/*
  You should call the function resolveCollsion, when a collision between two
  circles is detected.
*/

function rotate(velocity, angle) {
    const rotatedVelocities = {
        x: velocity.x * Math.cos(angle) - velocity.y * Math.sin(angle),
        y: velocity.x * Math.sin(angle) + velocity.y * Math.cos(angle)
    }

    return rotatedVelocities;
}

function resolveCollision(particle, otherParticle) {
      /*
        the particles passed as parameters should be objects with
           x and y position,
        a velocity object with x and y values,
        a mass value
        as attributes
        example of a class:
        
        function Circle() {
            this.x = x;
            this.y = y;
            this.velocity = {
                x: 5,
                y: 5
            };
            this.mass = 2;

        }
    */
    const xVelocityDiff = particle.velocity.x - otherParticle.velocity.x;
    const yVelocityDiff = particle.velocity.y - otherParticle.velocity.y;

    const xDist = otherParticle.x - particle.x;
    const yDist = otherParticle.y - particle.y;

    if (xVelocityDiff * xDist + yVelocityDiff * yDist >= 0) {
        const angle = -Math.atan2(otherParticle.y - particle.y, otherParticle.x - particle.x);

        const m1 = particle.mass;
        const m2 = otherParticle.mass;

        const u1 = rotate(particle.velocity, angle);
        const u2 = rotate(otherParticle.velocity, angle);

        const v1 = { x: u1.x * (m1 - m2) / (m1 + m2) + u2.x * 2 * m2 / (m1 + m2), y: u1.y };
        const v2 = { x: u2.x * (m1 - m2) / (m1 + m2) + u1.x * 2 * m2 / (m1 + m2), y: u2.y };

        const vFinal1 = rotate(v1, -angle);
        const vFinal2 = rotate(v2, -angle);

        particle.velocity.x = vFinal1.x;
        particle.velocity.y = vFinal1.y;

        otherParticle.velocity.x = vFinal2.x;
        otherParticle.velocity.y = vFinal2.y;
    }
}

/*
    source:
    https://youtu.be/789weryntzM
*/