// =======================================================
// HC-SR04 + BUZZER (NÂNG CAO - KHÔNG DELAY)
// =======================================================

#define TRIG 15
#define ECHO 14
#define BUZZER 16

unsigned long lastToggle = 0;
bool buzzerState = false;

float getDistance() {
  digitalWrite(TRIG, LOW);
  delayMicroseconds(2);

  digitalWrite(TRIG, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);

  long duration = pulseIn(ECHO, HIGH, 30000); // timeout 30ms

  float distance = duration * 0.034 / 2;
  return distance;
}

void setup() {
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  pinMode(BUZZER, OUTPUT);
}

void loop() {

  float d = getDistance();

  // --- VÙNG AN TOÀN ---
  if (d >= 30 || d == 0) {
    digitalWrite(BUZZER, LOW);
    return;
  }

  // --- TÍNH TỐC ĐỘ BEEP ---
  // càng gần → delay càng nhỏ
  int interval = d * 10;   // có thể chỉnh hệ số
  if (interval < 50) interval = 50;

  unsigned long now = millis();

  if (now - lastToggle >= interval) {
    buzzerState = !buzzerState;
    digitalWrite(BUZZER, buzzerState);
    lastToggle = now;
  }
}