Two functions to convert a base 10 NSNumber to a base 36 NSString and the other way around, using C

base36_c.m

/* Arno Richter, May 2014 */

- (NSString *)toBase10:(NSNumber *)fromValue withBase:(int)base {
    char base_digits[] = "0123456789abcdefghijklmnopqrstuvwxyz";
    
    unsigned long int number_to_convert = [fromValue unsignedLongValue];
    int converted_number[64];
    int index = 0;
    char return_string[64] = "";
    
    while(number_to_convert != 0) {
        converted_number[index] = number_to_convert % base;
        number_to_convert = number_to_convert / base;
        ++index;
    }

    --index;
    for( ; index>=0; index--) {
        int len = strlen(return_string);
        return_string[len] = base_digits[converted_number[index]];
        return_string[len+1] = '\0';
    }
    
    return [NSString stringWithFormat:@"%s", return_string];
}

- (NSNumber *)fromBase10:(NSString *)fromValue withBase:(int)base {
    const char *rawinput = [fromValue UTF8String];
    size_t len = strlen(rawinput) + 1;
    
    char mac [len];
    memcpy(mac, rawinput, len);
    long output = strtol(mac,NULL,base);
    
    return [NSNumber numberWithLong:output];
}

Usage:
NSLog(@"output base 10 to 36: %@", [ClassName toBase10:@42891 withBase:36]);
NSLog(@"output base 36 to 10: %@", [ClassName fromBase10:@"x3f" withBase:36]);