node_modules/d3-shape/src/curve/basis.js

export function point(that, x, y) {
  that._context.bezierCurveTo(
    (2 * that._x0 + that._x1) / 3,
    (2 * that._y0 + that._y1) / 3,
    (that._x0 + 2 * that._x1) / 3,
    (that._y0 + 2 * that._y1) / 3,
    (that._x0 + 4 * that._x1 + x) / 6,
    (that._y0 + 4 * that._y1 + y) / 6
  );
}

export function Basis(context) {
  this._context = context;
}

Basis.prototype = {
  areaStart: function() {
    this._line = 0;
  },
  areaEnd: function() {
    this._line = NaN;
  },
  lineStart: function() {
    this._x0 = this._x1 =
    this._y0 = this._y1 = NaN;
    this._point = 0;
  },
  lineEnd: function() {
    switch (this._point) {
      case 3: point(this, this._x1, this._y1); // falls through
      case 2: this._context.lineTo(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; break;
      case 2: this._point = 3; this._context.lineTo((5 * this._x0 + this._x1) / 6, (5 * this._y0 + this._y1) / 6); // falls through
      default: point(this, x, y); break;
    }
    this._x0 = this._x1, this._x1 = x;
    this._y0 = this._y1, this._y1 = y;
  }
};

export default function(context) {
  return new Basis(context);
}