constructor(x, y) {

    this.pos = createVector(x, y);

    this.vel = createVector(0, 0);

    this.r = BALL_RADIUS;

  }

​

  show() {

    noStroke();

    fill(255);

    ellipse(this.pos.x, this.pos.y, this.r);

  }

​

  update() {

    if (this.vel.mag() < 0.1) this.vel = createVector(0, 0);

    if (this.vel.mag() < 2) this.vel.mult(0.98);

    this.vel.mult(0.98);

    this.pos.add(this.vel.copy());

  }

​

  static transposeMatrix2x2(matrix) {

    return [matrix[0], matrix[2], matrix[1], matrix[3]];

  }

​

  static createRotationMatrixFromAngle(theta) {

    let c = Math.cos(theta);

    let s = Math.sin(theta);

    return [c, s, -s, c];

  }

​

  static matrix2x2MultiplyVector2(matrix, vector) {

    let MatrixRow1Vec = createVector(matrix[0], matrix[1]);

    let MatrixRow2Vec = createVector(matrix[2], matrix[3]);

​

    return createVector(vector.dot(MatrixRow1Vec), vector.dot(MatrixRow2Vec));

  }

​

  static collisionReboundSingleAxis(particle1, particle2) {

    let collisionForce =  particle2.vel.x;

    return collisionForce;

  }

​

  static billiardCollision(particle1, particle2) {

    let distVec = particle2.pos.copy().sub(particle1.pos);

​

    // calculate the angle

    let angle = Math.atan2(distVec.y, distVec.x);

​

    let mat = Ball.createRotationMatrixFromAngle(angle);

    let inverseRotation = Ball.transposeMatrix2x2(mat);

​

    // rotate the velocities of the particles so that the collision

    // happens on a single axis (x-axis) rather than 2 axes

    let vel1 = Ball.matrix2x2MultiplyVector2(mat, particle1.vel.copy());

    let vel2 = Ball.matrix2x2MultiplyVector2(mat, particle2.vel.copy());

​

    // collision reacton

    let vxTotal = vel1.x - vel2.x;

​

    vel1.x = Ball.collisionReboundSingleAxis(particle1, particle2, vel1, vel2);

    vel2.x = vxTotal + vel1.x;

​

    // particles often get stuck if they are going the same direction

    // a dot product greater than zero means they are going the same direction

    // reversing the velocity of one of the particles helps to keep them from getting stuck

    if (vel1.dot(vel2) > 0)  vel1.x *= -1;

​

    // position adjustment

    let particle1Adjustment = Ball.matrix2x2MultiplyVector2(inverseRotation, createVector(0, 0));

    particle1.pos = particle1.pos.copy().add(particle1Adjustment);

    particle2.pos = particle1.pos.copy().add(distVec);

​

    // rotate velocity back

    let reboundVelocity1 = Ball.matrix2x2MultiplyVector2(inverseRotation, vel1);

    let reboundVelocity2 = Ball.matrix2x2MultiplyVector2(inverseRotation, vel2);

​

    particle1.vel = reboundVelocity1;

    particle2.vel = reboundVelocity2;

  }

}