int frame_count, mouse_count;
boolean mouse_pressed;
Paint paint;

void setup(){
  size(400, 400);
  paint = new Paint(10, 20);
  smooth();
}

void draw(){
  background(255);
  paint.main();
  frame_count++;
}

void mousePressed(){
  mouse_count = frame_count;
  mouse_pressed = true;
}
void mouseReleased(){
  mouse_pressed = false;
}
class Line{
  PVector a, b;
  float vx, vy, dx, dy, length;
  Line(PVector a, PVector b){
    this.a = a;
    this.b = b;
    updateLine();
  }
  void updateLine(){
    vx = b.x - a.x;
    vy = b.y - a.y;
    length = sqrt(vx * vx + vy * vy);
    if(length > 0){
	dx = vx/length;
	dy = vy/length;
    } 
    else {
	dx = 0.0f;
	dy = 0.0f;
    }
  }
  void draw(){
    line(a.x, a.y, b.x, b.y);
  }
}

public class Paint {
  ArrayList lines, dots;
  PVector last;
  int length;
  float spacing;
  Paint(int spacing, int length){
    lines = new ArrayList();
    dots = new ArrayList();
    last = null;
    this.spacing = spacing;
    this.length = length;
  }
  // Adds dots inbetween dots that are too far apart
  void interpolate(PVector a, PVector b) {
    Line line = new Line(a, b);
    // the number of segments we will divide the gap into
    int denominator = (int)(line.length / spacing);
    // the distance between each new dot (note that it will be a little above spacing
    // this means it'll look slightly uneven - but no bunching and we get fast results)
    float step = line.length / denominator;
    for(float n = step; n < line.length; n += step) {
	// hack to deal with flash's bullshit floating point math
	// not sure if java does this too, but I'm playing safe
	if(line.length - n < step * 0.5) break;
	dots.add(new PVector(a.x + line.dx * n, a.y + line.dy * n));
    }
  }
  // Returns true if (x0,y0) is less than length from (x1,y1)
  boolean proximity(float x0, float y0, float x1, float y1, float length){
    return (x1 - x0) * (x1 - x0) + (y1 - y0) * (y1 - y0) <= length * length;
  }
  void main() {
    int i, j;
    // saves me putting this function in mousePressed()
    if(mouse_count == frame_count) {
	clear();
    }
    // painting routine
    if(mouse_pressed){
	float x = mouseX;
	float y = mouseY;
	PVector next = null;
	if(last == null){
	  // draw the first dot
	  last = new PVector(x, y);
	  dots.add(last);
	} else {
	  // add new dots, but only if they're further than 'spacing' apart
	  if(!proximity(x, y, last.x, last.y, spacing)){
	    next = new PVector(x, y);
	  }
	}
	if(next != null) {
	  // dots that are too far apart we break up
	  if(!proximity(last.x, last.y, next.x, next.y, spacing * 2)) {
	    interpolate(last, next);
	  }
	  dots.add(next);
	  // turn those dots into lines
	  while(dots.size() > 1) {
	    last = (PVector)dots.get(1);
	    lines.add(new Line((PVector)dots.remove(0), last));
	  }
	  // limit the length of the whole chain
	  while(lines.size() >= length) lines.remove(0);
	}
    }
    draw();
  }
  // clears the paint from the screen and memory
  void clear() {
    lines = new ArrayList();
    dots = new ArrayList();
    last = null;
  }
  void draw(){
    for(int i = 0; i < lines.size(); i++){
	Line temp = (Line)lines.get(i);
	temp.draw();
	ellipse(temp.a.x, temp.a.y, 5, 5);
    }
  }
}