base64Decoder.h

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#include <vector>
#include <string>

inline std::vector<unsigned char> base64_to_binary(const std::string& in)
{

  std::string out;
  std::string in_padded;

  // Add padding
  if (in_padded.length() % 4 == 2)
  {
    in_padded = in + "==";
  }

  else if (in_padded.length() % 4 == 3)
  {
    in_padded = in + "=";
  }

  else
  {
    in_padded = in;
  }

  std::vector<int> T(256, -1);
  for (int i = 0; i < 64; i++)
    T["ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"[i]] =
        i;

  int val = 0, valb = -8;
  for (unsigned char c : in_padded)
  {
    if (T[c] == -1)
      break;
    val = (val << 6) + T[c];
    valb += 6;
    if (valb >= 0)
    {
      out.push_back(char((val >> valb) & 0xFF));
      valb -= 8;
    }
  }
  return std::vector<unsigned char>(out.begin(), out.end());
}

inline unsigned long getValueAt(std::vector<unsigned char>& bytes,
                                const unsigned int startByteIndex,
                                const unsigned int endByteIndex)
{
  unsigned long out = 0;

  for (int i = startByteIndex; i <= endByteIndex; i++)
  {
    if (i >= bytes.size())
      throw std::out_of_range("Out of bounds error while decoding b64.");
    out = out | ((0x0000000000000000 | bytes[i]) << 8 * (i - startByteIndex));
  }
  return out;
}

inline long convertValue(const unsigned long value, const unsigned int size,
                         bool isSigned)
{
  if (!isSigned)
    return static_cast<unsigned int>(value);
  unsigned long mask = 0x0000000000000080;
  mask = mask << (8 * (size - 1));
  mask = mask & value;
  for (int i = 0; i < 64 - (size * 8); i++)
  {
    mask = mask | (mask << 1);
  }
  return value | mask;
}

inline long base64_decode(const std::string& in,
                          const unsigned int startByteIndex,
                          const unsigned int endByteIndex,
                          const unsigned int size, bool isSigned)
{
  std::vector<unsigned char> bytes = base64_to_binary(in);
  unsigned long value = getValueAt(bytes, startByteIndex, endByteIndex);
  return convertValue(value, size, isSigned);
}