codec.cpp

std::string DecodeBase64(std::string_view input) {
  static const uint8_t b64_decode[] = {
      64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
      64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 62,
      64, 64, 64, 63, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 64, 64, 64, 64, 64, 64, 64, 0,
      1,  2,  3,  4,  5,  6,  7,  8,  9,  10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22,
      23, 24, 25, 64, 64, 64, 64, 64, 64, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38,
      39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 64, 64, 64, 64, 64, 64, 64, 64, 64,
      64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
      64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
      64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
      64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
      64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64,
      64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64, 64};

  size_t in_len = input.size();
  ASSERT_DEBUG(in_len % 4 == 0);

  size_t out_len = in_len / 4 * 3;
  if (input[in_len - 1] == '=') out_len--;
  if (input[in_len - 2] == '=') out_len--;

  std::string out(out_len, 0);

  for (size_t i = 0, j = 0; i < in_len;) {
    uint32_t a = input[i] == '=' ? 0 & i++ : b64_decode[static_cast<int>(input[i++])];
    uint32_t b = input[i] == '=' ? 0 & i++ : b64_decode[static_cast<int>(input[i++])];
    uint32_t c = input[i] == '=' ? 0 & i++ : b64_decode[static_cast<int>(input[i++])];
    uint32_t d = input[i] == '=' ? 0 & i++ : b64_decode[static_cast<int>(input[i++])];

    uint32_t triple = (a << 3 * 6) + (b << 2 * 6) + (c << 1 * 6) + (d << 0 * 6);

    if (j < out_len) out[j++] = (triple >> 2 * 8) & 0xFF;
    if (j < out_len) out[j++] = (triple >> 1 * 8) & 0xFF;
    if (j < out_len) out[j++] = (triple >> 0 * 8) & 0xFF;
  }

  return out;
}

std::string EncodeBase64(std::string_view v) {
  size_t sz = v.size();
  size_t padding = sz % 3;
  if (padding) {
    padding = 3 - padding;
  }
  sz += padding;
  ASSERT_DEBUG(sz % 3 == 0);
  std::string out(sz * 4 / 3, 0);

  // base64 lookup table
  static const char b64_encode[] =
      "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";

  size_t p = 0;
  for (size_t i = 0; i < sz; i += 3) {
    uint8_t b = v[i] >> 2;
    ASSERT_DEBUG(b < 64);
    out[p++] = b64_encode[b];

    b = ((v[i] & 0x3) << 4) + (v[i + 1] >> 4);
    ASSERT_DEBUG(b < 64);
    out[p++] = b64_encode[b];

    b = ((v[i + 1] & 0xf) << 2) + (v[i + 2] >> 6);
    ASSERT_DEBUG(b < 64);
    out[p++] = b64_encode[b];

    b = v[i + 2] & 0x3f;
    ASSERT_DEBUG(b < 64);
    out[p++] = b64_encode[b];
  }

  // add padding bytes
  for (size_t i = 0; i < padding; i++) {
    out[p++] = '=';
  }

  ASSERT_DEBUG(p == out.size());
  return out;
}