// constants won't change:
const long interval = 25; // 25 ms * 6 tests  * 2 = 200 ms means 5 loops per second

int pushButton1 = 2;
int pushButton2 = 4;
int pushButton3 = 6;
int pushButton4 = 11;

int buttons[4] = {
  pushButton1,
  pushButton2,
  pushButton3,
  pushButton4
};

void setup() {
  // set the digital pin as output:
  Serial.begin(9600);
  pinMode(pushButton1, OUTPUT);
  digitalWrite(pushButton1, LOW);
  pinMode(pushButton2, INPUT_PULLUP);
  pinMode(pushButton3, INPUT_PULLUP);
  pinMode(pushButton4, INPUT_PULLUP);
}

bool testConnection2(int A, int B) {
  for (int i = 0; i < 4; i++) {
    if (i == A) {
      pinMode(buttons[i], OUTPUT);
      digitalWrite(buttons[i], LOW);
    } else {
      pinMode(buttons[i], INPUT_PULLUP);
    }
  }

  delay(interval);
  if (!digitalRead(buttons[B])) {
    return true;
  } else {
    return false;
  }
}
bool testConnection(int A, int B) {
  return testConnection2(A, B) && testConnection2(B, A);
}


void loop() {
  if (testConnection(0, 1)) {
    Serial.println("0011");
  } else if (testConnection(0, 2)) {
    Serial.println("0101");
  } else if (testConnection(0, 3)) {
    Serial.println("1001");
  } else if (testConnection(1, 2)) {
    Serial.println("0110");
  } else if (testConnection(1, 3)) {
    Serial.println("1010");
  } else if (testConnection(2, 3)) {
    Serial.println("1100");
  }
}