Making sure I get how base-64 encoding works.

main.m

int main(int argc, const char * argv[]) {
    
    @autoreleasepool {
        NSString *str = @"Base64 is a group of similar binary-to-text encoding schemes that represent binary data in an ASCII string format by translating it into a radix-64 representation. The term Base64 originates from a specific MIME content transfer encoding.";
        
        NSData *data = [str dataUsingEncoding:NSUTF8StringEncoding];
        
        char table[] = {
            'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
            'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
            'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
            'w', 'x', 'y', 'z', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '+', '/',
            '='
        };
        
        NSMutableString *final = [NSMutableString string];
        
        const char *bytes = [data bytes];
        NSUInteger bytesLength = [data length];
        NSUInteger bytesProcessed = 0;
        
        while (bytesProcessed < bytesLength) {
            char firstByte = bytes[bytesProcessed];
            char secondByte = bytesProcessed+1 < bytesLength ? bytes[bytesProcessed+1] : 0;
            char thirdByte = bytesProcessed+2 < bytesLength ? bytes[bytesProcessed+2] : 0;
            
            uint32_t nextThreeBytes = (firstByte << 16) | (secondByte << 8) | (thirdByte);
            
            char firstSix = (nextThreeBytes & 0xFC0000) >> 18;
            char secondSix = (nextThreeBytes & 0x3F000) >> 12;
            char thirdSix = secondByte != 0 ? (nextThreeBytes & 0xFC0) >> 6 : 64;
            char fourthSix = thirdByte != 0 ? (nextThreeBytes & 0x3F) : 64;
            
            [final appendFormat:@"%c%c%c%c", table[firstSix], table[secondSix], table[thirdSix], table[fourthSix]];
            bytesProcessed += 3;
        }
        
        NSLog(@"\n%@", final);
    }
    
    return 0;
}