class Position { int x = 0; int y = 0; }
class Delta { int dx = 0; int dy = 0; }

class RectArea
{
  Position position = new Position();
  Delta delta = new Delta();
  color fillColor;
  int opacity;

  RectArea(color fillColor, int opacity)
  {
    this.fillColor = fillColor;
    this.opacity = opacity;
  }

  void setSize(int dx, int dy)
  {
    delta.dx = dx;
    delta.dy = dy;
  }

  boolean hitTest(int posX, int posY)
  {
    return posX >=  position.x && posX <= (position.x + delta.dx) &&
           posY >=  position.y && posY <= (position.y + delta.dy);
  }

  void initialize() { placeAtCenter(this); }
  void setX(int x) { position.x = x; }
  void moveX(int dx) { position.x += dx; }
  int maybeMoveX(int dx) { return position.x + dx; }

  void paint()
  {
    fill(fillColor, opacity);
    rect(position.x, position.y, delta.dx, delta.dy);
  }
}

void placeAtCenter(RectArea area)
{
  area.position.x = height / 2 - area.delta.dx / 2;
  area.position.y = width / 2 - area.delta.dy / 2;
}

boolean drag = false;
boolean inDrag = false;

int lastMouseX = 0;

int canvasX = 400;
int canvasY = 400;

RectArea viewableArea = new RectArea(#93BAF7, 200);
RectArea backgroundArea = new RectArea(#CE7BCE, 255);

int scaleBy = 0;
int excessXs = 0;

int scrollEdgeX1_b = 0;
int scrollEdgeX2_b = 0;
int scrollEdgeX1_v = 0;
int scrollEdgeX2_v = 0;

void setup()
{
  size(canvasX, canvasY);
  background(0);
  noStroke();

  frameRate(25);
  lastMouseX = mouseX;

  viewableArea.setSize(100, 200);
  backgroundArea.setSize(250, 200);

  backgroundArea.initialize();
  backgroundArea.paint();

  viewableArea.initialize();
  viewableArea.paint();

  float vdx = viewableArea.delta.dx;
  float bdx = backgroundArea.delta.dx;
  int slack = (int)((3*vdx - bdx)/2);
  scaleBy = (int)(vdx / slack);

  println(scaleBy);

  scrollEdgeX1_v = viewableArea.position.x - viewableArea.delta.dx;
  scrollEdgeX2_v = viewableArea.position.x + viewableArea.delta.dx;
  scrollEdgeX1_b = scrollEdgeX1_v;
  scrollEdgeX2_b = backgroundArea.position.x + slack;
}

void putAtEdge(RectArea r, int x)
{
  r.setX(x);
  inDrag = false;
}

void draw()
{
  if (drag)
  {
    background(0);
    backgroundArea.paint();

    int dx = -1 * (lastMouseX - mouseX);
    boolean resetDrag = false;

    if ( viewableArea.maybeMoveX(dx) < scrollEdgeX1_v )
    {
      putAtEdge(viewableArea, scrollEdgeX1_v);
      putAtEdge(backgroundArea, scrollEdgeX1_b);
    }
    else if ( viewableArea.maybeMoveX(dx) > scrollEdgeX2_v )
    {
      putAtEdge(viewableArea, scrollEdgeX2_v);
      putAtEdge(backgroundArea, scrollEdgeX2_b);
    }
    else
    {
      viewableArea.moveX(dx);
      backgroundArea.moveX((dx + excessXs) / scaleBy);
      excessXs = (dx + excessXs) % scaleBy;
    }

    viewableArea.paint();
  }

  lastMouseX = mouseX;
}

void mouseDragged()
{
  if (inDrag)
    return;

  drag = viewableArea.hitTest(mouseX, mouseY);
  if (drag)
    inDrag = true;
}

void mouseReleased()
{
  inDrag = false;
  drag = false;
}