查找字符串的 KMP 算法

查找字符串是我们平常编程过程中经常遇到的，现在介绍一种查找字符串算法，增加程序的执行速度。

通常我们是这么写的：

/*
  content: search a string in a othor string
  author:  lw
  date:    2015-01-30
  target: kmp algorithm
                      */

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

void compare(char * sourcestr, char * targetstr)
{
    char *moveSource, *moveTarget, *headPoint;
    int num = 0;
    headPoint = sourcestr;
    moveSource = sourcestr;
    moveTarget = targetstr;
    while(*headPoint != *moveTarget)
            headPoint++;
    moveSource = headPoint;
    while(*moveSource != ‘\0‘ && *moveTarget != ‘\0‘)
    {
        num++;
        moveSource++;
        moveTarget++;
        if(num == 3)
            break;
        while(*moveSource != *moveTarget)//-->kmp
        {
            headPoint = headPoint + 1;
            moveSource = headPoint;
            num = 0;
            moveTarget = targetstr;
        }
    }
    printf("%s\n", headPoint);
}

int main()
{
    char *source = "mabveacabiabcy";
    char *target = "abc";
    char *sourcestr;
    char *targetstr;
    sourcestr = (char *)malloc(15 * sizeof(char));
    targetstr = (char *)malloc(4 * sizeof(char));
    memset(sourcestr, ‘\0‘, 15 * sizeof(char));
    memset(targetstr, ‘\0‘, 4 * sizeof(char));
    strncpy(sourcestr, source, 15);
    strncpy(targetstr, target, 4);
    compare(sourcestr, targetstr);
    return 0;
}

现在把函数  compare()  函数中的内 while() 中的内容改进一下：

说明：拿字符串 mabveacabiabcy 来说，当查找到字符 v 时发现和 abc 中的 c 不同，则指向字符串 mabveacabiabcy 中的第二个字符的指针就要移动，如果不使用 kmp 算法，那么指针移动一位，如果使用 kmp 算法，则指针移动两位，因为当比较到字符 v 时我们实际已经知道 v 以前的字符是什么了，所以可以断定不止要移动一位，具体移动几位就和字符串 abc 有关了，要判断其是否回文字符串，此例中 abc 对应数组 1,2,0 。

修改后的代码如下：

/*
  content: search a string in a othor string
  author:  lw
  date:    2015-01-30
  target: kmp algorithm
                      */

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

void compare(char * sourcestr, char * targetstr)
{
    char *moveSource, *moveTarget, *headPoint;
    int num = 0;
    headPoint = sourcestr;
    moveSource = sourcestr;
    moveTarget = targetstr;
    while(*headPoint != *moveTarget)
            headPoint++;
    moveSource = headPoint;
    while(*moveSource != ‘\0‘ && *moveTarget != ‘\0‘)
    {
        num++;
        moveSource++;
        moveTarget++;
        if(num == 3)
            break;
        while(*moveSource != *moveTarget)//-->kmp
        {
            if(num > 0)
                headPoint = headPoint + num;
            else
                headPoint = headPoint + 1;
            moveSource = headPoint;
            num = 0;
            moveTarget = targetstr;
        }
    }
    printf("%s\n", headPoint);
}

int main()
{
    char *source = "mabveacabiabcy";
    char *target = "abc";
    char *sourcestr;
    char *targetstr;
    sourcestr = (char *)malloc(15 * sizeof(char));
    targetstr = (char *)malloc(4 * sizeof(char));
    memset(sourcestr, ‘\0‘, 15 * sizeof(char));
    memset(targetstr, ‘\0‘, 4 * sizeof(char));
    strncpy(sourcestr, source, 15);
    strncpy(targetstr, target, 4);
    compare(sourcestr, targetstr);
    return 0;
}