1 files changed, 163 insertions, 0 deletions
diff --git a/alligator_render/src/instance.rs b/alligator_render/src/instance.rs
new file mode 100644
index 0000000..2d1808f
--- /dev/null
+++ b/alligator_render/src/instance.rs
@@ -0,0 +1,163 @@
+use std::mem::size_of;
+
+use bytemuck::{Pod, Zeroable};
+
+/// The ID for an instance
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+pub struct InstanceId(usize);
+
+/// A sprite, that can be used by the alligator shader
+#[repr(C)]
+#[derive(Copy, Clone, Debug, PartialEq, Pod, Zeroable)]
+pub struct Instance {
+	/// Position on the screen
+	pub position: [f32; 2],
+	/// Relative size
+	pub size: [f32; 2],
+	/// The location of the texture in the texture atlas
+	pub texture_coordinates: [f32; 2],
+	/// The size of the sprite's texture
+	pub texture_size: [f32; 2],
+	/// The index of the texture atlas to use
+	pub texture_atlas_index: u32,
+	/// Rotation, in radians
+	pub rotation: f32,
+	/// z-index
+	pub z_index: f32,
+}
+
+impl Default for Instance {
+	fn default() -> Self {
+		Self {
+			position: [0.0; 2],
+			size: [1.0; 2],
+			rotation: 0.0,
+			z_index: 0.0,
+			texture_coordinates: [0.0; 2],
+			texture_size: [1.0; 2],
+			texture_atlas_index: 0,
+		}
+	}
+}
+
+impl Instance {
+	// whenever this is updated, please also update `sprite.wgsl`
+	const ATTRIBUTES: [wgpu::VertexAttribute; 7] = wgpu::vertex_attr_array![
+		1 => Float32x2, 2 => Float32x2, 3 => Float32x2, 4 => Float32x2,
+		5 => Uint32, 6 => Float32, 7 => Float32
+	];
+
+	pub(crate) fn desc<'a>() -> wgpu::VertexBufferLayout<'a> {
+		// make sure these two don't conflict
+		debug_assert_eq!(
+			Self::ATTRIBUTES[0].shader_location as usize,
+			crate::Vertex::ATTRIBUTES.len()
+		);
+		wgpu::VertexBufferLayout {
+			array_stride: size_of::<Self>() as wgpu::BufferAddress,
+			step_mode: wgpu::VertexStepMode::Instance,
+			attributes: &Self::ATTRIBUTES,
+		}
+	}
+}
+
+/// A buffer of sprites, for both CPU and GPU memory
+pub struct InstanceBuffer {
+	instances: Vec<Instance>,
+	instance_buffer: wgpu::Buffer,
+	instance_buffer_size: usize,
+}
+
+fn create_buffer(device: &wgpu::Device, instances: &Vec<Instance>) -> wgpu::Buffer {
+	device.create_buffer(&wgpu::BufferDescriptor {
+		label: Some("Sprite Instance Buffer"),
+		size: (instances.capacity() * size_of::<Instance>()) as wgpu::BufferAddress,
+		usage: wgpu::BufferUsages::VERTEX | wgpu::BufferUsages::COPY_DST,
+		mapped_at_creation: false,
+	})
+}
+
+impl InstanceBuffer {
+	/// Create a new buffer with the given capacity
+	pub(crate) fn new(device: &wgpu::Device, capacity: usize) -> Self {
+		let instances = Vec::with_capacity(capacity);
+		let instance_buffer_size = instances.capacity();
+		let instance_buffer = create_buffer(device, &instances);
+
+		Self {
+			instances,
+			instance_buffer,
+			instance_buffer_size,
+		}
+	}
+
+	/// The number of sprites
+	pub fn len(&self) -> u32 {
+		self.instances
+			.len()
+			.try_into()
+			.expect("expected less than 3 billion instances")
+	}
+
+	/// Returns `true` if there are no sprites
+	pub fn is_empty(&self) -> bool {
+		self.instances.is_empty()
+	}
+
+	/// The capacity of the buffer
+	pub const fn buffer_size(&self) -> usize {
+		self.instance_buffer_size
+	}
+
+	/// Get a slice containing the entire buffer
+	pub(crate) fn buffer_slice(&self) -> wgpu::BufferSlice {
+		self.instance_buffer.slice(..)
+	}
+
+	/// Add a new sprite. The new sprite's `InstanceId` is returned. This ID
+	/// becomes invalid if the buffer is cleared.
+	pub fn push_instance(&mut self, instance: Instance) -> InstanceId {
+		let index = self.instances.len();
+		self.instances.push(instance);
+		InstanceId(index)
+	}
+
+	/// Get a specific instance
+	pub fn get_instance(&self, id: InstanceId) -> Option<&Instance> {
+		self.instances.get(id.0)
+	}
+
+	/// Get a mutable reference to a specific sprite
+	pub fn get_instance_mut(&mut self, id: InstanceId) -> Option<&mut Instance> {
+		self.instances.get_mut(id.0)
+	}
+
+	/// Clear the instance buffer. This invalidates all `InstanceId`'s
+	pub fn clear(&mut self) {
+		self.instances.clear();
+	}
+
+	/// Increase the capacity of the buffer
+	fn expand_buffer(&mut self, device: &wgpu::Device) {
+		self.instance_buffer_size = self.instances.capacity();
+		self.instance_buffer = create_buffer(device, &self.instances);
+	}
+
+	/// Fill the GPU buffer with the sprites in the CPU buffer.
+	#[profiling::function]
+	pub(crate) fn fill_buffer(&mut self, device: &wgpu::Device, queue: &wgpu::Queue) {
+		if self.instances.len() > self.instance_buffer_size {
+			self.expand_buffer(device);
+		}
+
+		// the instances must be sorted by z-index before being handed to the GPU
+		let mut sorted = self.instances.clone();
+		sorted.sort_by(|a, b| a.z_index.total_cmp(&b.z_index));
+
+		queue.write_buffer(
+			&self.instance_buffer,
+			0 as wgpu::BufferAddress,
+			bytemuck::cast_slice(&sorted),
+		);
+	}
+}