base64.cpp

#include <Rcpp.h>
using namespace Rcpp;

constexpr auto b64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

// [[Rcpp::export]]
std::string base64_encode(const RawVector& input) {
  std::string out;
  out.reserve(input.size() / 3 * 4 + 4);

  int remainder = 0;
  unsigned char leftover = 0;
  for (unsigned char c: input) {
    remainder %= 3; // which byte of 3-bytes sequence
    switch (remainder) {
    case 0: // 1 byte of 3
      out += b64[c >> 2];
      leftover = c << 6; // last 2 bits
      break;
    case 1: // 2 byte of 3
      out += b64[(leftover >> 2) + (c >> 4)];
      leftover = c << 4;
      break;
    case 2: // 3 by of 3
      out += b64[(leftover >> 2) + (c >> 6)];
      out += b64[c & 0x3F];
      break;
    }
    remainder++;
  }

  switch (remainder) {
  case 1:
    out += b64[leftover >> 2];
    out += "==";
    break;
  case 2:
    out += b64[leftover >> 2];
    out += "=";
    break;
  }

  return out;
}