color red = color(255, 0, 0);
color green = color(0, 255, 0);
color blue = color(0, 0, 255);
color black = color(0, 0, 0);

// circle 1
float x1, y1, dx1, dy1, diam1;

// circle 2
float x2, y2, dx2, dy2, diam2;

// circle 3
float x3, y3, dx3, dy3, diam3;

void setup() {
  size(500, 500);
  
  // Ball 1
  x1 = 100;
  y1 = 100;
  
  dx1 = 2;
  dy1 = 4;
  
  diam1 = 50;
  
  // Ball 2
  x2 = 400;
  y2 = 400;
  
  dx2 = 1;
  dy2 = -5;
  
  diam2 = 70;
  
  // Ball 3
  x3 = 250;
  y3 = 250;
  
  dx3 = -2;
  dy3 = -3;
  
  diam3 = 100;
}

void draw() {
  background(black);
  
  noStroke();
  
  // Ball 1
  drawBall(x1, y1, diam1, red);
  
  x1 += dx1;
  y1 += dy1;
  
  // Ball 2
  drawBall(x2, y2, diam2, green);
  
  x2 += dx2;
  y2 += dy2;
  
  // Ball 3
  drawBall(x3, y3, diam3, blue);
  
  x3 += dx3;
  y3 += dy3;
  
  // Ball 1?
  dy1 = hitTop(y1, diam1, dy1);
  dy1 = hitBottom(y1, diam1, dy1);
  dx1 = hitLeft(x1, diam1, dx1);
  dx1 = hitRight(x1, diam1, dx1);
  
  // Ball 2?
  dy2 = hitTop(y2, diam2, dy2);
  dy2 = hitBottom(y2, diam2, dy2);
  dx2 = hitLeft(x2, diam2, dx2);
  dx2 = hitRight(x2, diam2, dx2);
  
  dy3 = hitTop(y3, diam3, dy3);
  dy3 = hitBottom(y3, diam3, dy3);
  dx3 = hitLeft(x3, diam3, dx3);
  dx3 = hitRight(x3, diam3, dx3);
  
}

void drawBall(float x, float y, float diam, color fillColor) {
  fill(fillColor);
  ellipse(x, y, diam, diam);
}

float hitTop(float y, float diam, float dy) {
  if (y - diam / 2 <= 0) {
    return -dy;
  } else {
    return dy;
  }
}

float hitBottom(float y, float diam, float dy) {
  if (y + diam / 2 >= 499) {
    return -dy;
  } else {
    return dy;
  }
}

float hitLeft(float x, float diam, float dx) {
  if (x - diam / 2 <= 0) {
    return -dx;
  } else {
    return dx;
  }
}

float hitRight(float x, float diam, float dx) {
  if (x + diam / 2 >= 499) {
    return -dx;
  } else {
    return dx;
  }
}