const DouglasPeucker = {
  getProcessPoints: function (points, tolerance) {
    /// <summary>获取处理后的点</summary>
    /// <param name="points" type="Array">包含点的数组</param>
    /// <param name="tolerance" type="Float">取样临界值</param>
    /// <returns type="Array" />
    if (!Array.isArray(points) || !points.length) { //当points不是数组或没有值时，直接返回一个空数组
      return [];
    }
    if (points.length < 3) return points; //小于3个点时不抽稀，因为1个或2个点无法进行抽稀
    let firstPoint = 0,
      lastPoint = points.length - 1; //取开始点与结束点的下标
    let pointIndexsToKeep = []; //保存需要点下标的数组
    pointIndexsToKeep.push(firstPoint);
    pointIndexsToKeep.push(lastPoint); //开始与结束不进行处理，直接保留
    while (points[firstPoint] == points[lastPoint]) { //处理闭合情况，闭合时，强制断开
      lastPoint--;
    }
    this.reduce(points, firstPoint, lastPoint, tolerance, pointIndexsToKeep); //抽稀
    let resultPoints = []; //返回的点数组
    pointIndexsToKeep.sort(function (a, b) { //排序，这个可排可不排
      return a - b;
    });
    for (let i = 0; i < pointIndexsToKeep.length; i++) {
      resultPoints.push(points[pointIndexsToKeep[i]]);
    }
    return resultPoints;
  },
  reduce: function (points, firstPoint, lastPoint, tolerance, pointIndexsToKeep) {
    /// <summary>抽稀处理</summary>
    /// <param name="points" type="Array">待抽稀的数组</param>
    /// <param name="firstPoint" type="Integer">起点</param>
    /// <param name="lastPoint" type="Integer">终点</param>
    /// <param name="tolerance" type="Float">取样临界值</param>
    /// <param name="pointIndexsToKeep" type="Array">保留点下标的数组</param>
    let maxDis = 0,
      idxFarthest = 0; //定义最大长度及最远点的下标
    for (let i = firstPoint, dis; i < lastPoint; i++) {
      dis = this.perpendicularDistance(points[firstPoint], points[lastPoint], points[i]); //获取当前点到起点与
      if (dis > maxDis) { //保存最远距离
        maxDis = dis;
        idxFarthest = i;
      }
    }
    if (maxDis > tolerance && idxFarthest != 0) { //如果最远距离大于临界值
      pointIndexsToKeep.push(idxFarthest);
      this.reduce(points, firstPoint, idxFarthest, tolerance, pointIndexsToKeep);
      this.reduce(points, idxFarthest, lastPoint, tolerance, pointIndexsToKeep);
    }
  },
  perpendicularDistance: function (beginPoint, endPoint, comparePoint) {
    /// <summary>计算给出的comparePoint到beginPoint与endPoint组成的直线的垂直距离</summary>
    /// <param name="beginPoint" type="Object">起始点</param>
    /// <param name="endPoint" type="Object">结束点</param>
    /// <param name="comparePoint" type="Object">比较点</param>
    /// <returns type="Float" />
    //Area = |(1/2)(x1y2 + x2y3 + x3y1 - x2y1 - x3y2 - x1y3)|   *Area of triangle
    //Base = v((x1-x2)2+(y1-y2)2)                               *Base of Triangle*
    //Area = .5*Base*H                                          *Solve for height
    //Height = Area/.5/Base
    let area = Math.abs(0.5 * (beginPoint.x * endPoint.y + endPoint.x * comparePoint.y + comparePoint.x * beginPoint.y -
      endPoint.x * beginPoint.y - comparePoint.x * endPoint.y - beginPoint.x * comparePoint.y));
    let bottom = Math.sqrt(Math.pow(beginPoint.x - endPoint.x, 2) + Math.pow(beginPoint.y - endPoint.y, 2));
    let height = area / bottom * 2;
    return height;
  }
};
export default DouglasPeucker;