Add an initial implementation of the chaos game

enkou-shaders/Cargo.toml

@@ -10,6 +10,7 @@ repository.workspace = true
 workspace = true
 
 [dependencies]
-spirv-std.workspace = true
-glam.workspace = true
 bytemuck.workspace = true
+glam.workspace = true
+rand.workspace = true
+spirv-std.workspace = true

enkou-shaders/src/lib.rs

@@ -1,8 +1,12 @@
+//! # Enkou
 #![no_std]
+#![warn(missing_docs)]
 
 use bytemuck::{Pod, Zeroable};
 use core::f32::consts::PI;
-use glam::{Affine2, Vec3, Vec4, vec2, vec3};
+use glam::{Affine2, Vec3, Vec4, vec2, vec3, Vec2};
+use rand::{Rng, RngExt};
+use rand::distr::StandardUniform;
 #[cfg(target_arch = "spirv")]
 use spirv_std::num_traits::Float;
 use spirv_std::spirv;
@@ -69,6 +73,46 @@ impl Coefficients2 for Affine2 {
     }
 }
 
+#[derive(Copy, Clone, Pod, Zeroable)]
+#[repr(C)]
+pub struct Transform {
+    pub coefficients: Affine2,
+}
+
+impl Transform {
+    pub fn new(coefficients: Affine2) -> Self {
+        Transform { coefficients }
+    }
+
+    pub fn transform_point(&self, point: Vec2) -> Vec2 {
+        self.coefficients.transform_point2(point)
+    }
+}
+
+/// Iterate one step in the chaos game; choose the next transform, apply it,
+/// and return the resulting point. Also returns the transform index so that
+/// path-dependent weights (the "Xaos" table in Apophysis) can be chosen
+/// for the next iteration step.
+///
+/// # Arguments
+///
+/// * `weights` - Weights are assumed to be normalized; adding all elements together should return the value 1
+pub fn step_chaos_game<R: Rng>(rng: &mut R, point: Vec2, weights: &[f32], transforms: &[Transform]) -> (Vec2, u32) {
+    let mut choice_weight = rng.sample::<f32, _>(StandardUniform);
+    let mut transform_index: u32 = 0;
+
+    for weight in weights {
+        choice_weight -= weight;
+        if choice_weight <= 0.0 {
+            break;
+        }
+
+        transform_index += 1;
+    }
+
+    (transforms[transform_index as usize].transform_point(point), transform_index)
+}
+
 #[derive(Copy, Clone, Pod, Zeroable)]
 #[repr(C)]
 pub struct ShaderConstants {