int numberOfCircles;
int lineX;
//is the line moving right? To check if the line is moving left you just check if it's NOT moving right (!isLineMovingRight)
boolean isLineMovingRight;
//The index of the array is always the number of the circle. So if I do xPosition[5] I get the x value for the 6th circle. Remember, arrays start at 0, not 1!
int[] xPositions;
int[] yPositions;
int[] radii;

void setup() {
  size(500, 500);
  background(0);
  ellipseMode(CORNER);
  lineX = 0;
  numberOfCircles = 100;
  isLineMovingRight = true;
  xPositions = new int[numberOfCircles];
  yPositions = new int[numberOfCircles];
  radii = new int[numberOfCircles];
  //I've put the whole generating circle code into its own method, so I can call it in other places without re-writing it again. Specifically, we need it again when the line reaches one of the window borders.
  calculateCircles();
}

void draw() {
  background(50);
  //code to draw circles and check if they should be moved
  noStroke();
  fill(255, 125);
  for (int i = 0; i < numberOfCircles; i++) {
    if (isLineMovingRight && xPositions[i] == lineX) {
      xPositions[i]++;
    }
    //when the line is moving left, we have to add the radius to the xPosition of the circle. To see what happens if we don't, just remove the "+radii[i]". This is because the "corner" of each circle is in the top left.
    if (!isLineMovingRight && xPositions[i]+radii[i] == lineX) {
      xPositions[i]--;
    }
    //using the circle function instead of the ellipse function, well, because uh that's what it's made for. Of course you could just use ellipse and add another argument with radii[i] again *shrug*
    circle(xPositions[i], yPositions[i], radii[i]);
  }
  //code to draw and move line
  stroke(255, 0, 0);
  line(lineX, 0, lineX, height);
  //another way to write the following 5-line-if-else-statement would be:
  //lineX = isLineMovingRight ? lineX+1 : lineX-1
  //the question mark is called a "ternary operator". You'll use it all the time once you've learned about it because it unclutters the code greatly. In this case it saves 4 lines of code!
  if (isLineMovingRight) {
    lineX++;
  } else {
    lineX--;
  }
  lineX = isLineMovingRight ? lineX-1 : lineX+1
  //if the line hits either the right or left side, we need to switch direction and calculate new circle positions.
  if (lineX == width || lineX == 0) {
    isLineMovingRight = !isLineMovingRight;
    calculateCircles();
  }
}

void calculateCircles() {
  for (int i = 0; i < numberOfCircles; i++) {
    radii[i] = ceil(random(1, 30));
    yPositions[i] = ceil(random(0, height-radii[i]));
    xPositions[i] = ceil(random(0, width-radii[i]));
  }
}