strStr.cpp

// Source : https://oj.leetcode.com/problems/implement-strstr/
// Author : Hao Chen
// Date   : 2014-07-19

/********************************************************************************** 
* 
* Implement strStr().
* 
* Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.
* 
*               
**********************************************************************************/

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

char *strStr1(char *haystack, char *needle);
char *strStr2(char *haystack, char *needle);

char *strStr(char*haystack, char *needle) {
    if (random()%2){
        printf("---KMP---\n");
        return strStr1(haystack, needle);
    }
    printf("---brute-force---\n");
    return strStr2(haystack, needle);
}
//KMP
char *strStr1(char *haystack, char *needle) {
    if(!haystack || !needle ) {
        return NULL;
    }
    if (!*needle ) {
        return haystack;
    }

    char *ph = haystack;
    char *pn = needle;
    for( ;*ph && *pn ; ph++, pn++ );

    //len(haystack) < len(needle)
    if (!*ph && *pn){
        return NULL;    
    }

    for(ph=ph-1; *ph; haystack++, ph++) {
        char *q=needle;
        char *p=haystack;
        int n=0;
        while(*q && *p && *p==*q){
            p++; q++;
            if (n==0 && *p == *needle){
                n = p - haystack;
            }
        }
        if (!*q){
            return haystack;
        }
        haystack += (n>0 ? n-1 : n);
    }
    return NULL;
}

//brute-force
char *strStr2(char *haystack, char *needle) {

    if(!haystack || !needle ) {
        return NULL;
    }
    if (!*needle ) {
        return haystack;
    }

    char *ph = haystack;
    char* pn = needle;
    for( ;*ph && *pn ; ph++, pn++ );

    //len(haystack) < len(needle)
    if (!*ph && *pn){
        return NULL;    
    }
    ph--;

    for( ; *ph; haystack++, ph++) {
        char *q=needle;
        char *p=haystack;
        while(*q && *p && *p==*q){
            p++; q++;
        }
        if (!*q){
            return haystack;
        }
    }

    return NULL;
}

int main(int argc, char** argv)
{
    srand(time(0));
    const char* haystack = "mississippi";
    const char* needle = "issi";
    printf("%s, %s : %s\n", haystack, needle, strStr((char*)haystack, (char*)needle));

    haystack = "mississippi";
    needle = "issip";
    printf("%s, %s : %s\n", haystack, needle, strStr((char*)haystack, (char*)needle));


    haystack = "babbbbbabb";
    needle = "bbab";
    printf("%s, %s : %s\n", haystack, needle, strStr1((char*)haystack, (char*)needle));

    if (argc>2){
        haystack = argv[1];
        needle = argv[2];
        printf("%s, %s : %s\n", haystack, needle, strStr((char*)haystack, (char*)needle));
    }
    
    return 0;
}