KMP算法c语言实现

/**
* author:lingd
* date:2013-12-5
*/

#include<stdio.h>
#include<string.h>
#define String char*


void get_next(String ,int* );
int str_pos(String ,String  ,int );
void get_nextval(String T,int* next);

int main(void){
    int next[100],len,i;
    String s = "ababababdcabababababdc";
    String t = "abababc";
    printf("%d",str_pos(s,t,0));
    return 0;
}

void get_next(String T,int* next){
    int j,i,lenT;
    j = -1;
    i = 0;
    lenT = strlen(T);
    next[0] = -1;
    while(i < lenT){
        if(j == -1 || T[j] == T[i]){  //i是后缀  j是前缀
            i++;
            j++;
            next[i] = j;
        }else{
            j = next[j];
        }
    }
}

//改进
void get_nextval(String T,int* next){
    int j,i,lenT;
    j = -1;
    i = 0;
    lenT = strlen(T);
    next[0] = -1;
    while(i < lenT){
        if(j == -1 || T[j] == T[i]){ //i是后缀 j是前缀
            i++;
            j++;
            if(T[j] != T[i]){
                next[i] = j;
            }else{
                next[i] = next[j];
            }
        }else{
            j = next[j];
        }
    }
}

/**
    S主串
    T子串
    pos  开始匹配的子串起始位置
    return 位置  or  -1（无匹配）
*/
int str_pos(String S,String T ,int pos){
    int lenS,lenT,i,j;
    int next[255];
    lenS = strlen(S);
    lenT = strlen(T);
    i = pos;
    j = 0;
    //get_next(T,next);  //改进前
    get_nextval(T,next); //改进后
    while(i < lenS && j < lenT){
        if(j == -1 || T[j] == S[i]){
            i++;
            j++;
        }else{
            j = next[j]; //回朔j
        }
    }
    
    //有匹配
    if(j >= lenT){
        return i - lenT;
    }

    return -1; //没有匹配
}

/*
 0 1 2 3 4 5 6
 a b a b a b c
-1 0 0 1 2 3 4    //改进前的next
-1 0-1 0-1 0 4      //改进后的next
 */