1/*
2  You should call the function resolveCollsion, when a collision between two
3  circles is detected.
4*/
5
6function rotate(velocity, angle) {
7    const rotatedVelocities = {
8        x: velocity.x * Math.cos(angle) - velocity.y * Math.sin(angle),
9        y: velocity.x * Math.sin(angle) + velocity.y * Math.cos(angle)
10    }
11
12    return rotatedVelocities;
13}
14
15function resolveCollision(particle, otherParticle) {
16  	/*
17    	the particles passed as parameters should be objects with
18       	x and y position,
19        a velocity object with x and y values,
20        a mass value
21        as attributes
22        example of a class:
23        
24        function Circle() {
25    		this.x = x;
26    		this.y = y;
27    		this.velocity = {
28            	x: 5,
29                y: 5
30            };
31    		this.mass = 2;
32
33		}
34    */
35    const xVelocityDiff = particle.velocity.x - otherParticle.velocity.x;
36    const yVelocityDiff = particle.velocity.y - otherParticle.velocity.y;
37
38    const xDist = otherParticle.x - particle.x;
39    const yDist = otherParticle.y - particle.y;
40
41    if (xVelocityDiff * xDist + yVelocityDiff * yDist >= 0) {
42        const angle = -Math.atan2(otherParticle.y - particle.y, otherParticle.x - particle.x);
43
44        const m1 = particle.mass;
45        const m2 = otherParticle.mass;
46
47        const u1 = rotate(particle.velocity, angle);
48        const u2 = rotate(otherParticle.velocity, angle);
49
50        const v1 = { x: u1.x * (m1 - m2) / (m1 + m2) + u2.x * 2 * m2 / (m1 + m2), y: u1.y };
51        const v2 = { x: u2.x * (m1 - m2) / (m1 + m2) + u1.x * 2 * m2 / (m1 + m2), y: u2.y };
52
53        const vFinal1 = rotate(v1, -angle);
54        const vFinal2 = rotate(v2, -angle);
55
56        particle.velocity.x = vFinal1.x;
57        particle.velocity.y = vFinal1.y;
58
59        otherParticle.velocity.x = vFinal2.x;
60        otherParticle.velocity.y = vFinal2.y;
61    }
62}
63
64/*
65	source:
66    https://youtu.be/789weryntzM
67*/