Turned a lot of for loops into for_each loops, which should be easier
for LLVM to optimize currently. Also updated almost all the non-erosion
stuff in WorldGen to run in parallel (and take advantage of the cache,
in the case of TownGen), and hopefully improved performance somewhat for
chunk generation as well.
See the doc comments in `common/src/vol.rs` for more information on
the API itself.
The changes include:
* Consistent `Err`/`Error` naming.
* Types are named `...Error`.
* `enum` variants are named `...Err`.
* Rename `VolMap{2d, 3d}` -> `VolGrid{2d, 3d}`. This is in preparation
to an upcoming change where a “map” in the game related sense will
be added.
* Add volume iterators. There are two types of them:
* _Position_ iterators obtained from the trait `IntoPosIterator`
using the method
`fn pos_iter(self, lower_bound: Vec3<i32>, upper_bound: Vec3<i32>) -> ...`
which returns an iterator over `Vec3<i32>`.
* _Volume_ iterators obtained from the trait `IntoVolIterator`
using the method
`fn vol_iter(self, lower_bound: Vec3<i32>, upper_bound: Vec3<i32>) -> ...`
which returns an iterator over `(Vec3<i32>, &Self::Vox)`.
Those traits will usually be implemented by references to volume
types (i.e. `impl IntoVolIterator<'a> for &'a T` where `T` is some
type which usually implements several volume traits, such as `Chunk`).
* _Position_ iterators iterate over the positions valid for that
volume.
* _Volume_ iterators do the same but return not only the position
but also the voxel at that position, in each iteration.
* Introduce trait `RectSizedVol` for the use case which we have with
`Chonk`: A `Chonk` is sized only in x and y direction.
* Introduce traits `RasterableVol`, `RectRasterableVol`
* `RasterableVol` represents a volume that is compile-time sized and has
its lower bound at `(0, 0, 0)`. The name `RasterableVol` was chosen
because such a volume can be used with `VolGrid3d`.
* `RectRasterableVol` represents a volume that is compile-time sized at
least in x and y direction and has its lower bound at `(0, 0, z)`.
There's no requirement on he lower bound or size in z direction.
The name `RectRasterableVol` was chosen because such a volume can be
used with `VolGrid2d`.
