// Variance
int dia = 20;  //diameter
float[] posX = {10.0, 300.0};   //position X
float[] posY = {290.0, 290,0};   //position Y
//speed X = (290*9.8/59)/(2*spdY[0])
float[] spdX = {2.497244, 0.0};
//speed Y =SQRT(2*9.8/59*280)     
float[] spdY = {-9.644529, 0.0} ;     
int[][] col = {{255,128,0}, {0, 128, 255}};
float[] mass = {1.0, 1.0} ;     //weight
float[] elast = {1.0, 1.0} ;    //elast
int flagK = 0;  //flag by keyboard
float GRAV = 9.8;    //gravity

void setup(){   // Initialize
   size(400, 300);      //window
   background(0, 0, 0); //black
   noStroke();    //lineless
   frameRate(60);
}
void draw() {
  fill(0,0,0);  //clear color
  rect(0, 0, width, height);

if (flagK == 0) {  //watch at flag
    //draw balls
    for(int i=0;i<2;i++) {
      fill(col[i][0],col[i][1],col[i][2]);    //circle color
      ellipse(posX[i], posY[i], dia, dia);  //circle
    }
  } else {
    collisionDetect();
    
    posX[0] += spdX[0];
    spdY[0] += GRAV/60;   // gravity
    posY[0] += spdY[0];
    if( posY[0] >= height-dia/2 ) {
      posY[0] = height-dia/2;
    }
    fill(col[0][0],col[0][1],col[0][2]);    //circle color
    ellipse(posX[0], posY[0], dia, dia);  //circle
    
    posX[1] += spdX[1];
    posY[1] += spdY[1];
    fill(col[1][0],col[1][1],col[1][2]);    //circle color
    ellipse(posX[1], posY[1], dia, dia);  //circle
  }
}

void keyPressed() {
  if (key == ' ') {  //space
    flagK = 1;  //clear mode set
  }
}

void collisionDetect(){
 //distance of balls
  float bpx = posX[1] - posX[0];
  float bpy = posY[1] - posY[0];
  float dd = dist(0, 0, bpx, bpy);  

  if( dd <= dia ){
        //Angle along X axis
    float theta = atan2(bpy, bpx); 
    float sint = sin(theta);
    float cost = cos(theta);
    //Direction of ball0
    float tvx0 =   cost*spdX[0] + sint*spdY[0];
    float tvy0 = - sint*spdX[0] + cost*spdY[0];
    //Direction of ball1
    float tvx1 =   cost*spdX[1] + sint*spdY[1];
    float tvy1 = - sint*spdX[1] + cost*spdY[1];
    //Mass & coffeient
    float m0 = mass[0];
    float m1 = mass[1];
    float conf = elast[0]* elast[1];

    //Velocity based on conservation of momentum
    float tvx0n = ( ( m0 - conf*m1 ) * tvx0 + m1 * ( 1 + conf ) * tvx1 ) / ( m0 + m1 );
    float tvx1n = ( m0 * ( 1 + conf ) * tvx0 + ( m1 - conf*m0 ) * tvx1 ) / ( m0 + m1 );

    //Velocity each Ball
    spdX[0] = cost*tvx0n - sint*tvy0;
    spdY[0] = sint*tvx0n + cost*tvy0;
    spdX[1] = cost*tvx1n - sint*tvy1;
    spdY[1] = sint*tvx1n + cost*tvy1;
  }
}