size_t Base64EscapeInternal()

in absl/strings/internal/escaping.cc [73:176]

• 83 lines of code
• 15 McCabe index (conditional complexity)

size_t Base64EscapeInternal(const unsigned char* src, size_t szsrc, char* dest,
                            size_t szdest, const char* base64,
                            bool do_padding) {
  static const char kPad64 = '=';

  if (szsrc * 4 > szdest * 3) return 0;

  char* cur_dest = dest;
  const unsigned char* cur_src = src;

  char* const limit_dest = dest + szdest;
  const unsigned char* const limit_src = src + szsrc;

  // Three bytes of data encodes to four characters of cyphertext.
  // So we can pump through three-byte chunks atomically.
  if (szsrc >= 3) {                    // "limit_src - 3" is UB if szsrc < 3.
    while (cur_src < limit_src - 3) {  // While we have >= 32 bits.
      uint32_t in = absl::big_endian::Load32(cur_src) >> 8;

      cur_dest[0] = base64[in >> 18];
      in &= 0x3FFFF;
      cur_dest[1] = base64[in >> 12];
      in &= 0xFFF;
      cur_dest[2] = base64[in >> 6];
      in &= 0x3F;
      cur_dest[3] = base64[in];

      cur_dest += 4;
      cur_src += 3;
    }
  }
  // To save time, we didn't update szdest or szsrc in the loop.  So do it now.
  szdest = limit_dest - cur_dest;
  szsrc = limit_src - cur_src;

  /* now deal with the tail (<=3 bytes) */
  switch (szsrc) {
    case 0:
      // Nothing left; nothing more to do.
      break;
    case 1: {
      // One byte left: this encodes to two characters, and (optionally)
      // two pad characters to round out the four-character cypherblock.
      if (szdest < 2) return 0;
      uint32_t in = cur_src[0];
      cur_dest[0] = base64[in >> 2];
      in &= 0x3;
      cur_dest[1] = base64[in << 4];
      cur_dest += 2;
      szdest -= 2;
      if (do_padding) {
        if (szdest < 2) return 0;
        cur_dest[0] = kPad64;
        cur_dest[1] = kPad64;
        cur_dest += 2;
        szdest -= 2;
      }
      break;
    }
    case 2: {
      // Two bytes left: this encodes to three characters, and (optionally)
      // one pad character to round out the four-character cypherblock.
      if (szdest < 3) return 0;
      uint32_t in = absl::big_endian::Load16(cur_src);
      cur_dest[0] = base64[in >> 10];
      in &= 0x3FF;
      cur_dest[1] = base64[in >> 4];
      in &= 0x00F;
      cur_dest[2] = base64[in << 2];
      cur_dest += 3;
      szdest -= 3;
      if (do_padding) {
        if (szdest < 1) return 0;
        cur_dest[0] = kPad64;
        cur_dest += 1;
        szdest -= 1;
      }
      break;
    }
    case 3: {
      // Three bytes left: same as in the big loop above.  We can't do this in
      // the loop because the loop above always reads 4 bytes, and the fourth
      // byte is past the end of the input.
      if (szdest < 4) return 0;
      uint32_t in = (cur_src[0] << 16) + absl::big_endian::Load16(cur_src + 1);
      cur_dest[0] = base64[in >> 18];
      in &= 0x3FFFF;
      cur_dest[1] = base64[in >> 12];
      in &= 0xFFF;
      cur_dest[2] = base64[in >> 6];
      in &= 0x3F;
      cur_dest[3] = base64[in];
      cur_dest += 4;
      szdest -= 4;
      break;
    }
    default:
      // Should not be reached: blocks of 4 bytes are handled
      // in the while loop before this switch statement.
      ABSL_RAW_LOG(FATAL, "Logic problem? szsrc = %zu", szsrc);
      break;
  }
  return (cur_dest - dest);
}