#define PICKUP_PIN 2
#define IGNITION_PIN 13

volatile unsigned long lastPulse = 0;
volatile unsigned long interval = 0;

void setup() {
  pinMode(PICKUP_PIN, INPUT_PULLUP);
  pinMode(IGNITION_PIN, OUTPUT);

  attachInterrupt(digitalPinToInterrupt(PICKUP_PIN), pickupISR, FALLING);
}

void loop() {
  if (interval > 0) {
    float rpm = 60000000.0 / interval;

    if (rpm < 500 || rpm > 15000) return;

    float advanceDeg;

    if (rpm < 2000) advanceDeg = 5;
    else if (rpm < 5000) advanceDeg = 10;
    else if (rpm < 8000) advanceDeg = 18;
    else advanceDeg = 22;

    float delay_us = interval * (advanceDeg / 360.0);

    delayMicroseconds((int)delay_us);

    digitalWrite(IGNITION_PIN, HIGH);
    delayMicroseconds(100);
    digitalWrite(IGNITION_PIN, LOW);
  }
}

void pickupISR() {
  unsigned long now = micros();
  interval = now - lastPulse;
  lastPulse = now;
}