Crate 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.

Structs

GenMap	A collection of T's referred to by Handle's. When you add an object to the GenMap it will return a Handle, and you can look that item up by that Handle. You can also remove the item, which makes any old Handle's to it become invalid and attempting to get it will return None.
Handle	A small, easy-to-copy handle referring to a location in a particular GenMap.
Iterator	Iterator over keys in a GenMap.