uuid.h

// Library for generating a UUID4 on ESP32
// Mike Abbott - 2019
// MIT License

#ifndef UUID_GEN
#define UUID_GEN
// For a 32 bit int, returnVar must be of length 8 or greater.
void IntToHex(const unsigned int inInt, char *returnVar)
{
  const char *HEXMAP = "0123456789abcdef";
  for (int i = 0; i < 8; i++)
  {
    // Shift each hex digit to the right, and then map it to its corresponding value
    returnVar[7 - i] = HEXMAP[(inInt >> (i * 4)) & 0b1111];
  }
}

// returnUUID must be of length 37 or greater
// (For the null terminator)
void UUIDGen(char *returnUUID)
{
  for (int i = 0; i < 4; i++)
  {
    unsigned int chunk = esp_random();
    // UUID4 requires a few bits to be specific values.
    if (i == 1)
    {
      chunk &= 0xFFFF0FFF;
      chunk |= 0x00004000;
    }
    if (i == 2)
    {
      chunk &= 0b00111111111111111111111111111111;
      chunk |= 0b10000000000000000000000000000000;
    }
    char chunkChars[8];
    IntToHex(chunk, chunkChars);
    for (int p = 0; p < 8; p++)
    {
      returnUUID[p + 8 * i] = chunkChars[p];
    }
  }
  int dashOffset = 4;
  const int UUID_NUM_DIGITS = 32;
  for (int i = UUID_NUM_DIGITS - 1; i >= 0; i--)
  {
    if (i == 7 || i == 11 || i == 15 || i == 19) // location of dashes
    {
      returnUUID[i + dashOffset--] = '-';
    }
    returnUUID[i + dashOffset] = returnUUID[i];
  }
  returnUUID[36] = 0;
}

String StringUUIDGen()
{
  char returnUUID[37];
  UUIDGen(returnUUID);
  return String(returnUUID);
}
#endif