WIP: Add star model

models/star/Cargo.toml

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+[package]
+name    = "star"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies.fj]
+path = "../../fj"

models/star/README.md

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+# Fornjot - Star
+
+A model of a star that demonstrates sweeping a sketch to create a 3D shape, concave sketches, and how to generate sketches from code.
+
+To display this model, run the following from the repository root (model parameters are optional):
+``` sh
+cargo run -- --model start --parameters num_points=5 r1=1.0 r2=2.0 h=1.0
+```
+
+![Screenshot of the star model](star.png)

models/star/src/lib.rs

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+use std::{collections::HashMap, f64::consts::PI};
+
+#[no_mangle]
+pub extern "C" fn model(args: &HashMap<String, String>) -> fj::Shape {
+    // Number of points of the star
+    //
+    // "Points" in the sense of "pointy ends", not in the sense of geometrical
+    // points, or vertices.
+    let num_points: u64 = args
+        .get("num_points")
+        .map(|arg| arg.parse().unwrap())
+        .unwrap_or(5);
+
+    // Radius of the circle that all the vertices between the pointy ends are on
+    let r1: f64 = args
+        .get("r1")
+        .map(|arg| arg.parse().unwrap())
+        .unwrap_or(1.0);
+
+    // Radius of the circle that all the pointy ends are on
+    let r2: f64 = args
+        .get("r2")
+        .map(|arg| arg.parse().unwrap())
+        .unwrap_or(2.0);
+
+    // The height of the star
+    let h: f64 = args.get("h").map(|arg| arg.parse().unwrap()).unwrap_or(1.0);
+
+    // We need to figure out where to generate vertices, depending on the number
+    // of points the star is supposed to have. Let's generate an iterator that
+    // gives us the angle and radius for each vertex.
+    let num_vertices = num_points * 2;
+    let vertex_iter = (0..num_vertices).map(|i| {
+        let angle = 2. * PI / num_vertices as f64 * i as f64;
+        let radius = if i % 2 == 0 { r1 } else { r2 };
+        (angle, radius)
+    });
+
+    // Now that we got that iterator prepared, generating the vertices is just a
+    // bit of trigonometry.
+    let mut vertices = Vec::new();
+    for (angle, radius) in vertex_iter {
+        let (sin, cos) = angle.sin_cos();
+
+        let x = cos * radius;
+        let y = sin * radius;
+
+        vertices.push([x, y]);
+    }
+
+    let sketch = fj::Sketch::from_points(vertices);
+    let star = fj::Sweep {
+        shape: sketch.into(),
+        length: h,
+    };
+
+    star.into()
+}