Implement a basic Sierpinski Gasket IFS

enkou-shaders/src/chaos_game.rs

@@ -0,0 +1,68 @@
+use glam::{vec2, Vec2};
+use rand::distr::{Distribution, StandardUniform};
+use rand::{Rng, RngExt};
+use crate::transform::Transform;
+
+struct BiUnit;
+impl Distribution<f32> for BiUnit {
+    fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> f32 {
+        rng.sample::<f32, _>(StandardUniform) * 2.0 - 1.0
+    }
+}
+
+/// 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>(
+    point: Vec2,
+    rng: &mut R,
+    transforms: &[Transform],
+    weights: &[f32],
+) -> (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,
+    )
+}
+
+pub struct ChaosGame<'a, R: Rng> {
+    current_point: Vec2,
+    rng: &'a mut R,
+    transforms: &'a [Transform],
+    weights: &'a [f32],
+}
+
+impl<'a, R: Rng> ChaosGame<'a, R> {
+    pub fn new(rng: &'a mut R, transforms: &'a [Transform], weights: &'a [f32]) -> Self {
+        let current_point = vec2(rng.sample(BiUnit), rng.sample(BiUnit));
+        ChaosGame { current_point, rng, transforms, weights }
+    }
+}
+
+impl<'a, R: Rng> Iterator for ChaosGame<'a, R> {
+    type Item = Vec2;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let (next_point, _) = step_chaos_game(self.current_point, self.rng, self.transforms, self.weights);
+        self.current_point = next_point;
+
+        Some(next_point)
+    }
+}