# 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