Question

I have upto 10,000 randomly positioned points in a space and i need to be able to tell which the cursor is closest to at any given time. To add some context, the points are in the form of a vector drawing, so they can be constantly and quickly added and removed by the user and also potentially be unbalanced across the canvas space..

I am therefore trying to find the most efficient data structure for storing and querying these points. I would like to keep this question language agnostic if possible.

Answer 1

After the Update to the Question

1. Use two Red-Black Tree or Skip_list maps. Both are compact self-balancing data structures giving you O(log n) time for search, insert and delete operations. One map will use X-coordinate for every point as a key and the point itself as a value and the other will use Y-coordinate as a key and the point itself as a value.

2. As a trade-off I suggest to initially restrict the search area around the cursor by a square. For perfect match the square side should equal to diameter of your “sensitivity circle” around the cursor. I.e. if you’re interested only in a nearest neighbour within 10 pixel radius from the cursor then the square side needs to be 20px. As an alternative, if you’re after nearest neighbour regardless of proximity you might try finding the boundary dynamically by evaluating floor and ceiling relative to cursor.

3. Then retrieve two subsets of points from the maps that are within the boundaries, merge to include only the points within both sub sets.

4. Loop through the result, calculate proximity to each point (dx^2+dy^2, avoid square root since you’re not interested in the actual distance, just proximity), find the nearest neighbour.

5. Take root square from the proximity figure to measure the distance to the nearest neighbour, see if it’s greater than the radius of the “sensitivity circle”, if it is it means there is no points within the circle.

6. I suggest doing some benchmarks every approach; it’s two easy to go over the top with optimisations. On my modest hardware (Duo Core 2) naïve single-threaded search of a nearest neighbour within 10K points repeated a thousand times takes 350 milliseconds in Java. As long as the overall UI re-action time is under 100 milliseconds it will seem instant to a user, keeping that in mind even naïve search might give you sufficiently fast response.

Generic Solution

The most efficient data structure depends on the algorithm you’re planning to use, time-space trade off and the expected relative distribution of points:

• If space is not an issue the most efficient way may be to pre-calculate the nearest neighbour for each point on the screen and then store nearest neighbour unique id in a two-dimensional array representing the screen.
• If time is not an issue storing 10K points in a simple 2D array and doing naïve search every time, i.e. looping through each point and calculating the distance may be a good and simple easy to maintain option.
• For a number of trade-offs between the two, here is a good presentation on various Nearest Neighbour Search options available: http://dimacs.rutgers.edu/Workshops/MiningTutorial/pindyk-slides.ppt
• A bunch of good detailed materials for various Nearest Neighbour Search algorithms: http://simsearch.yury.name/tutorial.html, just pick one that suits your needs best.

So it’s really impossible to evaluate the data structure is isolation from algorithm which in turn is hard to evaluate without good idea of task constraints and priorities.

Sample Java Implementation

import java.util.*;
import java.util.concurrent.ConcurrentSkipListMap;

class Test
{

  public static void main (String[] args)
  {

      Drawing naive = new NaiveDrawing();
      Drawing skip  = new SkipListDrawing();

      long start;

      start = System.currentTimeMillis();
      testInsert(naive);
      System.out.println("Naive insert: "+(System.currentTimeMillis() - start)+"ms");
      start = System.currentTimeMillis();
      testSearch(naive);
      System.out.println("Naive search: "+(System.currentTimeMillis() - start)+"ms");


      start = System.currentTimeMillis();
      testInsert(skip);
      System.out.println("Skip List insert: "+(System.currentTimeMillis() - start)+"ms");
      start = System.currentTimeMillis();
      testSearch(skip);
      System.out.println("Skip List search: "+(System.currentTimeMillis() - start)+"ms");

  }

  public static void testInsert(Drawing d)
  {
      Random r = new Random();
      for (int i=0;i<100000;i++)
            d.addPoint(new Point(r.nextInt(4096),r.nextInt(2048)));
  }

  public static void testSearch(Drawing d)
  {
      Point cursor;
      Random r = new Random();
      for (int i=0;i<1000;i++)
      {
          cursor = new Point(r.nextInt(4096),r.nextInt(2048));
          d.getNearestFrom(cursor,10);
      }
  }


}

// A simple point class
class Point
{
    public Point (int x, int y)
    {
        this.x = x;
        this.y = y;
    }
    public final int x,y;

    public String toString()
    {
        return "["+x+","+y+"]";
    }
}

// Interface will make the benchmarking easier
interface Drawing
{
    void addPoint (Point p);
    Set<Point> getNearestFrom (Point source,int radius);

}


class SkipListDrawing implements Drawing
{

    // Helper class to store an index of point by a single coordinate
    // Unlike standard Map it's capable of storing several points against the same coordinate, i.e.
    // [10,15] [10,40] [10,49] all can be stored against X-coordinate and retrieved later
    // This is achieved by storing a list of points against the key, as opposed to storing just a point.
    private class Index
    {
        final private NavigableMap<Integer,List<Point>> index = new ConcurrentSkipListMap <Integer,List<Point>> ();

        void add (Point p,int indexKey)
        {
            List<Point> list = index.get(indexKey);
            if (list==null)
            {
                list = new ArrayList<Point>();
                index.put(indexKey,list);
            }
            list.add(p);
        }

        HashSet<Point> get (int fromKey,int toKey)
        {
            final HashSet<Point> result = new HashSet<Point> ();

            // Use NavigableMap.subMap to quickly retrieve all entries matching
            // search boundaries, then flatten resulting lists of points into
            // a single HashSet of points.
            for (List<Point> s: index.subMap(fromKey,true,toKey,true).values())
                for (Point p: s)
                 result.add(p);

            return result;
        }

    }

    // Store each point index by it's X and Y coordinate in two separate indices
    final private Index xIndex = new Index();
    final private Index yIndex = new Index();

    public void addPoint (Point p)
    {
        xIndex.add(p,p.x);
        yIndex.add(p,p.y);
    }


    public Set<Point> getNearestFrom (Point origin,int radius)
    {


          final Set<Point> searchSpace;
          // search space is going to contain only the points that are within
          // "sensitivity square". First get all points where X coordinate
          // is within the given range.
          searchSpace = xIndex.get(origin.x-radius,origin.x+radius);

          // Then get all points where Y is within the range, and store
          // within searchSpace the intersection of two sets, i.e. only
          // points where both X and Y are within the range.
          searchSpace.retainAll(yIndex.get(origin.y-radius,origin.y+radius));


          // Loop through search space, calculate proximity to each point
          // Don't take square root as it's expensive and really unneccessary
          // at this stage.
          //
          // Keep track of nearest points list if there are several
          // at the same distance.
          int dist,dx,dy, minDist = Integer.MAX_VALUE;

          Set<Point> nearest = new HashSet<Point>();

          for (Point p: searchSpace)
          {
             dx=p.x-origin.x;
             dy=p.y-origin.y;
             dist=dx*dx+dy*dy;

             if (dist<minDist)
             {
                   minDist=dist;
                   nearest.clear();
                   nearest.add(p);
             }
             else if (dist==minDist)
             {
                 nearest.add(p);
             }


          }

          // Ok, now we have the list of nearest points, it might be empty.
          // But let's check if they are still beyond the sensitivity radius:
          // we search area we have evaluated was square with an side to
          // the diameter of the actual circle. If points we've found are
          // in the corners of the square area they might be outside the circle.
          // Let's see what the distance is and if it greater than the radius
          // then we don't have a single point within proximity boundaries.
          if (Math.sqrt(minDist) > radius) nearest.clear();
          return nearest;
   }
}

// Naive approach: just loop through every point and see if it's nearest.
class NaiveDrawing implements Drawing
{
    final private List<Point> points = new ArrayList<Point> ();

    public void addPoint (Point p)
    {
        points.add(p);
    }

    public Set<Point> getNearestFrom (Point origin,int radius)
    {

          int prevDist = Integer.MAX_VALUE;
          int dist;

          Set<Point> nearest = Collections.emptySet();

          for (Point p: points)
          {
             int dx = p.x-origin.x;
             int dy = p.y-origin.y;

             dist =  dx * dx + dy * dy;
             if (dist < prevDist)
             {
                   prevDist = dist;
                   nearest  = new HashSet<Point>();
                   nearest.add(p);
             }
             else if (dist==prevDist) nearest.add(p);

          }

          if (Math.sqrt(prevDist) > radius) nearest = Collections.emptySet();

          return nearest;
   }
}