#include "Arduino.h"
#include <SPI.h>
#include <SD.h>

File myFile;

int peakVal = 900;
int timeCur = millis();

void setup()
{
  Serial.begin(9600);

  while (!Serial);
  Serial.print("Initializing SD card...");
  if (!SD.begin(10)) {
    Serial.println("initialization failed!");
    return;
  }
  Serial.println("initialization done.");
}

void loop()
{
  int val = analogRead(A6);
  int c = millis();

  if (val > peakVal && c - timeCur > 500) {
    myFile = SD.open("hrv.txt",  O_RDWR | O_APPEND);
    if (myFile) {
      int dif = c - timeCur;
      myFile.println(dif);
      myFile.close();
      Serial.println(dif);
    } else {
      if (!myFile) {
        myFile = SD.open("hrv.txt", O_RDWR | O_CREAT);
        if (!myFile) {
            Serial.println("Failed to open file");
        }
      }
    }
    timeCur = c;
  }
  delay(10);
}