hex conversion: hex string to integer

htoi.c

#include <stdio.h>
#include <ctype.h>

int htoi(char hex_string[]);
int expect(int got, int wanted);
void debuggerino(void);

int htoi(char hex_string[]) {

    /* calculate len first and check for 0x offset */
    int hex_offset = 0;
    int len = 0;
    for (int i = 0; hex_string[i] != '\0'; i++) { 
        if (i == 0 && hex_string[i] == '0')
            hex_offset++;
        if (i == 1 && tolower(hex_string[i]) == 'x')
            hex_offset++;
        len++;
    }

    int value = 0;
    int start = hex_offset == 2 ? hex_offset : 0;
    for (int i = start; i < len; i++) {
        char c = tolower(hex_string[i]);
        //printf("got %c %i\n", c, c);
        if (c >= '0' && c <= '9') {
            //printf("1-9 adding: %i\n", (c - 48) * (1 << (4*(len-1-i))));
            value += (c - 48) * (1 << (4*(len-1-i)));
        } else if (c >= 'a' && c <= 'f') {
            //printf("a-f adding: %i\n", ((c - 'a') + 10) * (1 << (4*(len-1-i))));
            value += ((c - 'a') + 10) * (1 << (4*(len-1-i)));
        } else {
            return -1; /* error */
        }
    }

    return value;
}

int expect(int got, int wanted) {
    if (got != wanted) {
        printf("FAIL: got: %i wanted: %i\n", got, wanted);
        return -1;
    } else {
        printf("PASS\n");
        return 0;
    }
}

void debuggerino() {
    int len = 2;
    int i = 1;
    printf("'f' - 'a' : %i\n", ('f' - 'a'));
    printf("('f' - 'a') + 10 : %i\n", (('f' - 'a') + 10));
    printf("(('f' - 'a') + 10) * (1 << (4*(len-1-i))) : %i\n", 
            (('f' - 'a') + 10) * (1 << (4*(len-1-i))));
}

int main() {
    //debuggerino();
    //return 0;

    expect(htoi("10"), 16);
    expect(htoi("100"), 256);
    expect(htoi("FF"), 255);
    expect(htoi("Fe"), 254);
    expect(htoi("0xFe"), 254);
    expect(htoi("0XFe"), 254);
    expect(htoi("0Xe3"), 227);
    expect(htoi("0x00"), 0);
    expect(htoi("0000000"), 0);
    expect(htoi("100"), 256);
}