/*
 * RECEIVER — XIAO ESP32-C3 + passive buzzer
 * Receives a frequency over ESP-NOW and plays it on the buzzer. If packets
 * stop arriving it silences the buzzer (watchdog) so it never gets stuck.
 *
 * Wiring (XIAO ESP32-C3):
 *   Buzzer +  -> D10 (GPIO10)
 *   Buzzer -  -> GND
 *
 * IMPORTANT: pitch control needs a PASSIVE (piezo) buzzer or small speaker.
 * An ACTIVE buzzer has its own oscillator and only does on/off at a fixed
 * pitch — tone() cannot change its frequency.
 *
 * Built for ESP32 Arduino core 3.x (current).
 */

#include <esp_now.h>
#include <WiFi.h>
#include <esp_wifi.h>

static const int     PIN_BUZZER  = 10;    // D10
static const uint8_t WIFI_CHANNEL = 1;    // must match the sender
static const uint32_t TIMEOUT_MS = 500;   // silence if no packet within this

typedef struct __attribute__((packed)) {
  uint16_t freq;     // Hz (0 = silent)
} message_t;

volatile uint16_t targetFreq = 0;
volatile uint32_t lastRxMs   = 0;

/* ---- ESP-NOW receive callback (core 3.x signature) ----
 * If you are on the OLDER core 2.x, replace the line below with:
 *   void onRecv(const uint8_t *mac, const uint8_t *data, int len) {
 */
void onRecv(const esp_now_recv_info_t *info, const uint8_t *data, int len) {
  if (len < (int)sizeof(message_t)) return;
  message_t m;
  memcpy(&m, data, sizeof(m));
  targetFreq = m.freq;
  lastRxMs   = millis();
}

void setup() {
  Serial.begin(115200);
  delay(300);

  pinMode(PIN_BUZZER, OUTPUT);
  noTone(PIN_BUZZER);

  WiFi.mode(WIFI_STA);
  WiFi.disconnect();
  esp_wifi_set_channel(WIFI_CHANNEL, WIFI_SECOND_CHAN_NONE);

  if (esp_now_init() != ESP_OK) {
    Serial.println("ESP-NOW init failed; restarting");
    ESP.restart();
  }
  esp_now_register_recv_cb(onRecv);

  Serial.println("Receiver ready");
}

uint16_t curFreq = 0;

void loop() {
  uint16_t f    = targetFreq;
  uint32_t last = lastRxMs;

  // watchdog: go quiet if the sender has gone silent
  if (millis() - last > TIMEOUT_MS) f = 0;

  if (f != curFreq) {
    curFreq = f;
    if (f == 0) noTone(PIN_BUZZER);
    else        tone(PIN_BUZZER, f);
  }
  delay(5);
}