In the past:
* hexops/mach#156 was the initial ECS implementation detailed in https://devlog.hexops.com/2022/lets-build-ecs-part-1
* hexops/mach#157 was the second major redesign in which we:
* Eliminated major limitations (e.g. inability to add/remove components at runtime)
* Investigated sparse sets
* Began thinking in terms of databases
* Enabled runtime introspection
Our second revision of the ECS, however, still had _archetypes_ exposed as a public-facing
user concern. When a new component was added to an entity, say a weapon, the table storing
entities of that archetype changed to effectively have a new column `?Weapon` with a null
value for _all existing entities of that archetype_. We can say that our ECS had archetypes
as a user-facing concern AND this made performance worse: when iterating all entities with
a weapon, we needed to check if the component value was `null` or not because every column
was `?Weapon` instead of a guaranteed non-null value like `Weapon`. This was a key learning
that I got from [discussing ECS tradeoffs with the Bevy team](https://github.com/hexops/mach/pull/157#issuecomment-1022916117).
This third revision of our ECS has some big benefits:
* Entities are now just IDs proper, you can add/remove arbitrary components at runtime.
* You don't have an "entity which always belongs to one archetype table which changes"
* Rather, you have an "entity of one archetype" and adding a component means that entity _moves_ from one archetype table to another.
* Archetypes are now an implementation detail, not something you worry about as a consumer of the API.
* Performance
* We benefit from the fact that we no longer need check if a component on an entity is `null` or not.
* Introspection
* Previously iterating the component names/values an entity had was not possible, now it is.
* Querying & multi-threading
* Very very early stages into this, but we now have a general plan for how querying will work and multi-threading.
* Effectively, it will look much like interfacing with a database: you have a connection (we call it an adapter)
and you can ask for information through that. More work to be done here.
* Systems, we now have a (very) basic starting point for how systems will work.
Some examples of how the API looks today:
* 979240135b/ecs/src/main.zig (L49)
* 979240135b/ecs/src/entities.zig (L625-L656)
Much more work to do, I will do a blog post detailing this step-by-step first though.
Signed-off-by: Stephen Gutekanst <stephen@hexops.com>
