## gl-block

### Using

Primitive types should have the `Block` instance.
With that in place, you can build structures and get Storable instances derived generically, according to intented usage.

```haskell
import GHC.Generics (Generic)
import Graphics.Gl.Block (Block, Packed(..), Std140(..), Std430(..))

-- | Attribute streams can be packed tightly together
data VertexAttrs = VertexAttrs
  { color :: Vec4
  , texCoords :: Vec2
  }
  deriving stock (Eq, Ord, Show, Generic) -- The regular stuff and Generic
  deriving anyclass Block -- The layout class, with Generic defaults
  deriving Storable via (Packed VertexAttrs) -- Free goodies!

-- | Uniform data require jumping through padding and alignment flaming hoop.
-- You can use derive-storable-plugin or hs2c instead, but there are gotchas.
data SceneUniform = SceneUniform
  { projection :: Mat4
  , viewPosition :: Vec3 -- Here comes the jazz
  , viewDirection :: Vec3
  }
  deriving stock (Eq, Ord, Show, Generic)
  deriving anyclass Block
  deriving Storable via (Std140 VertexAttrs) -- With comfy padding

-- | Shader buffer objects are less vacuous, but the rules are specific to the domain.
data Material = Material
  { baseColor :: Vec4
  , metallicRoughness :: Vec2
  , emission :: Vec4
  }
  deriving stock (Eq, Ord, Show, Generic)
  deriving anyclass Block
  deriving Storable via (Std430 VertexAttrs) -- Less alignment, less calculations
```

* [Shader Buffer Memory Layout Info](https://gist.github.com/teoxoy/936891c16c2a3d1c3c5e7204ac6cd76c)



### Benchmarks

The benchmark consists of filling a Storable vector.

* Packed layout is on par with manual instances.
* Std140 is slower, but not catastrophically so.
* Std430 seems to regain some performance due to being a tad simpler.

> There's only one "manual" case standing for all the layouts since it would only be different in pointer offsets.
> And no way in hell I'm going to calculate them by hand!

```python
  struct
    10
      manual: OK (2.26s)
        60.6 ns ± 5.4 ns
      packed: OK (1.21s)
        63.7 ns ± 5.8 ns
      std140: OK (1.19s)
        256  ns ±  26 ns
      std430: OK (0.13s)
        269  ns ±  24 ns
    1000
      manual: OK (1.95s)
        3.34 μs ± 244 ns
      packed: OK (1.96s)
        3.34 μs ± 235 ns
      std140: OK (1.97s)
        6.70 μs ± 425 ns
      std430: OK (1.44s)
        4.89 μs ±  93 ns
    1000000
      manual: OK (2.59s)
        2.22 ms ± 176 μs
      packed: OK (1.50s)
        2.47 ms ±  24 μs
      std140: OK (2.70s)
        4.54 ms ± 431 μs
      std430: OK (1.08s)
        3.28 ms ± 244 μs
```

> Caveat: nested structures have degraded performance.