String txt = "Hello, world! ";
boolean dir = true;

void draw(){
  background(128);
  text(txt, 10, height/2);
  if(frameCount%10==0){
    if(dir) {
      txt = rotate(txt, 1);
    } else {
      txt = rotate(txt, txt.length()-1);
    }
  }
}

void mouseReleased(){
  dir = !dir;
}

String rotate(String text, int startIdx) {
  char[] rotated = new char[text.length()];
  for (int i = 0; i < text.length(); i++) {
    rotated[i] = text.charAt((i + startIdx) % text.length());
  }
  return String.valueOf(rotated);
}