/*
  matrix-scan a set of LEDs

  pins:

  "rows"
  12 & 13 go to a 220Ω resistor, then to 2 LEDs each (positive / long stem side)

  "columns"
  14 & 15 go to 2 LEDs each (negative / short stem side)

  so that given one of 12|13 and one of 14|15, one LED is identified
 */

int currentLed = 0;

void setup() {
  Serial.begin(19200);
  pinMode(12, OUTPUT);
  pinMode(13, OUTPUT);
  pinMode(14, OUTPUT);
  pinMode(15, OUTPUT);
  currentLed = 0;
}

void tristate(int pin) {
  pinMode(pin,OUTPUT);
  digitalWrite(pin,LOW);
  pinMode(pin,INPUT);
}

void power(int pin) {
  pinMode(pin,OUTPUT);
  digitalWrite(pin,HIGH);
}

void ground(int pin) {
  pinMode(pin,OUTPUT);
  digitalWrite(pin,LOW);
}

void enableRowCol(int row, int col) {
  Serial.print("enabling ");
  Serial.print(row);
  Serial.print(" ");
  Serial.println(col);

  if (row==0) {
    power(12); ground(13);
  }
  else {
    ground(12); power(13);
  }

  if (col==0) {
    ground(14); tristate(15);
  }
  else {
    tristate(14); ground(15);
  }
}

void loop() {
  Serial.print("current led ");
  Serial.println(currentLed);
  enableRowCol(currentLed/2,currentLed%2);
  currentLed = (currentLed+1)%4;
  delay(10);
}