        if (j == -1 || t[i]==t[j])
        {
            j++;
            i++;
            if (t[j]!=t[i])
            {
                nextval[i] = j; 
            }
            else
            {
                nextval[i] = nextval[j]; //回溯到下标为nextval[j]但元素与下标为nextval[i]的元素不相同的位置
            }
        }
        else
        {
            j = nextval[j];
        }
    }

    return nextval;    
}


int main()
{

    char s[] = "acabaabaabcacaabc";
    char t[] = "abaabcac";
    char t2[] = "abcdabd";
    int tl = strlen(t);
    int sl = strlen(s);
    /*int *next = get_next(t, tl);
    int c = Index_KMP(s, t, sl, tl, next); //返回开始正确匹配的下标（s的下标）
    printf("%d
", c);*/
    int l = strlen(t2);
    int * nextval = get_nextval(t,tl);
    /*for(int i = 0; i<l;i++)
    {
        printf("%d ",nextval[i]);
    }*/
    int c = Index_KMP(s, t, sl, tl, nextval);
    printf("%d
",c);

    printf("
");
    return 0;
}

理解：

模式匹配就是将主串中下标为i的元素与模式串中下标为j的元素进行比较（比较过程中i不会回溯 而j的值会按照next对应的值进行回溯）