export default function(x, y) {
  if (isNaN(x = +x) || isNaN(y = +y)) return this; // ignore invalid points

  var x0 = this._x0,
      y0 = this._y0,
      x1 = this._x1,
      y1 = this._y1;

  // If the quadtree has no extent, initialize them.
  // Integer extent are necessary so that if we later double the extent,
  // the existing quadrant boundaries don’t change due to floating point error!
  if (isNaN(x0)) {
    x1 = (x0 = Math.floor(x)) + 1;
    y1 = (y0 = Math.floor(y)) + 1;
  }

  // Otherwise, double repeatedly to cover.
  else {
    var z = x1 - x0,
        node = this._root,
        parent,
        i;

    while (x0 > x || x >= x1 || y0 > y || y >= y1) {
      i = (y < y0) << 1 | (x < x0);
      parent = new Array(4), parent[i] = node, node = parent, z *= 2;
      switch (i) {
        case 0: x1 = x0 + z, y1 = y0 + z; break;
        case 1: x0 = x1 - z, y1 = y0 + z; break;
        case 2: x1 = x0 + z, y0 = y1 - z; break;
        case 3: x0 = x1 - z, y0 = y1 - z; break;
      }
    }

    if (this._root && this._root.length) this._root = node;
  }

  this._x0 = x0;
  this._y0 = y0;
  this._x1 = x1;
  this._y1 = y1;
  return this;
}