/*
  matrix-scan a set of LEDs

  pins:

  "rows" 12 & 13 go to a 220Ω resistor, then to 2 LEDs each (positive
  / long stem side); also to a 10kΩ resistor then to 2
  phototransistors (collector / long stem side)

  also, from between the 10kΩ and the phototransistors, wire goes to
  A1 & A2

  "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;
int lastSeen = 100;

void setup() {
  Serial.begin(19200);
  pinMode(12, OUTPUT);
  pinMode(13, OUTPUT);
  pinMode(14, OUTPUT);
  pinMode(15, OUTPUT);
  pinMode(A1, INPUT);
  pinMode(A2, INPUT);
  currentLed = 0;
  lastSeen = 100;
}

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 enableLed(int led) {
  int row = led/2;
  int col = led%2;
  /* 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);
  }
}

int sense(int led) {
  int row = led/2;

  int value = analogRead(row == 0 ? A1 : A2);

  //Serial.println(value);
  return value < 500;
}

void loop() {
  /* Serial.print("current led "); */
  /* Serial.println(currentLed); */

  enableLed(currentLed);
  delay(1);
  if (sense(currentLed)) {
    if (lastSeen != currentLed) {
      lastSeen = currentLed;
      Serial.print(currentLed);
      Serial.println(" proximity!");
    }
  }
  else if (lastSeen == currentLed) {
    lastSeen = 100;
  }

  currentLed = (currentLed+1)%4;
}