// 1️⃣  Request a HR‑monitor device
const hrOptions = {
  // Show only devices that advertise the Heart Rate service (0x180D)
  filters: [{ services: [0x180d] }],

  // After the user picks a device we also want to read the Device
  // Information service (optional, no extra prompt)
  optionalServices: [0x180a], // Device Information
};

navigator.bluetooth
  .requestDevice(hrOptions)
  .then((device) => {
    console.log("✅ Selected:", device.name);
    // 2️⃣ Connect to the GATT server
    return device.gatt.connect();
  })
  .then((server) => {
    // 3️⃣ Get the Heart Rate service
    return server.getPrimaryService(0x180d);
  })
  .then((hrService) => {
    // 4️⃣ Get the Heart Rate Measurement characteristic (0x2A37)
    return hrService.getCharacteristic(0x2a37);
  })
  .then((hrChar) => {
    // 5️⃣ Enable notifications – the monitor will push new readings
    return hrChar.startNotifications().then(() => hrChar);
  })
  .then((hrChar) => {
    console.log("🔔 Listening for heart‑rate measurements...");
    hrChar.addEventListener("characteristicvaluechanged", (ev) => {
      const value = ev.target.value; // DataView
      const heartRate = parseHeartRate(value);
      console.log("❤️  Heart Rate:", heartRate, "bpm");
    });
  })
  .catch((err) => console.error("❌ Bluetooth error:", err));

/**
 * Parse the Heart Rate Measurement characteristic (Bluetooth SIG spec).
 * Returns the BPM as a Number.
 */
function parseHeartRate(dataView) {
  // Flags are in the first byte
  const flags = dataView.getUint8(0);
  const hrFormatUint16 = flags & 0x01; // 0 = 8‑bit, 1 = 16‑bit

  if (hrFormatUint16) {
    return dataView.getUint16(1, /*littleEndian=*/ true);
  }
  return dataView.getUint8(1);
}

// After the HR connection succeeds, you can also fetch the Device Info service:
navigator.bluetooth
  .requestDevice(hrOptions) // reuse the same options
  .then((d) => d.gatt.connect())
  .then((server) =>
    Promise.all([
      server.getPrimaryService(0x180a), // Device Information
      server.getPrimaryService(0x180d), // Heart Rate
    ]),
  )
  .then(([infoService, hrService]) =>
    Promise.all([
      // Manufacturer Name (0x2A29)
      infoService.getCharacteristic(0x2a29).then((c) => c.readValue()),
      // Model Number (0x2A24)
      infoService.getCharacteristic(0x2a24).then((c) => c.readValue()),
      // Heart Rate Measurement (as before)
      hrService
        .getCharacteristic(0x2a37)
        .then((c) => c.startNotifications().then(() => c)),
    ]),
  )
  .then(([manufVal, modelVal, hrChar]) => {
    const dec = new TextDecoder("utf-8");
    console.log("🏭 Manufacturer:", dec.decode(manufVal));
    console.log("📦 Model:", dec.decode(modelVal));

    hrChar.addEventListener("characteristicvaluechanged", (ev) =>
      console.log("❤️", parseHeartRate(ev.target.value), "bpm"),
    );
  })
  .catch(console.error);