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;
const MAX_STRETCH = 1.5;

const options = {
  wind: true,
};

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

  satisfy() {
    /** calculate current spring length */
    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));

    /** calculate necessary correction length and direction */
    const correction = diff.multiplyScalar((currentDist - this.restDist) / currentDist);
    correction.multiplyScalar(K);
    const correctionHalf = correction.multiplyScalar(0.5);

    let p1movable = this.p1.movable && this.p1.movableTmp;
    let p2movable = this.p2.movable && this.p2.movableTmp;

    /** apply correction if masses aren't fixed */
    /** divide correction if both are movable */
    if (p1movable && p2movable) {
      this.p1.position.add(correctionHalf);
      this.p2.position.sub(correctionHalf);
    } else if (! p1movable && p2movable) {
      this.p2.position.sub(correction);
    } else if (p1movable && ! p2movable) {
      this.p1.position.add(correction);
    }
  }
}

class Mass {
  movableTmp = true;
  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)
    /** calculate velocity */
    const nextPosition = this.position.clone().sub(this.previous);
    /** apply drag */
    nextPosition.multiplyScalar(DRAG);
    /** add to current position and add acceleration */
    nextPosition.add(this.position);
    nextPosition.add(this.acceleration.multiplyScalar(dt*dt));

    if (this.movable && this.movableTmp) {
      this.previous = this.position;
      this.position = nextPosition;
    }

    /** reset for next frame */
    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(3, 2, 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 "masses"
     */
    this.masses = [];
    for (let y = 0; y < numPointsHeight; y++) {
      for (let x = 0; x < numPointsWidth; x++) {
        this.masses.push(
          new Mass(
            (x - ((numPointsWidth-1)/2)) * stepWidth,
            height - (y + ((numPointsHeight-1)/2)) * stepHeight,
            0,
            MASS)
        );
      }
    }

    /** attach cloth to flag pole */
    const n = 3;
    for (let i = 0; i < numPointsHeight; i++)
      this.masses[this.getVertexIndex(0, i)].movable = false;

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

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

    const vertices = [];
    const indices = [];
    const uvs = [];

    /** create one vertex and one uv coordinate per mass */
    for (let i in this.masses) {
      let particle = this.masses[i];
      vertices.push(
        particle.position.x,
        particle.position.y,
        particle.position.z);
      uvs.push(
        this.getX(i) / (this.numPointsWidth-1),
        1 - (this.getY(i) / (this.numPointsHeight-1))
      );
    }

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

    /** set up geometry */
    geometry.setIndex(indices);
    geometry.setAttribute('position', new THREE.Float32BufferAttribute(vertices, 3));
    geometry.setAttribute('uv', new THREE.Float32BufferAttribute(uvs, 2));
    geometry.computeBoundingSphere();
    geometry.computeVertexNormals();

    return geometry;
  }
  updateGeometry(geometry) {
    /** update vertex positions in place */
    const positions = geometry.attributes.position.array;
    for (let i in this.masses) {
      let p = this.masses[i];
      positions[i*3+0] = p.position.x;
      positions[i*3+1] = p.position.y;
      positions[i*3+2] = p.position.z;
    }
    /** update internally and recalculate bounding volume */
    geometry.attributes.position.needsUpdate = true;
    geometry.computeBoundingSphere();
    geometry.computeVertexNormals();
  }
  simulate(dt) {
    let now = performance.now();
    for (let mass of this.masses) {
      /** accumulate acceleration:
       *  - wind
       *  - gravity
       */
      let vertex = mass.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?
      if (options.wind)
        mass.addForce(fWind);
      // calculate wind with normal?

      mass.addForce(GRAVITY);

      /** integrate motion */
      mass.verlet(dt);
    }

    /** run satisfy step */
    for (let constraint of this.springs) {
      constraint.satisfy();
    }

    /** prevent self-intersections */
    this.intersect();
  }

  intersect() {
    for (let i in this.masses) {
      for (let j in this.masses) {  
        let p1 = this.masses[i];
        let p2 = this.masses[j];

        p1.movableTmp = true;
        p2.movableTmp = true;

        /** skip if i == j or if masses are adjacent */
        if (i == j || (Math.abs(this.getX(i) - this.getX(j)) == 1 && Math.abs(this.getY(i) - this.getY(j)) == 1))
          continue;

        /** calculate distance of points  */
        let dist = p1.position.distanceTo(p2.position);
        /** calculate minimal resting distance (largest distance that should not be fallen below) */
        let collisionDistance = Math.min(this.width / this.numPointsWidth, this.height / this.numPointsHeight);
        // collisionDistance /= 2;
        /** calculate "sphere intersection" */
        if (dist < collisionDistance) {
          // p1.movableTmp = false;
          // p2.movableTmp = false;

          /** vectors from p1 to p2 and the other way round */
          let diffP2P1 = p1.position.clone().sub(p2.position).normalize();
          diffP2P1.multiplyScalar((collisionDistance - dist) * 1.001 / 2);
          let diffP1P2 = diffP2P1.clone().multiplyScalar(-1);

          // let v1 = p1.position.clone().sub(p1.previous).normalize();
          // let v2 = p2.position.clone().sub(p2.previous).normalize();

          // let factor1 = (Math.PI - Math.acos(v1.dot(diffP2P1))) / Math.PI * 2;
          // let factor2 = (Math.PI - Math.acos(v2.dot(diffP1P2))) / Math.PI * 2;

          /** move masses apart */
          if (p1.movable)
            p1.position.add(diffP2P1);
            //p1.position.add(diffP2P1.multiplyScalar(factor1));
          if (p2.movable)
            p2.position.add(diffP1P2);
            //p2.position.add(diffP1P2.multiplyScalar(factor2));
        }
      }
    }
  }
  blow(camPos, intersects) {
    let face = intersects[0].face;
    /** vector from cam to intersection (wind) */
    let dir = intersects[0].point.clone().sub(camPos).multiplyScalar(100);
    /** apply to all vertices of affected face */
    this.masses[face.a].addForce(dir);
    this.masses[face.b].addForce(dir);
    this.masses[face.c].addForce(dir);
  }
  drag(mousePosWorld, index) {
    /** calculate vector from vertex to cursor */
    let dir = mousePosWorld.clone().sub(this.masses[index].position).multiplyScalar(200);
    /** apply to grabbed vertex */
    this.masses[index].addForce(dir);
  }

  /**
   * 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;
  }
  getX(i) { return i % this.numPointsWidth; }
  getY(i) { return Math.floor(i / this.numPointsWidth); }
}