const DAMPING = 0.03;
const DRAG = 1 - DAMPING;
const MASS = 0.1;
const GRAVITY = new THREE.Vector3(0, -9.81 * MASS, 0);
const K = 1;

let tmpCorrection;

class Constraint {
  constructor(p1, p2, restDist) {
    this.p1 = p1;
    this.p2 = p2;
    this.restDist = restDist;
  }

  satisfy() {
    const diff = this.p2.position.clone().sub(this.p1.position);
    const currentDist = diff.length();
    if (currentDist == 0) return;
    if (currentDist <= this.restDist) return;
    const correction = diff.multiplyScalar(1 - (this.restDist / currentDist));
    correction.multiplyScalar(K);
    tmpCorrection = correction;
    const correctionHalf = correction.multiplyScalar(0.5);
    this.p1.position.add(correctionHalf);
    this.p2.position.sub(correctionHalf);
  }
}

class Particle {
  movable = true;

  constructor(x, y, z, mass) {
    this.position = new THREE.Vector3(x, y, z);
    this.previous = new THREE.Vector3(x, y, z);
    this.acceleration = new THREE.Vector3(0, 0, 0);
    this.mass = mass;
  }
  addForce(force) {
    this.acceleration.add(
      force.clone().multiplyScalar(1/this.mass)
    );
  }
  verlet(dt) {
    // verlet algorithm
    // next position = 2 * current Position - previous position + acceleration * (passed time)^2
    // acceleration (dv/dt) = F(net)
    const nextPosition = this.position.clone().sub(this.previous);
    nextPosition.multiplyScalar(DRAG);
    nextPosition.add(this.position);
    nextPosition.add(this.acceleration.multiplyScalar(dt*dt));

    this.previous = this.position;
    this.position = nextPosition;

    this.acceleration.set(0, 0, 0);
  }
}

class Cloth {
  constructor(width, height, numPointsWidth, numPointsHeight) {
    this.width = width;
    this.height = height;
    this.numPointsWidth = numPointsWidth;
    this.numPointsHeight = numPointsHeight;
    this.windFactor = new THREE.Vector3(0.5, 0.2, 0.2);

    /**
     * distance between two vertices horizontally/vertically
     * divide by the number of points minus one
     * because there are (n - 1) lines between n vertices
     */
    let stepWidth = width / (numPointsWidth - 1);
    let stepHeight = height / (numPointsHeight - 1);

    /**
     * iterate over the number of vertices in x/y axis
     * and add a new Particle to "particles"
     */
    this.particles = [];
    for (let y = 0; y < numPointsHeight; y++) {
      for (let x = 0; x < numPointsWidth; x++) {
        this.particles.push(
          new Particle(
            (x - ((numPointsWidth-1)/2)) * stepWidth,
            height - (y + ((numPointsHeight-1)/2)) * stepHeight,
            0,
            MASS)
        );
      }
    }

    this.particles[this.getVertexIndex(0, 0)].movable = false;
    this.particles[this.getVertexIndex(0, numPointsHeight-1)].movable = false;
    this.particles[this.getVertexIndex(numPointsWidth-1, 0)].movable = false;

    const REST_DIST_X = width / (numPointsWidth-1);
    const REST_DIST_Y = height / (numPointsHeight-1);

    /**
     * generate constraints (springs)
     */
    this.constraints = [];
    for (let y = 0; y < numPointsHeight; y++) {
      for (let x = 0; x < numPointsWidth; x++) {
        if (x < numPointsWidth-1) {
          this.constraints.push(new Constraint(
            this.particles[this.getVertexIndex(x, y)],
            this.particles[this.getVertexIndex(x+1, y)],
            REST_DIST_X
          ));
        }
        if (y < numPointsHeight-1) {
          this.constraints.push(new Constraint(
            this.particles[this.getVertexIndex(x, y)],
            this.particles[this.getVertexIndex(x, y+1)],
            REST_DIST_Y
          ));
        }
      }
    }
  }
  generateGeometry() {
    const geometry = new THREE.BufferGeometry();

    const vertices = [];
    const normals = [];
    const indices = [];

    for (let particle of this.particles) {
      vertices.push(
        particle.position.x,
        particle.position.y,
        particle.position.z);
    }

    const numPointsWidth = this.numPointsWidth;
    const numPointsHeight = this.numPointsHeight;

    /**
     * generate faces based on 4 vertices
     * and 6 springs each
     */
    for (let y = 0; y < numPointsHeight - 1; y++) {
      for (let x = 0; x < numPointsWidth - 1; x++) {
        indices.push(
          this.getVertexIndex(x, y),
          this.getVertexIndex(x+1, y),
          this.getVertexIndex(x+1, y+1)
        );
        indices.push(
          this.getVertexIndex(x, y),
          this.getVertexIndex(x+1, y+1),
          this.getVertexIndex(x, y+1)
        );
      }
    }

    geometry.setIndex(indices);
    geometry.setAttribute('position', new THREE.Float32BufferAttribute(vertices, 3));
    //geometry.setAttribute('normal', new THREE.Float32BufferAttribute(normals, 3));
    geometry.computeBoundingSphere();
    geometry.computeVertexNormals();

    return geometry;
  }
  updateGeometry(geometry) {
    const positions = geometry.attributes.position.array;
    for (let i in this.particles) {
      let p = this.particles[i];
      if (p.movable) {
        positions[i*3+0] = p.position.x;
        positions[i*3+1] = p.position.y;
        positions[i*3+2] = p.position.z;
      } else {
        p.position = p.previous;
      }
    }
    geometry.attributes.position.needsUpdate = true;
    geometry.computeBoundingSphere();
    geometry.computeVertexNormals();
  }
  simulate(dt) {
    let now = performance.now();
    for (let particle of this.particles) {
      let vertex = particle.position;
      let fWind = new THREE.Vector3(
        this.windFactor.x * (Math.sin(vertex.x * vertex.y * now)+1),
        this.windFactor.y * Math.cos(vertex.z * now),
        this.windFactor.z * Math.sin(Math.cos(5 * vertex.x * vertex.y * vertex.z))
      );
      // normalize then multiply?
      particle.addForce(fWind);
      // calculate wind with normal?

      particle.addForce(GRAVITY);

      particle.verlet(dt);
    }

    
    for (let constraint of this.constraints) {
      constraint.satisfy();
    }
    //console.log(tmpCorrection);
  }
  /**
   * helper function to calculate index of vertex
   * in "vertices" array based on its x and y positions
   * in the mesh
   * @param {number} x - x index of vertex
   * @param {number} y - y index of vertex
   */
  getVertexIndex(x, y) {
    return y * this.numPointsWidth + x;
  }
}