// https://docs.arduino.cc/built-in-examples/display/RowColumnScanning/

// https://forum.arduino.cc/t/form-a-digit-on-a-led-matrix/1231824
// https://forum.arduino.cc/t/led-matrix-not-working-properly/1244654
// https://forum.arduino.cc/t/connect-many-leds-to-the-board/1301617

byte aPin[] = {18, 17, 16, 15, 14, 12, 11, 10}; // anode pins
byte aSize = sizeof(aPin)/sizeof(aPin[0]);
byte cPin[] = { 9,  8,  7,  6,  5,  4,  3,  2}; // cathode pins
byte cSize = sizeof(cPin)/sizeof(cPin[0]);
bool once;
int x, y, xdir, ydir;

void setup() {
  randomSeed(analogRead(A0));

  for (int i = 0; i < aSize; i++) {
    pinMode(aPin[i], OUTPUT);
    digitalWrite(aPin[i], LOW);
  }

  for (int i = 0; i < cSize; i++) {
    pinMode(cPin[i], OUTPUT);
    digitalWrite(cPin[i], HIGH);
  }
  clearMatrix();
}

void loop() {
  // dvd();
  randomLED();
  // firstLast();
}

void clearMatrix() { // reverse bias LEDs
  for (int i = 0; i < aSize; i++) {
    digitalWrite(aPin[i], LOW); // Anodes off
  }
  for (int i = 0; i < cSize; i++) {
    digitalWrite(cPin[i], HIGH); // Cathodes off
  }
}

void randomLED() {
  clearMatrix();
  digitalWrite(aPin[random(aSize)], HIGH); // random column
  digitalWrite(cPin[random(cSize)], LOW); // random row
  delay(250); // if they match, stay lit briefly
}

void firstLast() { // causes opposite corner LEDs to turn ON unless cycled off/on
  clearMatrix();
  digitalWrite(aPin[0], HIGH);
  digitalWrite(cPin[0], LOW);
  clearMatrix();
  digitalWrite(aPin[aSize - 1], HIGH);
  digitalWrite(cPin[cSize - 1], LOW);
}

void dvd() {
  if (!once) {
    once = 1;
    x = random(8), y = random(8), xdir = 1, ydir = 1;
  }

  x = x + xdir;
  y = y + ydir;
  if (x == 7 || x == 0) xdir = -xdir;
  if (y == 6 || y == 1) ydir = -ydir;

  digitalWrite(aPin[x], HIGH);
  digitalWrite(cPin[y], LOW);
  delay(250);
  clearMatrix();
}