#include <SD.h>
#define CS_PIN 10
#define MAX_LEN 32

char line[MAX_LEN];

size_t pos = 0;
uint32_t t0;

size_t openFile(const char* filename) {
  File file = SD.open(filename);
  if (file) {    
    while (file.available()) {
      Serial.write(file.read());
    }
    size_t size = file.size();
    file.close();
    return size;
  } else {
    Serial.println("error opening wokwi.txt!");
  }
  return 0;
}


void appendValue(const char* filename) {
  File file = SD.open(filename, FILE_WRITE);  
  if (file) {
    file.seek(pos);
    snprintf(line, MAX_LEN, "% 4d; % 4d; % 4d; % 4d;\n", 
      analogRead(A0), 
      analogRead(A1), 
      analogRead(A2), 
      analogRead(A3)
    );
    pos += file.write(line);
    file.close();
  }
}


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

  Serial.print("Initializing SD card... ");

  if (!SD.begin(CS_PIN)) {
    Serial.println("Card initialization failed!");
    while (true);
  }

  Serial.println("initialization done.\n");
  pos = openFile("wokwi.csv");
}

void loop() {
  t0 = millis();
  appendValue("wokwi.csv");
  Serial.println(millis() - t0);
}