// ====== PACKET PROTOCOL ======
// [START][LENGTH][SENSOR_ID][DATA_H][DATA_L][CHECKSUM][END]

#define START_BYTE  0xAA
#define END_BYTE    0xFF
#define TMP         0x01  // Temperature sensor ID

// ====== GLOBAL STORAGE ======
uint8_t packet[32];
uint8_t packetLength = 0;

// ====== SENSOR DATA STRUCT ======
struct SensorData {
  uint8_t id;
  uint16_t rawValue;
  float value;
  bool valid;
};

// ====== PACKET CREATOR ======
void createPacket(uint8_t sensorId, uint16_t value) {
  uint8_t index = 0;
  packet[index++] = START_BYTE;   // START
  packet[index++] = 0;            // LENGTH placeholder
  packet[index++] = sensorId;     // SENSOR ID
  packet[index++] = (value >> 8) & 0xFF; // DATA_H
  packet[index++] = value & 0xFF;        // DATA_L

  // Update LENGTH = number of bytes between SENSOR_ID and DATA_L
  packet[1] = index - 2;

  // Compute checksum (from LENGTH up to DATA_L)
  uint8_t checksum = 0;
  for (int i = 1; i < index; i++) {
    checksum ^= packet[i];
  }

  packet[index++] = checksum;
  packet[index++] = END_BYTE;

  packetLength = index;
}


// ====== PACKET DECODER ======
SensorData decodePacket(uint8_t* pkt, uint8_t length) {
  SensorData data = {0, 0, 0.0, false};

  if (length < 7) return data;              // too short
  if (pkt[0] != START_BYTE || pkt[length-1] != END_BYTE) return data;

  // Recompute checksum
  uint8_t checksum = 0;
  for (int i = 1; i < length-2; i++) {
    checksum ^= pkt[i];
  }
  if (checksum != pkt[length-2]) return data; // checksum fail

  // Extract values
  data.id = pkt[2];
  data.rawValue = (pkt[3] << 8) | pkt[4];
  data.value = data.rawValue / 10.0;  // assuming ×10 scaling
  data.valid = true;

  return data;
}

// ====== DISPLAY PACKET (HEX) ======
void displayPacket() {
  Serial.print("Packet: ");
  for (uint8_t i = 0; i < packetLength; i++) {
    if (packet[i] < 0x10) Serial.print("0");
    Serial.print(packet[i], HEX);
    Serial.print(" ");
  }
  Serial.println();
}

// ====== MAIN ======
void setup() {
  Serial.begin(115200);
  delay(1000);

  // Example: Temperature = 25.6°C (scaled ×10 → 256)
  createPacket(TMP, 256);
  displayPacket();

  // Decode packet
  SensorData s = decodePacket(packet, packetLength);
  if (s.valid) {
    Serial.print("Decoded -> ID: 0x");
    Serial.print(s.id, HEX);
    Serial.print("  Value: ");
    Serial.println(s.value);
  } else {
    Serial.println("Invalid packet");
  }
}

void loop() {
  // Nothing here for now
}