#define PIN_A 0
#define PIN_B 1
#define PIN_C 2
#define PIN_D 4
const int pause = 500;

void setup()
{
  // first set all pins to input, or high impedance
  // (not strictly necessary as all pins are inputs by default)
  pinMode(PIN_A, INPUT);
  pinMode(PIN_B, INPUT);
  pinMode(PIN_C, INPUT);
  pinMode(PIN_D, INPUT);
}

void loop()
{
  // run through a sample loop, lighting each LED

  // in turn and holding for half a second.
  set_pins(PIN_A, PIN_B);
  delay(pause);

  set_pins(PIN_B, PIN_A);
  delay(pause);

  set_pins(PIN_A, PIN_C);
  delay(pause);

  set_pins(PIN_C, PIN_A);
  delay(pause);

  set_pins(PIN_A, PIN_D);
  delay(pause);

  set_pins(PIN_D, PIN_A);
  delay(pause);

  set_pins(PIN_B, PIN_C);
  delay(pause);

  set_pins(PIN_C, PIN_B);
  delay(pause);

  set_pins(PIN_B, PIN_D);
  delay(pause);

  set_pins(PIN_D, PIN_B);
  delay(pause);

    set_pins(PIN_C, PIN_D);
  delay(pause);

    set_pins(PIN_D, PIN_C);
  delay(pause);

}

void set_pins(int high_pin, int low_pin)
{
  // reset all the pins
  reset_pins();

  // set the high and low pins to output
  pinMode(high_pin, OUTPUT);
  pinMode(low_pin, OUTPUT);

  // set high pin to logic high, low to logic low
  digitalWrite(high_pin, HIGH);
  digitalWrite(low_pin, LOW);
}

void reset_pins()
{
  // start by ensuring all pins are at input and low
  pinMode(PIN_A, INPUT);
  pinMode(PIN_B, INPUT);
  pinMode(PIN_C, INPUT);
  pinMode(PIN_D, INPUT);

  digitalWrite(PIN_A, LOW);
  digitalWrite(PIN_B, LOW);
  digitalWrite(PIN_C, LOW);
  digitalWrite(PIN_D, LOW);
}