base64.cpp

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#include <stdexcept>

#include <foxglove_bridge/base64.hpp>

namespace foxglove {

// Adapted from:
// https://gist.github.com/tomykaira/f0fd86b6c73063283afe550bc5d77594
// https://github.com/protocolbuffers/protobuf/blob/01fe22219a0/src/google/protobuf/compiler/csharp/csharp_helpers.cc#L346
std::string base64Encode(const std::string_view& input) {
  constexpr const char ALPHABET[] =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
  std::string result;
  // Every 3 bytes of data yields 4 bytes of output
  result.reserve((input.size() + (3 - 1 /* round up */)) / 3 * 4);

  // Unsigned values are required for bit-shifts below to work properly
  const unsigned char* data = reinterpret_cast<const unsigned char*>(input.data());

  size_t i = 0;
  for (; i + 2 < input.size(); i += 3) {
    result.push_back(ALPHABET[data[i] >> 2]);
    result.push_back(ALPHABET[((data[i] & 0b11) << 4) | (data[i + 1] >> 4)]);
    result.push_back(ALPHABET[((data[i + 1] & 0b1111) << 2) | (data[i + 2] >> 6)]);
    result.push_back(ALPHABET[data[i + 2] & 0b111111]);
  }
  switch (input.size() - i) {
    case 2:
      result.push_back(ALPHABET[data[i] >> 2]);
      result.push_back(ALPHABET[((data[i] & 0b11) << 4) | (data[i + 1] >> 4)]);
      result.push_back(ALPHABET[(data[i + 1] & 0b1111) << 2]);
      result.push_back('=');
      break;
    case 1:
      result.push_back(ALPHABET[data[i] >> 2]);
      result.push_back(ALPHABET[(data[i] & 0b11) << 4]);
      result.push_back('=');
      result.push_back('=');
      break;
  }

  return result;
}

// Adapted from:
// https://github.com/mvorbrodt/blog/blob/cd46051e180/src/base64.hpp#L55-L110
std::vector<unsigned char> base64Decode(const std::string& input) {
  if (input.length() % 4) {
    throw std::runtime_error("Invalid base64 length!");
  }

  constexpr char kPadCharacter = '=';

  std::size_t padding{};

  if (input.length()) {
    if (input[input.length() - 1] == kPadCharacter) padding++;
    if (input[input.length() - 2] == kPadCharacter) padding++;
  }

  std::vector<unsigned char> decoded;
  decoded.reserve(((input.length() / 4) * 3) - padding);

  std::uint32_t temp{};
  auto it = input.begin();

  while (it < input.end()) {
    for (std::size_t i = 0; i < 4; ++i) {
      temp <<= 6;
      if (*it >= 0x41 && *it <= 0x5A)
        temp |= *it - 0x41;
      else if (*it >= 0x61 && *it <= 0x7A)
        temp |= *it - 0x47;
      else if (*it >= 0x30 && *it <= 0x39)
        temp |= *it + 0x04;
      else if (*it == 0x2B)
        temp |= 0x3E;
      else if (*it == 0x2F)
        temp |= 0x3F;
      else if (*it == kPadCharacter) {
        switch (input.end() - it) {
          case 1:
            decoded.push_back((temp >> 16) & 0x000000FF);
            decoded.push_back((temp >> 8) & 0x000000FF);
            return decoded;
          case 2:
            decoded.push_back((temp >> 10) & 0x000000FF);
            return decoded;
          default:
            throw std::runtime_error("Invalid padding in base64!");
        }
      } else
        throw std::runtime_error("Invalid character in base64!");

      ++it;
    }

    decoded.push_back((temp >> 16) & 0x000000FF);
    decoded.push_back((temp >> 8) & 0x000000FF);
    decoded.push_back((temp)&0x000000FF);
  }

  return decoded;
}

}  // namespace foxglove