Determine if string contains all unique characters using a bitmap

unqchars.c

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

int is_unique(char* str) {
        /*
         * Bit check array for 256 values
         * Rationale:  256 / 32 = 8. 32 for 32 bits in an integer
         */ 
        int check[8] = {0};
        int i, val, idx1, idx2;
        int tmp;

        for (i = 0; i < strlen(str); i++) {
                /*
                 * To determine which bit is related to the val,
                 * we first divide the value by 8 to locate which "integer"                 * in the array of 8 we are dealing with
                 */
                val = (int)str[i];
                idx1 = val / 32;

                /*
                 * Once the first level map is located, now we need to 
                 * determine which index in that integer corresponds to
                 * the value we are dealing with.  We do by using the 
                 * modulo operator: divide and get remainder, in this case
                 * divide by 32 since we have 32 bits in an integer
                 */
                idx2 = val % 32;
                tmp = check[idx1];

                if (tmp & (1 << idx2)) {
                        /* If we found a match, return immediately */
                        return 0;
                } else {
                        /*
                         * If we didn't find a match, preare a fresh
                         * integer with the one bit set to where we need it.
                         * Then use the 'or' operator to set it in the map
                         */
                        tmp |= (1 << idx2);
                        check[idx1] = tmp;
                }       
        }

        return 1;
}

int main() {
        char *str1 = "abcdefgh";
        printf(">> str1: %d\n", is_unique(str1));
        char *str2 = "ello";
        printf(">> str2: %d\n", is_unique(str2));

        return 0;
}