I need a spatial index in C

Question

I'm working on my gEDA fork and want to get rid of the existing simple tile-based system¹ in favour of a real spatial index².

An algorithm that efficiently finds points is not enough: I need to find objects with non-zero extent. Think in terms of objects having bounding rectangles, that pretty much captures the level of detail I need in the index. Given a search rectangle, I need to be able to efficiently find all objects whose bounding rectangles are inside, or that intersect, the search rectangle.

The index can't be read-only: gschem is a schematic capture program, and the whole point of it is to move things around the schematic diagram. So things are going to be a'changing. So while I can afford insertion to be a bit more expensive than searching, it can't be too much more expensive, and deleting must also be both possible and reasonably cheap. But the most important requirement is the asymptotic behaviour: searching should be O(log n) if it can't be O(1). Insertion / deletion should preferably be O(log n), but O(n) would be okay. I definitely don't want anything > O(n) (per action; obviously O(n log n) is expected for an all-objects operation).

What are my options? I don't feel clever enough to evaluate the various options. Ideally there'd be some C library that will do all the clever stuff for me, but I'll mechanically implement an algorithm I may or may not fully understand if I have to. gEDA uses glib by the way, if that helps to make a recommendation.

Footnotes:

¹ Standard gEDA divides a schematic diagram into a fixed number (currently 100) of "tiles" which serve to speed up searches for objects in a bounding rectangle. This is obviously good enough to make most schematics fast enough to search, but the way it's done causes other problems: far too many functions require a pointer to a de-facto global object. The tiles geometry is also fixed: it would be possible to defeat this tiling system completely simply by panning (and possibly zooming) to an area covered by only one tile.

² A legitimate answer would be to keep elements of the tiling system, but to fix its weaknesses: teaching it to span the entire space, and to sub-divide when necessary. But I'd like others to add their two cents before I autocratically decide that this is the best way.

Answer 1

A nice data structure for a mix of points and lines would be an R-tree or one of its derivatives (e.g. R*-Tree or a Hilbert R-Tree). Given you want this index to be dynamic and serializable, I think using SQLite's R*-Tree module would be a reasonable approach.

If you can tolerate C++, libspatialindex has a mature and flexible R-tree implementation which supports dynamic inserts/deletes and serialization.

Answer 2

Your needs sound very similar to what is used in collision detection algorithms for games and physics simulations. There are several open source C++ libraries that handle this in 2-D (Box2D) or 3-D (Bullet physics). Although your question is for C, you may find their documentation and implementations useful.

Usually this is split into a two phases:

1. A fast broad phase that approximates objects by their axis-aligned bounding box (AABB), and determines pairs of AABBs that touch or overlap.
2. A slower narrow phase that calculates the points of geometric overlap for pairs of objects whose AABBs touch or overlap.

Physics engines also use spatial coherence to further reduce the pairs of objects that are compared, but this optimization probably won't help your application.

The broadphase is usually implemented with an O(N log N) algorithm like Sweep and prune. You may be able to accelerate this by using it in conjunction with the current tile approach (one of Nvidia's GPUGems describes this hybrid approach). The narrow phase is quite costly for each pair, and may be overkill for your needs. The GJK algorithm is often used for convex objects in this step, although faster algorithms exist for more specialized cases (e.g.: box/circle and box/sphere collisions).