genmap

A Rust crate for a generational map, handle map, whatever you want to call it. Whatever it is, this is a random-access data structure that stores an unordered bag of items and gives you a handle for each specific item you insert. Looking things up by handle is O(1) -- the backing storage is just a Vec -- and items can be removed as well, which is also O(1). Handles are small (two usize's) and easy to copy, similar to a slice. The trick is that there is a generation number stored with each item, so that a "dangling" handle that refers to an item that has been removed is invalid. Unlike array indices, if you remove an item from the array and a new one gets put in its place, the old stale handle does not refer to the new one and trying to use it will fail at runtime.

This is useful for various things: managing lots of uniform things with shared ownership (such as video game resources), interning strings, that sort of thing. Essentially by using handles to items you have runtime-checked memory safety: you can get an item from the map using the handle, since it's basically just an array index.

In practice this is not unrelated to Rc, it's just that Rc does the accounting of memory on cloning the Rc, and this does it on "dereferencing" by looking up an object's handle. Referencing counting, threading garbage collection and this sort of map are all different methods of achieving the same thing (checking for memory safety at runtime) with different tradeoffs. With a reference count you don't have to explicitly free items and can't have stale handles, but loops require special handling and you pay for the accounting cost on cloning a handle. With this you have to free items explicitly, but stale handles can be safely detected and the accounting happens when you look the item up. You can also use it as a slab allocator type thing, where you pre-allocate a large amount of storage and free it all at once.

Limitations

• It never frees memory until the GenMap is dropped (though it does reuse empty slots).
• Overflowing generations is an error. Should it be a u64 rather than usize?

Other things like this:

• generational-arena -- I couldn't find this and so wrote genmap
• instead. You should probably just use generational-arena.
• slotmap -- requires your items be Copy, which I feel is too restrictive. Its DenseSlotMap gets around this, but really at that point there's too many options and tradeoffs to consider. I just want something that Works.
• handy -- Nice but I don't like some of the design decisions it makes: https://github.com/thomcc/handy/pull/1
• slab -- Not really a generational map IMO, since it doesn't track generations.
• specs -- A Storage in specs is basically a type of generational map. specs adds tons of other stuff though.

License

MIT/Apache-2.0