/**
 *  Convenience Function for calculating the distance between two vectors
 *  because THREE JS Vector functions mutate variables
 * @param {Vector3} a - Vector A
 * @param {Vector3} b - Vector B
 */
function vectorLength(a, b) {
  let v1 = new THREE.Vector3();
  v1.set(a.x, a.y, a.z);
  let v2 = new THREE.Vector3();
  v2.set(b.x, b.y, b.z);

  return v1.sub(v2).length();
}

/**
 * Class representing a quad face
 * Each face consists of two triangular mesh faces
 * containts four indices for determining vertices
 * and six springs, one between each of the vertices
 */
export class Face {
  a;
  b;
  c;
  d;

  springs = [];

  constructor(a, b, c, d) {
    this.a = a;
    this.b = b;
    this.c = c;
    this.d = d;
  }
}

/**
 * Class representing a single spring
 * has a current and resting length
 * and indices to the two connected vertices
 */
export class Spring {
  restLength;
  currentLength;
  index1;
  index2;

 
  /**
   * set vertex indices
   * and calculate inital length based on the
   * vertex positions
   * @param {Array of Vector3} vertices 
   * @param {number} index1 
   * @param {number} index2 
   */
  constructor(vertices, index1, index2) {
    this.index1 = index1;
    this.index2 = index2;

    let length = vectorLength(vertices[index1], vertices[index2]);
    this.restLength = length;
    this.currentLength = length;
  }
}

/**
 * Class representing a single piece of cloth
 * contains THREE JS geometry,
 * logically represented by an array of adjacent faces
 * and vertex weights which are accessed by the same
 * indices as the vertices in the Mesh
 */
export class Cloth {
  VertexWeight = 1;

  geometry = new THREE.Geometry();

  faces = [];

  vertexWeights = [];

  
  /**
   * creates a rectangular piece of cloth
   * takes the size of the cloth
   * and the number of vertices it should be composed of
   * @param {number} width - width of the cloth
   * @param {number} height - height of the cloth
   * @param {number} numPointsWidth - number of vertices in horizontal direction
   * @param {number} numPointsHeight  - number of vertices in vertical direction
   */
  createBasic(width, height, numPointsWidth, numPointsHeight) {
    /** resulting vertices and faces */
    let vertices = [];
    let faces = [];

    /**
     * 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 Vector3 to "vertices"
     */
    for (let y = 0; y < numPointsHeight; y++) {
      for (let x = 0; x < numPointsWidth; x++) {
        vertices.push(
          new THREE.Vector3(x * stepWidth, height - y * stepHeight, 0)
        );
      }
    }

    /**
     * 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
     */
    function getVertexIndex(x, y) {
      return y * numPointsWidth + x;
    }
    
    /**
     * 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++) {
        let newFace = new Face(
          getVertexIndex(x, y),
          getVertexIndex(x, y + 1),
          getVertexIndex(x + 1, y),
          getVertexIndex(x + 1, y + 1),
        );

        newFace.springs.push(new Spring(vertices, getVertexIndex(x, y), getVertexIndex(x + 1, y)));
        newFace.springs.push(new Spring(vertices, getVertexIndex(x, y), getVertexIndex(x, y + 1)));
        newFace.springs.push(new Spring(vertices, getVertexIndex(x, y), getVertexIndex(x + 1, y + 1)));
        newFace.springs.push(new Spring(vertices, getVertexIndex(x + 1, y), getVertexIndex(x, y + 1)));
        newFace.springs.push(new Spring(vertices, getVertexIndex(x + 1, y), getVertexIndex(x + 1, y + 1)));
        newFace.springs.push(new Spring(vertices, getVertexIndex(x, y + 1), getVertexIndex(x + 1, y + 1)));
        
        faces.push(newFace);
      }
    }

    /**
     * call createExplicit
     * with generated vertices and faces
     */
    this.createExplicit(vertices, faces);
  }

  /**
   * Generate THREE JS Geometry
   * (list of vertices and list of indices representing triangles)
   * and calculate the weight of each face and split it between
   * surrounding vertices
   * @param {Array of Vector3} vertices 
   * @param {Array of Face} faces 
   */
  createExplicit(vertices, faces) {
    /**
     * Copy vertices and initialize vertex weights to 0
     */
    for (let i in vertices) {
      this.geometry.vertices.push(vertices[i]);
      this.vertexWeights.push(0);
    }
    /**
     * copy faces,
     * generate two triangles per face,
     * calculate weight of face as its area
     * and split between the 4 vertices
     */
    for (let i in faces) {
      let face = faces[i];

      /** copy faces to class member */
      this.faces.push(face);

      /** generate triangles */
      this.geometry.faces.push(new THREE.Face3(
        face.a, face.b, face.c
      ));
      this.geometry.faces.push(new THREE.Face3(
        face.c, face.b, face.d
      ));
      
      /**
       * calculate area of face as combined area of
       * its two composing triangles
       */
      let xLength = vectorLength(this.geometry.vertices[face.b], this.geometry.vertices[face.a]);
      let yLength = vectorLength(this.geometry.vertices[face.c], this.geometry.vertices[face.a]);
      let weight = xLength * yLength / 2;

      xLength = vectorLength(this.geometry.vertices[face.b], this.geometry.vertices[face.d]);
      yLength = vectorLength(this.geometry.vertices[face.c], this.geometry.vertices[face.d]);
      weight += xLength * yLength / 2;

      /**
       * split weight equally between four surrounding vertices
       */
      this.vertexWeights[face.a] += weight / 4;
      this.vertexWeights[face.b] += weight / 4;
      this.vertexWeights[face.c] += weight / 4;
      this.vertexWeights[face.d] += weight / 4;
    }

    /**
     * let THREE JS compute bounding sphere around generated mesh
     * needed for View Frustum Culling internally
     */
    this.geometry.computeBoundingSphere();
  }

  /**
   * generate a debug mesh for visualizing
   * vertices and springs of the cloth
   * and add it to scene for rendering
   * @param {Scene} scene - Scene to add Debug Mesh to
   */
  createDebugMesh(scene) {
    /**
     * helper function to generate a single line
     * between two Vertices with a given color
     * @param {Vector3} from 
     * @param {Vector3} to 
     * @param {number} color 
     */
    function addLine(from, to, color) {
      let geometry = new THREE.Geometry();
      geometry.vertices.push(from);
      geometry.vertices.push(to);
      let material = new THREE.LineBasicMaterial( { color: color, linewidth: 10 } );
      let line = new THREE.Line(geometry, material);
      line.renderOrder = 1;
      scene.add(line);
    }
    /**
     * helper function to generate a small sphere
     * at a given Vertex Position with color
     * @param {Vector3} point 
     * @param {number} color 
     */
    function addPoint(point, color) {
      const geometry = new THREE.SphereGeometry( 0.05, 32, 32 );
      const material = new THREE.MeshBasicMaterial( { color: color } );
      const sphere = new THREE.Mesh( geometry, material );
      sphere.position.set(point.x, point.y, point.z);
      scene.add( sphere );
    }

    let lineColor = 0x000000;
    let pointColor = 0xff00000;

    /**
     * generate one line for each of the 6 springs
     * and one point for each of the 4 vertices
     * for all of the faces
     */
    for (let i in this.faces) {
      let face = this.faces[i];
      addLine(this.geometry.vertices[face.a], this.geometry.vertices[face.b], lineColor);
      addLine(this.geometry.vertices[face.a], this.geometry.vertices[face.c], lineColor);
      addLine(this.geometry.vertices[face.a], this.geometry.vertices[face.d], lineColor);
      addLine(this.geometry.vertices[face.b], this.geometry.vertices[face.c], lineColor);
      addLine(this.geometry.vertices[face.b], this.geometry.vertices[face.d], lineColor);
      addLine(this.geometry.vertices[face.c], this.geometry.vertices[face.d], lineColor);

      addPoint(this.geometry.vertices[face.a], pointColor);
      addPoint(this.geometry.vertices[face.b], pointColor);
      addPoint(this.geometry.vertices[face.c], pointColor);
      addPoint(this.geometry.vertices[face.d], pointColor);
    }
  }
}