Improve hash function to reduce the number of comparisons in strstr

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C

hash

This implementation of strstr uses hashing, the choice of hash function below is obviously not perfect, that's why I want to improve it; the full program is the following:

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

const char *
strstr(const char *str1, const char *str2)
{
    unsigned hash1 = 0, hash2 = 0;
    unsigned i, j, str2_len;

    for (i = 0; str2[i] != '\0'; i++)
    {
        hash2 += (unsigned) str2[i];
        if (str1[i] == '\0')
            return NULL;
        hash1 += (unsigned) str1[i];
    }

    str2_len = i;

    for (i=0; str1[i] != '\0'; i++)
    {
        if (hash1 == hash2)
        {
            for (j=0; j < str2_len; j++)
                if (str1[i+j] != str2[j])
                    break;
            if (j == str2_len)
                return str1+i;
        }
        if (str1[i+str2_len] == '\0')
            return NULL;
        hash1 = (hash1 - ((unsigned)str1[i])) + ((unsigned)str1[i+str2_len]);
    }
}

int
main(int argc, char **argv)
{
    const char *match;

    char buffer[] = "abc abcdef";
    char sub[] = "abcdef";

    match = strstr(buffer, sub);

    if (match)
        printf("%s\n", match);
    else
        printf("NOT FOUND\n");

    return 0;
}

A better hash function would improve the run time of this algorithm (reduce the number of times str2 is compared to a substring of str1).

Any suggestions that contributes to the improvement the algorithm, It will be well received

Thanks.