Question

This is my fourth attempt at doing base64 encoding. My first tries work but it isn’t standard. It’s also extremely slow!!! I used vectors and push_back and erase a lot.

So I decided to re-write it and this is much much faster! Except that it loses data. -__-
I need as much speed as I can possibly get because I’m compressing a pixel buffer and base64 encoding the compressed string. I’m using ZLib. The images are 1366 x 768 so yeah.

I do not want to copy any code I find online because… Well, I like to write things myself and I don’t like worrying about copyright stuff or having to put a ton of credits from different sources all over my code..

Anyway, my code is as follows below. It’s very short and simple.

const static std::string Base64Chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

inline bool IsBase64(std::uint8_t C)
{
    return (isalnum(C) || (C == '+') || (C == '/'));
}

std::string Copy(std::string Str, int FirstChar, int Count)
{
    if (FirstChar <= 0)
        FirstChar = 0;
    else
        FirstChar -= 1;
    return Str.substr(FirstChar, Count);
}

std::string DecToBinStr(int Num, int Padding)
{
    int Bin = 0, Pos = 1;
    std::stringstream SS;
    while (Num > 0)
    {
        Bin += (Num % 2) * Pos;
        Num /= 2;
        Pos *= 10;
    }
    SS.fill('0');
    SS.width(Padding);
    SS << Bin;
    return SS.str();
}

int DecToBinStr(std::string DecNumber)
{
    int Bin = 0, Pos = 1;
    int Dec = strtol(DecNumber.c_str(), NULL, 10);

    while (Dec > 0)
    {
        Bin += (Dec % 2) * Pos;
        Dec /= 2;
        Pos *= 10;
    }
    return Bin;
}

int BinToDecStr(std::string BinNumber)
{
    int Dec = 0;
    int Bin = strtol(BinNumber.c_str(), NULL, 10);

    for (int I = 0; Bin > 0; ++I)
    {
        if(Bin % 10 == 1)
        {
            Dec += (1 << I);
        }
        Bin /= 10;
    }
    return Dec;
}

std::string EncodeBase64(std::string Data)
{
    std::string Binary = std::string();
    std::string Result = std::string();

    for (std::size_t I = 0; I < Data.size(); ++I)
    {
        Binary += DecToBinStr(Data[I], 8);
    }

    for (std::size_t I = 0; I < Binary.size(); I += 6)
    {
        Result += Base64Chars[BinToDecStr(Copy(Binary, I, 6))];
        if (I == 0) ++I;
    }

    int PaddingAmount = ((-Result.size() * 3) & 3);
    for (int I = 0; I < PaddingAmount; ++I)
        Result += '=';

    return Result;
}

std::string DecodeBase64(std::string Data)
{
    std::string Binary = std::string();
    std::string Result = std::string();

    for (std::size_t I = Data.size(); I > 0; --I)
    {
        if (Data[I - 1] != '=')
        {
            std::string Characters = Copy(Data, 0, I);
            for (std::size_t J = 0; J < Characters.size(); ++J)
                Binary += DecToBinStr(Base64Chars.find(Characters[J]), 6);
            break;
        }
    }

    for (std::size_t I = 0; I < Binary.size(); I += 8)
    {
        Result += (char)BinToDecStr(Copy(Binary, I, 8));
        if (I == 0) ++I;
    }

    return Result;
}

I’ve been using the above like this:

int main()
{
    std::string Data = EncodeBase64("IMG." + ::ToString(677) + "*" + ::ToString(604));  //IMG.677*604
    std::cout<<DecodeBase64(Data);        //Prints IMG.677*601
}

As you can see in the above, it prints the wrong string. It’s fairly close but for some reason, the 4 is turned into a 1!

Now if I do:

int main()
{
    std::string Data = EncodeBase64("IMG." + ::ToString(1366) + "*" + ::ToString(768));  //IMG.1366*768
    std::cout<<DecodeBase64(Data);        //Prints IMG.1366*768
}

It prints correctly.. I’m not sure what is going on at all or where to begin looking.

Just in-case anyone is curious and want to see my other attempts (the slow ones): http://pastebin.com/Xcv03KwE

I’m really hoping someone could shed some light on speeding things up or at least figuring out what’s wrong with my code :l

Answer 1

The main encoding issue is that you are not accounting for data that is not a multiple of 6 bits. In this case, the final 4 you have is being converted into 0100 instead of 010000 because there are no more bits to read. You are supposed to pad with 0s.

After changing your Copy like this, the final encoded character is Q, instead of the original E.

std::string data = Str.substr(FirstChar, Count);
while(data.size() < Count) data += '0';
return data;

Also, it appears that your logic for adding padding = is off because it is adding one too many = in this case.

As far as comments on speed, I’d focus primarily on trying to reduce your usage of std::string. The way you are currently converting the data into a string with 0 and 1 is pretty inefficent considering that the source could be read directly with bitwise operators.