#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

#define SCREEN_WIDTH 128
#define SCREEN_HEIGHT 64
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, -1);

// Base text
String prefix = "   Simon says: ";   // space before Simon
String baseMsg = "Hello world";
String msg = baseMsg;

int x = SCREEN_WIDTH;

void setup() {
  if(!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) {
    for(;;); // stop if no display
  }
  display.clearDisplay();
}

void loop() {
  // Scroll current message across the screen
  display.clearDisplay();
  display.setTextSize(1);
  display.setTextColor(SSD1306_WHITE);
  display.setCursor(x, 20);
  display.print(prefix + msg);
  display.display();

  // Move left
  x--;
  if (x < -( (prefix.length() + msg.length()) * 6 )) {
    // Finished scrolling
    x = SCREEN_WIDTH;

    // Drop first character from msg, reset if empty
    if (msg.length() > 1) {
      msg.remove(0, 1);
    } else {
      msg = baseMsg;
    }
  }

  delay(30);
}