source: node_modules/d3-shape/src/curve/cardinal.js

export function point(that, x, y) {
  that._context.bezierCurveTo(
    that._x1 + that._k * (that._x2 - that._x0),
    that._y1 + that._k * (that._y2 - that._y0),
    that._x2 + that._k * (that._x1 - x),
    that._y2 + that._k * (that._y1 - y),
    that._x2,
    that._y2
  );
}

export function Cardinal(context, tension) {
  this._context = context;
  this._k = (1 - tension) / 6;
}

Cardinal.prototype = {
  areaStart: function() {
    this._line = 0;
  },
  areaEnd: function() {
    this._line = NaN;
  },
  lineStart: function() {
    this._x0 = this._x1 = this._x2 =
    this._y0 = this._y1 = this._y2 = NaN;
    this._point = 0;
  },
  lineEnd: function() {
    switch (this._point) {
      case 2: this._context.lineTo(this._x2, this._y2); break;
      case 3: point(this, this._x1, this._y1); break;
    }
    if (this._line || (this._line !== 0 && this._point === 1)) this._context.closePath();
    this._line = 1 - this._line;
  },
  point: function(x, y) {
    x = +x, y = +y;
    switch (this._point) {
      case 0: this._point = 1; this._line ? this._context.lineTo(x, y) : this._context.moveTo(x, y); break;
      case 1: this._point = 2; this._x1 = x, this._y1 = y; break;
      case 2: this._point = 3; // falls through
      default: point(this, x, y); break;
    }
    this._x0 = this._x1, this._x1 = this._x2, this._x2 = x;
    this._y0 = this._y1, this._y1 = this._y2, this._y2 = y;
  }
};

export default (function custom(tension) {

  function cardinal(context) {
    return new Cardinal(context, tension);
  }

  cardinal.tension = function(tension) {
    return custom(+tension);
  };

  return cardinal;
})(0);