Literate Flatbush

Understanding a fast, elegant RTree implementation

Kyle Barron
January 8, 2025
Source code.

Spatial indexes are at the core of geospatial software engineering. Given a spatial query (“What items are within this bounding box“ or “What are the closest items to this point”), they allow for weeding out the vast majority of data, making a search massively faster than naively checking all items.

An RTree is one of the most common types of spatial indexes. An RTree indexes axis-aligned bounding boxes, and so can flexibly manage a variety of geospatial vector data, like points, lines, and polygons (by recording the bounding box represented by the minimum and maximum extents of a geometry’s coordinates).

But ever wondered how an RTree is actually implemented?

In this post we’ll dive into the implementation of Flatbush, a blazing-fast, memory-efficient RTree written in JavaScript by Volodymyr Agafonkin. While this implementation is written in JavaScript, it’s the algorithm that’s important here. Don’t get too caught up in the JavaScript; it should be easy to follow no matter what language you’re most familiar with.

I ported Flatbush to Rust with Python bindings, and this post is the result of my efforts to better understand and document how the algorithm works.

This post is a “literate” fork of the upstream Flatbush library. I’ve added comments to the code, and docco is used to generate the HTML file you’re reading now. Documentation and code are interspersed, letting you follow along with the code. No code modifications have been made in this fork; only comments have been added. The source for this fork is here.

All credit for this code included here goes to Volodymyr Agafonkin and other contributors to the Flatbush project, forked here under the ISC license. Any errors in explanation are mine alone.

Overview

The Flatbush algorithm generates a static, packed, ABI-stable RTree. Let’s break that down:

• RTree: a spatial index for storing geospatial vector data that allows for fast spatial queries.

  It’s a form of a “tree”. There’s one root node that has nodeSize children. Each of those nodes have their own nodeSize children, and so on. The tree structure allows you to avoid superfluous checks and quickly find matching candidates for your query. In particular, an RTree stores a bounding box for each geometry.

• static: the index is immutable. All geometries need to be added to the index before any searches can be done. Geometries can’t be added to an existing index later.

• packed: all nodes are at full capacity (except for the last node at each tree level). Because the tree is static, we don’t need to reserve space in each node for future additions. This improves memory efficiency.

• ABI-stable: the entire tree is stored in a single underlying memory buffer, with a well-defined, stable memory layout. This enables zero-copy sharing between threads (Web Workers in the browser) or, as in my Rust port, between two languages like Rust and Python.

Why Flatbush?

There are several nice features about Flatbush:

• Speed: This is likely the fastest static spatial index in JavaScript. Ports of the algorithm are among the fastest spatial indexes in other languages, too.
• Single, contiguous underlying buffer: The index is contained in a single ArrayBuffer, which makes it easy to share across multiple threads or persist and use later. In the process of building the index, there are only two buffer allocations: one for the main data buffer and a second intermediate one for the hilbert values.
• Memory-efficiency: because the index is fully packed, it’s highly memory efficient.
• Bounded-memory: for any given number of items and node size, you can infer the total memory that will be used by the RTree.
• Elegant and concise: Under 300 lines of JavaScript code and in my opinion it’s quite elegant how the structure of the tree implicitly maintains the insertion index.
• Used as the basis for other projects, like the FlatGeobuf geospatial file format.

What’s not to like? Keep in mind there are a few restrictions:

• Only two-dimensional data. Because the algorithm uses powers of two, only two-dimensional data is supported. It can be used with higher-dimensional input as long as you only index two of the dimensions.
• The index is immutable. After creating the index, items can no longer be added or removed.

Buffer layout

All bounding box and index data is stored in a single, contiguous buffer, with three parts:

• Header: an 8-byte header containing the coordinate array type, node size, and number of items.
• Boxes: the bounding box data for each input geometry and intermediate tree nodes.
• Indices: An ordering of boxes to allow for traversing the tree and retrieving the original insertion index.

Diving into the code

import FlatQueue from "flatqueue";

Flatbush supports a variety of TypedArray types to store box coordinate data. Flatbush uses Float64Array by default.

const ARRAY_TYPES = [
  Int8Array,
  Uint8Array,
  Uint8ClampedArray,
  Int16Array,
  Uint16Array,
  Int32Array,
  Uint32Array,
  Float32Array,
  Float64Array,
];

The Flatbush serialized format version is bumped whenever the binary layout of the index changes

const VERSION = 3; // serialized format version

/** @typedef {Int8ArrayConstructor | Uint8ArrayConstructor | Uint8ClampedArrayConstructor | Int16ArrayConstructor | Uint16ArrayConstructor | Int32ArrayConstructor | Uint32ArrayConstructor | Float32ArrayConstructor | Float64ArrayConstructor} TypedArrayConstructor */

Flatbush

The Flatbush class is the only export from the Flatbush library. It contains functions to create and query the spatial index.

export default class Flatbush {

Flatbush.from

One of Flatbush’s goals is to support zero-copy usage, meaning that you can take an ArrayBuffer backing a Flatbush index and transfer it between threads at virtually zero cost.

The from static method on the class reconstructs a Flatbush instance from a raw ArrayBuffer.

  /**
   * Recreate a Flatbush index from raw `ArrayBuffer` or `SharedArrayBuffer` data.
   * @param {ArrayBuffer | SharedArrayBuffer} data
   * @returns {Flatbush} index
   */
  static from(data) {
    if (!data || data.byteLength === undefined || data.buffer) {
      throw new Error(
        "Data must be an instance of ArrayBuffer or SharedArrayBuffer."
      );
    }

The first 8 bytes contain a header:

• byte 1: a “magic byte” set to 0xfb.
• byte 2: four bits for the serialized format version and four bits for the array type used for storing coordinates
• byte 3-4: a uint16-encoded number representing the size of each node
• byte 5-8: a uint32-encoded number representing the total number of items in the index.

We read each of these bytes from the provided data buffer, then pass the relevant parameters to the class constructor. Because the data argument (passed last) is not undefined, the constructor will not create a new underlying buffer, but rather reuse the existing buffer.

    const [magic, versionAndType] = new Uint8Array(data, 0, 2);
    if (magic !== 0xfb) {
      throw new Error("Data does not appear to be in a Flatbush format.");
    }
    const version = versionAndType >> 4;
    if (version !== VERSION) {
      throw new Error(`Got v${version} data when expected v${VERSION}.`);
    }
    const ArrayType = ARRAY_TYPES[versionAndType & 0x0f];
    if (!ArrayType) {
      throw new Error("Unrecognized array type.");
    }
    const [nodeSize] = new Uint16Array(data, 2, 1);
    const [numItems] = new Uint32Array(data, 4, 1);

    return new Flatbush(numItems, nodeSize, ArrayType, undefined, data);
  }

Constructor

The Flatbush constructor initializes the memory space (ArrayBuffer) for a Flatbush tree given the number of items the tree will contain and the number of elements per tree node.

  /**
   * Create a Flatbush index that will hold a given number of items.
   * @param {number} numItems
   * @param {number} [nodeSize=16] Size of the tree node (16 by default).
   * @param {TypedArrayConstructor} [ArrayType=Float64Array] The array type used for coordinates storage (`Float64Array` by default).
   * @param {ArrayBufferConstructor | SharedArrayBufferConstructor} [ArrayBufferType=ArrayBuffer] The array buffer type used to store data (`ArrayBuffer` by default).
   * @param {ArrayBuffer | SharedArrayBuffer} [data] (Only used internally)
   */
  constructor(
    numItems,
    nodeSize = 16,
    ArrayType = Float64Array,
    ArrayBufferType = ArrayBuffer,
    data
  ) {
    if (numItems === undefined)
      throw new Error("Missing required argument: numItems.");
    if (isNaN(numItems) || numItems <= 0)
      throw new Error(`Unexpected numItems value: ${numItems}.`);

    this.numItems = +numItems;
    this.nodeSize = Math.min(Math.max(+nodeSize, 2), 65535);

This do-while loop calculates the total number of nodes at each level of the R-tree (and thus also the total number of nodes). This will be used to allocate space for each level of the tree.

The tree is laid out in memory from bottom (leaves) to top (root). _levelBounds is an array that stores the offset within the coordinates array where each level ends. The first element of _levelBounds is n * 4, meaning that the slice of the coordinates array from 0 to n * 4 contains the bottom (leaves) of the tree.

Then the slice of the coordinates array from _levelBounds[0] to _levelBounds[1] represents the boxes of the first level of the tree, that is, the direct parent nodes of the leaves. And so on, _levelBounds[1] to _levelBounds[2] represents the nodes at level 2, the grandparent nodes of the leaf nodes.

So for example if numItems is 10,000 and nodeSize is 16, levelBounds will be:

[40000, 42500, 42660, 42672, 42676]

That is:

• The first 40,000 elements (10,000 nodes) are coordinates of the leaf nodes (4 coordinates per node).
• 2,500 coordinates and 625 nodes one level higher
• 160 coordinates and 40 nodes two levels higher
• 12 coordinates and 3 nodes three levels higher
• 1 root node four levels higher, at the top of the tree, with a single 4-coordinate box.

Keep in mind that because this is a packed tree, every node within a single level will be completely full (contain exactly nodeSize elements) except for the last node.

numNodes ends up as the total number of nodes in the tree, including all leaves.

    let n = numItems;
    let numNodes = n;
    this._levelBounds = [n * 4];
    do {
      n = Math.ceil(n / this.nodeSize);
      numNodes += n;
      this._levelBounds.push(numNodes * 4);
    } while (n !== 1);

Flatbush doesn’t manage references to objects directly. Rather, it operates in terms of the insertion index. Flatbush only maintains these insertion indices.

IndexArrayType will be used to create the indices array, to store the ordering of the input boxes. If possible, a Uint16Array will be used to save space. If the values would overflow a Uint16Array, a Uint32Array is used. The largest number a Uint16Array can hold is 2^16 = 65,536. Since each node holds four values, this gets divided by 4 and 65,536 / 4 = 16,384. This is why the check here is for 16,384.

    this.ArrayType = ArrayType;
    this.IndexArrayType = numNodes < 16384 ? Uint16Array : Uint32Array;

In order to accurately interpret the index from raw bytes, we need to record in the header which index type we’re using.

    const arrayTypeIndex = ARRAY_TYPES.indexOf(this.ArrayType);

The number of bytes needed to store all box coordinate data for all nodes.

    const nodesByteSize = numNodes * 4 * this.ArrayType.BYTES_PER_ELEMENT;

    if (arrayTypeIndex < 0) {
      throw new Error(`Unexpected typed array class: ${ArrayType}.`);
    }

This if statement switches on whether the data argument was passed in (i.e. this constructor is called by Flatbush.from). If data exists, this will create the _boxes and _indices arrays as views on the existing ArrayBuffer without allocating any new memory.

    if (data && data.byteLength !== undefined && !data.buffer) {
      this.data = data;

this._boxes is created as a view on this.data starting after the header (8 bytes) and with numNodes * 4 elements. this._indices is created as a view on this.data starting after the end of this._boxes and containing numNodes elements.

      this._boxes = new this.ArrayType(this.data, 8, numNodes * 4);
      this._indices = new this.IndexArrayType(
        this.data,
        8 + nodesByteSize,
        numNodes
      );

The coordinate data in the _boxes array is stored from the leaves up. So the last box is the single node that contains all data. The index of the last box is the four values in _boxes up to numNodes * 4.

This sets the total bounds on the Flatbush instance to the extent of that box.

We also set this._pos as the total number of coordinates. this._pos is a pointer into the this._boxes array, used while adding new boxes to the instance. This also allows for inferring whether the Flatbush instance has been “finished” (sorted) or not.

If the instance has already been sorted, adding more data is not allowed. Conversely, if the instance has not yet been sorted, query methods may not be called.

      this._pos = numNodes * 4;
      this.minX = this._boxes[this._pos - 4];
      this.minY = this._boxes[this._pos - 3];
      this.maxX = this._boxes[this._pos - 2];
      this.maxY = this._boxes[this._pos - 1];

In the else case, a data buffer was not provided, so we need to allocate data for the backing buffer.

this.data is a new ArrayBuffer with space for the header plus all box data plus all index data. Then this._boxes is created as a view on this.data starting after the header and with numNodes * 4 elements. this._indices is created as a view on this.data starting after the end of this._boxes.

    } else {
      this.data = new ArrayBufferType(
        8 + nodesByteSize + numNodes * this.IndexArrayType.BYTES_PER_ELEMENT
      );
      this._boxes = new this.ArrayType(this.data, 8, numNodes * 4);
      this._indices = new this.IndexArrayType(
        this.data,
        8 + nodesByteSize,
        numNodes
      );

We set this._pos to 0. This means that no boxes have yet been added to the index, and it tells any query methods to throw until finish has been called.

      this._pos = 0;

The RTree needs to maintain its total bounds (the global bounding box of all values) in order to set the bounds for the hilbert space.

We initialize these bounds to Infinity values that will be corrected when adding data. The minimum x/y of any box will be less than positive infinity and the maximum x/y of any box will be greater than negative infinity. The add() call will adjust these bounds if necessary.

      this.minX = Infinity;
      this.minY = Infinity;
      this.maxX = -Infinity;
      this.maxY = -Infinity;

Next we set the header values with metadata from the instance.

The first byte, 0xfb is a “magic byte”, used as basic validation that this buffer is indeed a Flatbush index.

Since arrayTypeIndex is known to have only 9 values, it doesn’t need to take up a a full byte. Here it shares a single byte with the Flatbush format version.

      new Uint8Array(this.data, 0, 2).set([
        0xfb,
        (VERSION << 4) + arrayTypeIndex,
      ]);
      new Uint16Array(this.data, 2, 1)[0] = nodeSize;
      new Uint32Array(this.data, 4, 1)[0] = numItems;
    }

We initialize a priority queue used for k-nearest-neighbors queries in the neighbors method.

    /** @type FlatQueue<number> */
    this._queue = new FlatQueue();
  }

Flatbush.Add

Add a given rectangle to the index.

  /**
   * Add a given rectangle to the index.
   * @param {number} minX
   * @param {number} minY
   * @param {number} maxX
   * @param {number} maxY
   * @returns {number} A zero-based, incremental number that represents the newly added rectangle.
   */
  add(minX, minY, maxX, maxY) {

We need to know the insertion index of the box presently being added.

In the constructor, this._pos is initialized to 0 and in each call to add(), this._pos is incremented by 4. Dividing this._pos by 4 retrieves the 0-based index of the box about to be inserted.

This bit shift:

this._pos >> 2

is equivalent to

this._pos / 4

but the bit shift is faster because it informs the JS engine that we expect the output to be an integer.

Because there are 4 values for each item, using _pos is an easy way to infer the insertion index without having to maintain a separate counter.

    const index = this._pos >> 2;
    const boxes = this._boxes;

We set the value of this._indices at the current index’s position to the value of the current index. So this._indices stores the insertion index of each box.

Later, inside the finish method, we’ll sort the boxes by their hilbert value and jointly reorder the values in _indices, ensuring that we keep the indices and boxes in sync.

This means that for any box representing a leaf node at position i (where i points to a box not a coordinate inside a box), this._indices[i] retrieves the original insertion-order index of that box.

    this._indices[index] = index;

We set the coordinates of this box into the boxes array. Note that this._pos++ is evaluated after the box index is set. So

boxes[this._pos++] = minX;

is equivalent to

boxes[this._pos] = minX;
this._pos += 1;

    boxes[this._pos++] = minX;
    boxes[this._pos++] = minY;
    boxes[this._pos++] = maxX;
    boxes[this._pos++] = maxY;

Update the total bounds of this instance if this rectangle is larger than the existing bounds.

    if (minX < this.minX) this.minX = minX;
    if (minY < this.minY) this.minY = minY;
    if (maxX > this.maxX) this.maxX = maxX;
    if (maxY > this.maxY) this.maxY = maxY;

    return index;
  }

Flatbush.finish

A spatial index needs to sort input data so that elements can be found quickly later.

The simplest way of sorting values is on a single dimension, where if a is less than b, a should be placed before b. But that presents a problem because we have two dimensions, not one.

One way to solve this is to map values from two-dimensional space into a one-dimensional range. A common way to perform this mapping is by using space-filling curves. In our case, we’ll use a hilbert curve, a specific type of space-filling curve that’s useful with geospatial data because it generally preserves locality.

First six iterations of the Hilbert curve, from Wikipedia, CC BY-SA.

Note that using a space-filling curve to map values into one dimension isn’t the only way of sorting multi-dimensional data. There are other algorithms, like sort-tile-recursive (STR) that first sort into groups on one dimension, then the other, recursively.

While this canonical Flatbush implementation chooses to sort based on hilbert value, that’s actually not necessary to maintain ABI-stability: any two-dimensional sort will work. My Rust port defines an extensible trait for sorting and provides both hilbert and STR sorting implementations.

  /** Perform indexing of the added rectangles. */
  finish() {

Recall that in the add method, we increment this._pos by 1 for each coordinate of each box. Here we validate that we’ve added the same number of boxes as we provisioned in the constructor. Remember that >> 2 is equivalent to / 4.

    if (this._pos >> 2 !== this.numItems) {
      throw new Error(
        `Added ${this._pos >> 2} items when expected ${this.numItems}.`
      );
    }
    const boxes = this._boxes;

If the total number of items in the tree is less than the node size, that means we’ll only have a single non-leaf node in the tree. In that case, we don’t even need to sort by hilbert value. We can just assign the total bounds of the tree to the following box and return.

    if (this.numItems <= this.nodeSize) {
      boxes[this._pos++] = this.minX;
      boxes[this._pos++] = this.minY;
      boxes[this._pos++] = this.maxX;
      boxes[this._pos++] = this.maxY;
      return;
    }

Using the total bounds of the tree, we compute the height and width of the hilbert space and instantiate space for the hilbert values.

    const width = this.maxX - this.minX || 1;
    const height = this.maxY - this.minY || 1;
    const hilbertValues = new Uint32Array(this.numItems);
    const hilbertMax = (1 << 16) - 1;

Map box centers into Hilbert coordinate space and calculate Hilbert values using the hilbert function defined below.

This for loop iterates over every box. At the beginning of each loop iteration, pos is equal to i * 4.

    for (let i = 0, pos = 0; i < this.numItems; i++) {
      const minX = boxes[pos++];
      const minY = boxes[pos++];
      const maxX = boxes[pos++];
      const maxY = boxes[pos++];
      const x = Math.floor(
        (hilbertMax * ((minX + maxX) / 2 - this.minX)) / width
      );
      const y = Math.floor(
        (hilbertMax * ((minY + maxY) / 2 - this.minY)) / height
      );
      hilbertValues[i] = hilbert(x, y);
    }

Up until this point, the values in boxes and in this._indices are still in insertion order. We now jointly sort the boxes and indices according to their hilbert values.

    sort(
      hilbertValues,
      boxes,
      this._indices,
      0,
      this.numItems - 1,
      this.nodeSize
    );

Now the leaves of the tree have been sorted, but we still need to construct the rest of the tree.

For each level of the tree, we need to generate parent nodes that contain nodeSize child nodes. We do this starting from the leaves, working from the bottom up.

Here the iteration variable, i, refers to the positional tree level, which is also an index into the this._levelBounds array.

• When i == 0, we’re iterating over the original geometry boxes.
• When i == 1, we’re iterating over the parent nodes one level up that we previously generated from the first loop iteration.
• And so on, i represents the number of parents from the original geometry boxes.

As elsewhere, pos is a local variable that points to a coordinate within a box at the given level i of the tree. Note this syntax: it’s unusual for two variables to be defined in the for loop binding: here both i and pos are only defined within the scope of this loop. But only i is incremented by the loop. pos is incremented separately within the body of the loop (four times for each box).

    for (let i = 0, pos = 0; i < this._levelBounds.length - 1; i++) {

Next, we want to scan through all nodes at this level of the tree, generating a parent node for each group of consecutive nodeSize boxes.

Here, end is the index of the first coordinate at the next level above the current level. So the range up to end includes all coordinates at the current tree level.

We then scan over all of these box coordinates in this while loop.

      const end = this._levelBounds[i];
      while (pos < end) {

We record the pos pointing to the first element of the first box in each group of consecutive nodeSize boxes, in order to later record it in the indices array.

        const nodeIndex = pos;

Calculate the bounding box for the new parent node.

We initialize the bounding box to the first box and then expand the box while looping over the rest of the elements that together are the children of this parent node we’re creating.

Note the j = 1 in the loop; this is a small optimization because we initialize the node* variables to the first element, rather than initializing with positive and negative infinity.

Also note that in the loop we constrain the iteration variable j to be both less than the node size and for pos < end. The former ensures we have only a maximum of nodeSize elements informing the parent node’s boundary. The latter ensures that we don’t accidentally overflow the current tree level.

        let nodeMinX = boxes[pos++];
        let nodeMinY = boxes[pos++];
        let nodeMaxX = boxes[pos++];
        let nodeMaxY = boxes[pos++];
        for (let j = 1; j < this.nodeSize && pos < end; j++) {
          nodeMinX = Math.min(nodeMinX, boxes[pos++]);
          nodeMinY = Math.min(nodeMinY, boxes[pos++]);
          nodeMaxX = Math.max(nodeMaxX, boxes[pos++]);
          nodeMaxY = Math.max(nodeMaxY, boxes[pos++]);
        }

Now that we know the extent of the parent node, we can add the new node’s information to the tree data.

Recall that nodeIndex, stored above, points to the first element of the first box in each group of consecutive nodeSize nodes.

The nodeIndex is always a multiple of 4 because there are 4 coordinates in each 2D box. This means we can divide by 4 to store the node index information more compactly. Again, we use >> 2 instead of / 4 as a performance optimization.

When we’re at the base (leaf) level of the tree, nodeIndex represents the insertion index of the first box in this group.

Similarly, when we’re at higher levels of the tree, nodeIndex represents the offset of the first box in this group.

These two facts allow us to traverse the tree in a search query, as we’ll see below in Flatbush.search.

Note that we’re setting the parent node into this._indices and boxes according to this._pos, which is a different variable than the local pos variable that’s incremented in this loop. this._pos is a global counter that keeps track of the new nodes we’re inserting into the index. In contrast, pos is a local counter for aggregating the information for the parent node.

Impressively, these loops do all the hard work of constructing the tree! That’s it! The structure of the tree and the coordinates of all the parent nodes are now fully contained within this._indices and boxes, which are both views on this.data!

        this._indices[this._pos >> 2] = nodeIndex;
        boxes[this._pos++] = nodeMinX;
        boxes[this._pos++] = nodeMinY;
        boxes[this._pos++] = nodeMaxX;
        boxes[this._pos++] = nodeMaxY;
      }
    }
  }

Flatbush.search

The primary API for searching an index by a bounding box query.

  /**
   * Search the index by a bounding box.
   * @param {number} minX
   * @param {number} minY
   * @param {number} maxX
   * @param {number} maxY
   * @param {(index: number) => boolean} [filterFn] An optional function for filtering the results.
   * @returns {number[]} An array of indices of items intersecting or touching the given bounding box.
   */
  search(minX, minY, maxX, maxY, filterFn) {

A simple check to ensure that this index has been finished/sorted.

    if (this._pos !== this._boxes.length) {
      throw new Error("Data not yet indexed - call index.finish().");
    }

nodeIndex is initialized to the root node, the parent of all other nodes. Since the tree is laid out from bottom to top, the root node is the last node in this._boxes. We subtract 4 so that nodeIndex points to the first coordinate of the box.

Note that nodeIndex will always point to the first box within a group of (usually nodeSize) boxes.

queue holds integers that represent the position within this._indices of intermediate nodes that still need to be searched. That is, queue represents nodes whose parents intersected the search predicate.

results holds integers that represent the insertion indexes that match the search predicate.

    /** @type number | undefined */
    let nodeIndex = this._boxes.length - 4;
    const queue = [];
    const results = [];

Now we have our search loop.

while (nodeIndex !== undefined)

will be true as long as there are still elements remaining in queue (note that the last line of the while loop is nodeIndex = queue.pop();).

    while (nodeIndex !== undefined) {

Find the end index of the current node.

Most of the time, the node contains nodeSize elements. At the end of each level, the node will contain fewer elements. In the first case, the end of the node will be the current index plus 4 coordinates for each box. We check if we’re in the second case by checking the value of this._levelBounds for the current level of the tree.

      const end = Math.min(
        nodeIndex + this.nodeSize * 4,
        upperBound(nodeIndex, this._levelBounds)
      );

Then we search through each box of the current node, checking whether each matches our predicate. The loop ranges from the first node of the level (nodeIndex) to the last (end). We increment pos by 4 for each loop step because there are 4 coordinates.

      for (let /** @type number */ pos = nodeIndex; pos < end; pos += 4) {

Check if the current box does not intersect with query box. If the current box does not intersect, then we can continue on to the next element of this node.

If we reach past these four lines, then we know the current box does intersect with the query box.

        if (maxX < this._boxes[pos]) continue; // maxX < nodeMinX
        if (maxY < this._boxes[pos + 1]) continue; // maxY < nodeMinY
        if (minX > this._boxes[pos + 2]) continue; // minX > nodeMaxX
        if (minY > this._boxes[pos + 3]) continue; // minY > nodeMaxY

pos is a pointer to the first coordinate of the given box. Recall in Flatbush.finish that we set:

this._indices[this._pos >> 2] = nodeIndex;

This stored a mapping from parent to child node, where this._pos >> 2 was the parent node and nodeIndex was the child node. Now is the time when we want to use this mapping.

• If the current box is not a leaf, index is the pos of the first box of the child node. This child is a node that we should evaluate later, so we add it to the queue array.
• If the current box is a leaf, then index is the original insertion index, and we add it to the results array.

Again, pos >> 2 is a faster way of expressing pos / 4, where we can inform the JS engine that the output will be an integer.

I believe | 0 is just a JS engine optimization that doesn’t affect the output of the operation?

Then we can add the index to either the intermediate queue or results arrays as necessary.

        const index = this._indices[pos >> 2] | 0;

        if (nodeIndex >= this.numItems * 4) {
          queue.push(index); // node; add it to the search queue
        } else if (filterFn === undefined || filterFn(index)) {
          results.push(index); // leaf item
        }
      }

Set the nodeIndex to the next item in the queue so that we continue the while loop.

      nodeIndex = queue.pop();
    }

    return results;
  }

Flatbush.neighbors

The primary API for searching an index by nearest neighbors to a point.

This has significant overlap with Flatbush.search, and so we’ll only touch on the differences.

  /**
   * Search items in order of distance from the given point.
   * @param {number} x
   * @param {number} y
   * @param {number} [maxResults=Infinity]
   * @param {number} [maxDistance=Infinity]
   * @param {(index: number) => boolean} [filterFn] An optional function for filtering the results.
   * @returns {number[]} An array of indices of items found.
   */
  neighbors(x, y, maxResults = Infinity, maxDistance = Infinity, filterFn) {
    if (this._pos !== this._boxes.length) {
      throw new Error("Data not yet indexed - call index.finish().");
    }

Instead of using an array as a queue, here we use a priority queue. This is a data structure that maintains the queue in sorted order, and which allows us to ensure that the first element of the queue is indeed the closest to the provided point.

    /** @type number | undefined */
    let nodeIndex = this._boxes.length - 4;
    const q = this._queue;
    const results = [];
    const maxDistSquared = maxDistance * maxDistance;

    outer: while (nodeIndex !== undefined) {
      const end = Math.min(
        nodeIndex + this.nodeSize * 4,
        upperBound(nodeIndex, this._levelBounds)
      );

Add child nodes to the queue.

dx and dy are computed as the one-dimensional change in x and y needed to reach one of the sides of the box from the query point. Then dist is the squared distance to reach the corner of the box closest to the query point.

If this distance is less than the provided maximum distance, we add it to the queue. Since we add both intermediate nodes and results to the same queue, we need a way to distinguish the two. When the index represents an intermediate node, we multiply by two (i.e. << 1) so that we have an even id. When the index represents a leaf item, we multiply by two and then add one (i.e. (<< 1) + 1), so that we have an odd id.

      for (let pos = nodeIndex; pos < end; pos += 4) {
        const index = this._indices[pos >> 2] | 0;

        const dx = axisDist(x, this._boxes[pos], this._boxes[pos + 2]);
        const dy = axisDist(y, this._boxes[pos + 1], this._boxes[pos + 3]);
        const dist = dx * dx + dy * dy;
        if (dist > maxDistSquared) continue;

        if (nodeIndex >= this.numItems * 4) {
          q.push(index << 1, dist); // node (use even id)
        } else if (filterFn === undefined || filterFn(index)) {
          q.push((index << 1) + 1, dist); // leaf item (use odd id)
        }
      }

Now that we’ve added all child nodes to the queue, we can move queue items to the results array and/or break out of the outer loop completely.

Since this queue is a priority queue, we can be assured that the first item of the queue is the closest to the query point. The nearest corner of the box of that item is closer than any other node or result.

While the queue is non-empty and the first (closest) item in the queue is a leaf item (odd), if that item’s distance is less than the maximum query distance, we can break out of the outer loop, since there cannot be any more nodes that are closer than that distance. If the item’s distance is less than the maximum query distance, we add it to the results array because it must be the next closest result.

If the first (closest) item of the queue is an intermediate node (not odd), then we need to evaluate the items of that node before knowing which one is the next closest. In this case, the while condition is false, and we set the nodeIndex to that intermediate node for the next iteration of the outer while loop.

      while (q.length && q.peek() & 1) {
        const dist = q.peekValue();
        if (dist > maxDistSquared) break outer;
        results.push(q.pop() >> 1);
        if (results.length === maxResults) break outer;
      }

      nodeIndex = q.length ? q.pop() >> 1 : undefined;
    }

We clear the queue because this queue is reused for all queries in this index.

    q.clear();
    return results;
  }
}

The remaining code is “just” utility functions.

I won’t document these in detail because they tend to be self explanatory or easily found online and this post is focused more on the RTree implementation itself.

axisDist: 1D distance from a value to a range.

/**
 * 1D distance from a value to a range.
 * @param {number} k
 * @param {number} min
 * @param {number} max
 */
function axisDist(k, min, max) {
  return k < min ? min - k : k <= max ? 0 : k - max;
}

upperBound: Binary search for the first value in the array bigger than the given.

/**
 * Binary search for the first value in the array bigger than the given.
 * @param {number} value
 * @param {number[]} arr
 */
function upperBound(value, arr) {
  let i = 0;
  let j = arr.length - 1;
  while (i < j) {
    const m = (i + j) >> 1;
    if (arr[m] > value) {
      j = m;
    } else {
      i = m + 1;
    }
  }
  return arr[i];
}

sort: Custom quicksort that partially sorts bbox data alongside the hilbert values.

/**
 * Custom quicksort that partially sorts bbox data alongside the hilbert values.
 * @param {Uint32Array} values
 * @param {InstanceType<TypedArrayConstructor>} boxes
 * @param {Uint16Array | Uint32Array} indices
 * @param {number} left
 * @param {number} right
 * @param {number} nodeSize
 */
function sort(values, boxes, indices, left, right, nodeSize) {
  if (Math.floor(left / nodeSize) >= Math.floor(right / nodeSize)) return;

  const pivot = values[(left + right) >> 1];
  let i = left - 1;
  let j = right + 1;

  while (true) {
    do i++;
    while (values[i] < pivot);
    do j--;
    while (values[j] > pivot);
    if (i >= j) break;
    swap(values, boxes, indices, i, j);
  }

  sort(values, boxes, indices, left, j, nodeSize);
  sort(values, boxes, indices, j + 1, right, nodeSize);
}

swap: Swap two values and two corresponding boxes.

/**
 * Swap two values and two corresponding boxes.
 * @param {Uint32Array} values
 * @param {InstanceType<TypedArrayConstructor>} boxes
 * @param {Uint16Array | Uint32Array} indices
 * @param {number} i
 * @param {number} j
 */
function swap(values, boxes, indices, i, j) {
  const temp = values[i];
  values[i] = values[j];
  values[j] = temp;

  const k = 4 * i;
  const m = 4 * j;

  const a = boxes[k];
  const b = boxes[k + 1];
  const c = boxes[k + 2];
  const d = boxes[k + 3];
  boxes[k] = boxes[m];
  boxes[k + 1] = boxes[m + 1];
  boxes[k + 2] = boxes[m + 2];
  boxes[k + 3] = boxes[m + 3];
  boxes[m] = a;
  boxes[m + 1] = b;
  boxes[m + 2] = c;
  boxes[m + 3] = d;

  const e = indices[i];
  indices[i] = indices[j];
  indices[j] = e;
}

hilbert: compute hilbert codes.

This is the function that takes a position in 2D space, x and y, and returns the hilbert value for that position.

Umm yeah sorry I can’t say anything else about this… it’s black magic.

Refer to the C++ source and the original blog post for any hope of understanding what’s going on here!

/**
 * Fast Hilbert curve algorithm by http://threadlocalmutex.com/
 * Ported from C++ https://github.com/rawrunprotected/hilbert_curves (public domain)
 * @param {number} x
 * @param {number} y
 */
function hilbert(x, y) {
  let a = x ^ y;
  let b = 0xffff ^ a;
  let c = 0xffff ^ (x | y);
  let d = x & (y ^ 0xffff);

  let A = a | (b >> 1);
  let B = (a >> 1) ^ a;
  let C = (c >> 1) ^ (b & (d >> 1)) ^ c;
  let D = (a & (c >> 1)) ^ (d >> 1) ^ d;

  a = A;
  b = B;
  c = C;
  d = D;
  A = (a & (a >> 2)) ^ (b & (b >> 2));
  B = (a & (b >> 2)) ^ (b & ((a ^ b) >> 2));
  C ^= (a & (c >> 2)) ^ (b & (d >> 2));
  D ^= (b & (c >> 2)) ^ ((a ^ b) & (d >> 2));

  a = A;
  b = B;
  c = C;
  d = D;
  A = (a & (a >> 4)) ^ (b & (b >> 4));
  B = (a & (b >> 4)) ^ (b & ((a ^ b) >> 4));
  C ^= (a & (c >> 4)) ^ (b & (d >> 4));
  D ^= (b & (c >> 4)) ^ ((a ^ b) & (d >> 4));

  a = A;
  b = B;
  c = C;
  d = D;
  C ^= (a & (c >> 8)) ^ (b & (d >> 8));
  D ^= (b & (c >> 8)) ^ ((a ^ b) & (d >> 8));

  a = C ^ (C >> 1);
  b = D ^ (D >> 1);

  let i0 = x ^ y;
  let i1 = b | (0xffff ^ (i0 | a));

  i0 = (i0 | (i0 << 8)) & 0x00ff00ff;
  i0 = (i0 | (i0 << 4)) & 0x0f0f0f0f;
  i0 = (i0 | (i0 << 2)) & 0x33333333;
  i0 = (i0 | (i0 << 1)) & 0x55555555;

  i1 = (i1 | (i1 << 8)) & 0x00ff00ff;
  i1 = (i1 | (i1 << 4)) & 0x0f0f0f0f;
  i1 = (i1 | (i1 << 2)) & 0x33333333;
  i1 = (i1 | (i1 << 1)) & 0x55555555;

  return ((i1 << 1) | i0) >>> 0;
}