float rx=0, ry=0, rz=1;    // view angles
PImage wood;   
float dt=0;
int n = 20;
BALL B1 = new BALL(), B2 = new BALL();
BALL B[] = new BALL[n];
v3 W[] = new v3[n];
int method = 1;
float g=0.05;

void setup() {
  size(600, 600, P3D); sphereDetail(12);  textureMode(NORMALIZED);  
  PFont font = loadFont("Courier-14.vlw"); textFont(font, 14);      // load font
  wood = loadImage("floor.jpg");  // load texture
  for (int i=0; i<n; i++)  B[i] = new BALL(); 
  for (int i=0; i<n; i++)  W[i] = new v3(0,0,0); 
  n=5;
  setBalls(false);
  } 
 
void draw() {         // rendering loop to refresh the window
  background(255);  lights();
  directionalLight(256,256,256, 0, 1, 0);
  text("Method: 1, 2, 3; Reset: l, h or H; Add: + or -; Drag view: r or z",10,20);
  fill(50); 
     if (method==1) text("1: Wrong Collision",40,50); 
     if (method==2) text("2: Correct Collision",40,50); 
     if (method==3) text("3: Correct Collision and bounce",40,50); 
     noFill();
  if (keyPressed&&(key=='r')) {rx+=(mouseY-pmouseY)/800.;ry-=(mouseX-pmouseX)/800.;}; // rotate
  if (keyPressed&&(key=='z')) {rz+=(mouseY-pmouseY)/1.;rz+=(mouseX-pmouseX)/1.;}; // zoom
  translate(0,0,-100+rz);
  translate(width/2,0,0); rotateY(ry);  translate(-width/2,0,0); 
  rotateX(rx); 
  noStroke(); fill(0); 
  for (int i=0; i<n; i++) { B[i].move(); B[i].bounce(); }; 
  for (int i=0; i<n-1; i++) for (int j=i+1; j<n; j++)  if (clash(B[i],B[j])) shock(i,j);
  showFloor(); 
  fill(100,100,100); for(int i=0; i<n; i++) B[i].showShadow(); 
  fill(100,150,50);   for(int i=0; i<n; i++) B[i].show();
  stroke(0,0,255); for(int i=0; i<n; i++) B[i].showV(); noStroke();
 };

void showFloor() {  
  beginShape();    texture(wood);          // activate texture
  vertex( 600, height+2, 300, 1, 1); 
  vertex(600, height+2, -300, 1, 0); 
  vertex(0,height+2,-300, 0, 0);  
  vertex( 0, height+2, 300, 0, 1);  
  endShape(); 
  }
  
class BALL {
  p3 C = new p3(0,0,0); v3 V = new v3(0,0,0); float r=40;
  BALL() {reset();}
  void setTo(BALL B) {C.setTo(B.C); V.setTo(B.V); r=B.r; }
  void reset() {C.setTo(random(r,width-r),random(r,height-r),0); V.setTo(random(-4,4),random(-3,3),0);};
  void reset3D() {C.setTo(random(2*r,width-2*r),random(2*r,height-2*r),random(-height/2+2*r,height/2.-2*r)); V.setTo(random(-4,4),random(-3,3),random(-3,3));};
  void show() {pushMatrix(); translate(C.x,C.y,C.z);   sphere(r);  popMatrix();}
  void showV() { V.show(20,C);}
  void showShadow() {pushMatrix(); translate(0,height,0); scale(1,0.001,1);  translate(C.x,0,C.z); fill(30,30,30); sphere(r); popMatrix(); } 
  void move(float t) {C.addVec(t,V);  }
  void move() {C.addVec(V); V.y+=g; }
  void bounce() {
    if ((C.y>height-r)&&(V.y>0.1)) {float dt=(C.y-height+r)/V.y; move(-dt); V.y=-0.95*V.y;  move(dt);}; // backs to collision time, turns, & advances
    if((C.x>width-r)&&(V.x>0)) V.x=-V.x; 
    if((V.x<0)&&(C.x<r)) V.x=-V.x; 
    if((C.z>300-r)&&(V.z>0)) V.z=-V.z; 
    if((V.z<0)&&(C.z<-300+r)) V.z=-V.z; }
  }  // end BALL class

void row () {for (int i=0; i<n; i++) {B[i].C.setTo(60+80*i, height/2, 0); B[i].V.setTo(0,0,0); } B[0].V.x=-6;}
void setBalls(boolean threeD) {  
 for (int i=1; i<n; i++) if(threeD) B[i].reset3D(); else B[i].reset();
 repell();
  }
void repell() { for (int i=1; i<10; i++) distribute();}
void distribute () {
   for (int i=0; i<n; i++) W[i].setTo(0,0,0);
   for (int i=0; i<n; i++) for (int j=0; j<n; j++)  if ( (i!=j) && clash(B[i],B[j])) W[i].add(B[j].C.vecTo(B[i].C));
   for (int i=0; i<n; i++) B[i].C.addVec(0.3,W[i]);
  }
void shock(int i, int j)    { 
  if (method==1) {v3 T=B[i].V.make(); B[i].V.setTo(B[j].V); B[j].V.setTo(T); B[i].move(); B[j].move(); }; // wrong implementation
  if (method==2) turn(B[i],B[j]);
  if (method==3) {float dt=backUp(B[i],B[j]); turn(B[i],B[j]); advance(B[i],B[j],-dt); };
  } 
boolean clash(BALL B1, BALL B2) {return B1.C.disTo(B2.C) < B1.r+B2.r ; } 
float collisionTime(BALL B1, BALL B2) { return 0; /* compute negative time offset to most recent collision ??? */ }  
float backUp (BALL B1, BALL B2) {float dt=collisionTime(B1,B2);  advance(B1,B2,dt); return dt;}
void advance (BALL B1, BALL B2, float dt) {B1.move(dt);  B2.move(dt);}
void turn (BALL B1, BALL B2) {/* change velocities by swapping the normal velocity components (with attenuation) ??? */  }

void keyPressed() {  
   if (key=='l') {n=5; row();}
   if (key=='p') {repell();}
   if (key=='+') {n++; repell();}
   if (key=='-') {n--; if(n<1) n=1; }
   if (key=='h') { setBalls(false);}
   if (key=='H') { setBalls(true);}
   if (key=='1') {method=1;}
   if (key=='2') {method=2;}
   if (key=='3') {method=3;}
   if (key=='X') {String S="bounce"+"-####.tif"; saveFrame(S);};   // makes a picture
   };