Heartbeat monitor on an Arduino Uno

// Heartbeat monitor: a Pulse Sensor and an SSD1306 OLED.
//
//   A0       the Pulse Sensor's S (purple); VCC and GND from the rails
//   A4, A5   the OLED's SDA and SCL (address 0x3C)
//   pin 13   the Uno's own LED, blinking with each beat
//
// PulseSensor Playground reads A0 every 2 ms on Timer2 and finds the beats,
// so loop() only has to draw. Every 20 ms it takes the latest sample and
// plots it one column further along the bottom of the screen, scaled from
// the 0 to 1023 of the converter to the 48 rows under the heart rate; at the
// right edge it starts again from the left. The screen is sent every fifth
// column, since a whole frame over I2C takes a while.

#define USE_ARDUINO_INTERRUPTS true
#include <PulseSensorPlayground.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

const int PULSE_PIN = A0;
const int LED = 13;
const int THRESHOLD = 550;

Adafruit_SSD1306 display(128, 64, &Wire, -1);
PulseSensorPlayground pulseSensor;

int column = 0;
int lastY = 40;
int bpm = 0;

void header() {
  display.fillRect(0, 0, 128, 16, SSD1306_BLACK);
  display.setCursor(0, 0);
  display.setTextSize(2);
  display.setTextColor(SSD1306_WHITE);
  if (bpm > 0) {
    display.print(bpm);
    display.print(" BPM");
  } else {
    display.print("-- BPM");
  }
}

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: heartbeat monitor");
  if (!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) {
    Serial.println("no OLED: check SDA, SCL and power");
  }
  display.clearDisplay();
  header();
  display.display();
  pulseSensor.analogInput(PULSE_PIN);
  pulseSensor.blinkOnPulse(LED);
  pulseSensor.setThreshold(THRESHOLD);
  pulseSensor.begin();
}

void loop() {
  if (pulseSensor.sawStartOfBeat()) {
    bpm = pulseSensor.getBeatsPerMinute();
    if (bpm > 0) {
      Serial.print("beat: ");
      Serial.print(bpm);
      Serial.print(" BPM ibi ");
      Serial.println(pulseSensor.getInterBeatIntervalMs());
    }
    header();
  }

  // One column of the wave: 0..1023 onto rows 63 (low) to 16 (high).
  int sample = pulseSensor.getLatestSample();
  int y = 63 - (int)((long)sample * 47 / 1023);
  display.drawFastVLine(column, 16, 48, SSD1306_BLACK);
  if (column > 0) display.drawLine(column - 1, lastY, column, y, SSD1306_WHITE);
  lastY = y;
  column++;
  if (column >= 128) column = 0;
  if (column % 5 == 0) display.display();
  delay(20);
}