Double Density Relaxation

class Simulation{
    constructor(){
        this.particles = [];
        

        this.AMOUNT_PARTICLES = 2000;
        this.VELOCITY_DAMPING = 1;
        this.GRAVITY = new Vector2(0,1);
        this.REST_DENSITY = 10;
        this.K_NEAR = 3;
        this.K = 0.5;
        this.INTERACTION_RADIUS = 25;

        this.fluidHashGrid = new FluidHashGrid(this.INTERACTION_RADIUS);
        this.instantiateParticles();
        this.fluidHashGrid.initialize(this.particles);
    }

    instantiateParticles(){
        let offsetBetweenParticles = 10;
        let offsetAllParticles = new Vector2(750, 100);

        let xParticles = Math.sqrt(this.AMOUNT_PARTICLES);
        let yParticles = xParticles;


        for(let x=0; x< xParticles; x++){
            for(let y=0; y< yParticles; y++){
                let position = new Vector2(
                    x*offsetBetweenParticles + offsetAllParticles.x,
                    y*offsetBetweenParticles + offsetAllParticles.y);

                let particle = new Particle(position);
                //particle.velocity = Scale(new Vector2(-0.5 + Math.random(),-0.5 + Math.random()), 200);


                this.particles.push(particle);
            }
        }
    }

    neighbourSearch(){
        this.fluidHashGrid.clearGrid();
        this.fluidHashGrid.mapParticleToCell();
    }

    update(dt){
        this.applyGravity(dt);

        this.predictPositions(dt);

        this.neighbourSearch();

        this.doubleDensityRelaxation(dt);

        this.worldBoundary();

        this.computeNextVelocity(dt);
    }

    doubleDensityRelaxation(dt){
        for(let i=0; i< this.particles.length; i++){
			let density = 0;
			let densityNear = 0;
			let neighbours = this.fluidHashGrid.getNeighbourOfParticleIdx(i);
			let particleA = this.particles[i];

			for(let j = 0; j < neighbours.length;j++){
				let particleB = neighbours[j];
				if(particleA == particleB) continue;
				
				
				let rij = Sub(particleB.position,particleA.position);
				let q = rij.Length() / this.INTERACTION_RADIUS;
				
				if(q < 1){
					let oneMinusQ = (1-q);
					density += oneMinusQ*oneMinusQ;
					densityNear += oneMinusQ*oneMinusQ*oneMinusQ;					
				}
			}

            let pressure = this.K  (density - this.REST_DENSITY);
            let pressureNear = this.K_NEAR  densityNear;
            let particleADisplacement = Vector2.Zero();

            
            for(let j=0; j< neighbours.length; j++){
                let particleB = neighbours[j];
                if(particleA == particleB){
                    continue;
                }

                let rij = Sub(particleB.position, particleA.position);
                let q = rij.Length() / this.INTERACTION_RADIUS;

                if(q < 1.0){
                    rij.Normalize();
                    let displacementTerm = Math.pow(dt, 2)  
                        (pressure  (1-q) + pressureNear  Math.pow(1-q, 2));
                    let D = Scale(rij, displacementTerm);

                    particleB.position = Add(particleB.position, Scale(D,0.5));
                    particleADisplacement = Sub(particleADisplacement, Scale(D,0.5));
                }
            }
            particleA.position = Add(particleA.position, particleADisplacement);
        }
    }

    applyGravity(dt){
        for(let i=0; i< this.particles.length; i++){
            this.particles[i].velocity = Add(this.particles[i].velocity, Scale(this.GRAVITY, dt));
        }
    }

    predictPositions(dt){
        for(let i=0; i< this.particles.length; i++){
            this.particles[i].prevPosition = this.particles[i].position.Cpy();
            let positionDelta = Scale(this.particles[i].velocity, dt  this.VELOCITY_DAMPING);
            this.particles[i].position = Add(this.particles[i].position, positionDelta);
        }
    }

    computeNextVelocity(dt){
        for(let i=0; i< this.particles.length; i++){
            let velocity = Scale(Sub(this.particles[i].position, this.particles[i].prevPosition), 1.0 / dt);
            this.particles[i].velocity = velocity;
        }
    }

    worldBoundary(){
        for(let i=0; i< this.particles.length; i++){
            let pos = this.particles[i].position;

            if(pos.x < 0){
                this.particles[i].position.x = 0;
                this.particles[i].prevPosition.x = 0;
            }
            if(pos.y < 0){
                this.particles[i].position.y = 0;
                this.particles[i].prevPosition.y = 0;
            }
            if(pos.x > canvas.width){
                this.particles[i].position.x = canvas.width-1;
                this.particles[i].prevPosition.x = canvas.width-1;
            }
            if(pos.y > canvas.height){
                this.particles[i].position.y = canvas.height-1;
                this.particles[i].prevPosition.y = canvas.height-1;
            }
        }
    }


    draw(){
        for(let i=0; i< this.particles.length; i++){
            let position = this.particles[i].position;
            let color = this.particles[i].color;
            DrawUtils.drawPoint(position, 5, color);
        }
    }
}