Quadtree Search by mbostock

This demonstrates finding the closest point to the mouse by searching a quadtree. As you descend into the quadtree, you track the closest point yet found and avoid searching cells that cannot contain closer points. Importantly, you must descend preferentially into closer cells so as to restrict the search more quickly.

Patrick Surry implemented a similar solution months earlier!

index.html

<!DOCTYPE html>
<meta charset="utf-8">
<style>

.point {
  fill: #000;
  fill-opacity: 0.4;
}

.point--selected {
  fill: red;
  fill-opacity: 1;
  stroke: red;
  stroke-width: 5px;
}

.node {
  fill: none;
  stroke: #ccc;
  shape-rendering: crispEdges;
}

</style>
<svg width="960" height="500"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script>

var svg = d3.select("svg").on("mousemove", mousemoved),
    width = +svg.attr("width"),
    height = +svg.attr("height"),
    selected;

var random = Math.random,
    data = d3.range(2500).map(function() { return [random() * width, random() * height]; });

var quadtree = d3.quadtree()
    .extent([[-1, -1], [width + 1, height + 1]])
    .addAll(data);

svg.selectAll(".node")
  .data(nodes(quadtree))
  .enter().append("rect")
    .attr("class", "node")
    .attr("x", function(d) { return d.x0; })
    .attr("y", function(d) { return d.y0; })
    .attr("width", function(d) { return d.y1 - d.y0; })
    .attr("height", function(d) { return d.x1 - d.x0; });

var point = svg.selectAll(".point")
  .data(data)
  .enter().append("circle")
    .attr("class", "point")
    .attr("cx", function(d) { return d[0]; })
    .attr("cy", function(d) { return d[1]; })
    .attr("r", 2);

function mousemoved() {
  var m = d3.mouse(this), p = quadtree.find(m[0], m[1]);
  point.classed("point--selected", function(d) { return d === p; });
}

// Collapse the quadtree into an array of rectangles.
function nodes(quadtree) {
  var nodes = [];
  quadtree.visit(function(node, x0, y0, x1, y1) {
    node.x0 = x0, node.y0 = y0;
    node.x1 = x1, node.y1 = y1;
    nodes.push(node);
  });
  return nodes;
}

</script>