flow like the river

BACK_BACK/node_modules/d3-quadtree/src/cover.js

@@ -0,0 +1,43 @@
+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 || 1,
+        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;
+}