From cd58c0ee0665a116c9881781c1c5aa4e008ead88 Mon Sep 17 00:00:00 2001
From: lisk77 <lisk77@protonmail.com>
Date: Sun, 27 Jul 2025 02:49:39 +0200
Subject: [PATCH] feat(math): added acceleration, curvature and arclength to
 the members of the Bezier curve. Split tangent and velocity functions.

---
 crates/comet_math/src/bezier.rs | 60 +++++++++++++++++++++++++++++++--
 1 file changed, 58 insertions(+), 2 deletions(-)

diff --git a/crates/comet_math/src/bezier.rs b/crates/comet_math/src/bezier.rs
index b9034b7..6ca3ee3 100644
--- a/crates/comet_math/src/bezier.rs
+++ b/crates/comet_math/src/bezier.rs
@@ -12,6 +12,7 @@ impl<V: InnerSpace + Clone> Bezier<V> {
         Self { points, degree }
     }
 
+    /// Returns the point at the position `t` along the curve.
     pub fn evaluate(&self, t: f32) -> V {
         let mut new_points = self.points.clone();
         for i in 0..self.degree {
@@ -22,7 +23,8 @@ impl<V: InnerSpace + Clone> Bezier<V> {
         new_points[0].clone()
     }
 
-    pub fn evaluate_tangent(&self, t: f32) -> V {
+    /// Returns the velocity at the position `t` along the curve.
+    pub fn velocity(&self, t: f32) -> V {
         let n = self.degree as f32;
 
         let mut d_pts: Vec<V> = self
@@ -37,6 +39,60 @@ impl<V: InnerSpace + Clone> Bezier<V> {
             }
         }
 
-        d_pts[0].clone().normalize()
+        d_pts[0].clone()
+    }
+
+    /// Returns the tangent at the position `t` along the curve.
+    pub fn tangent(&self, t: f32) -> V {
+        self.velocity(t).normalize()
+    }
+
+    /// Returns the acceleration at the position `t` along the curve.
+    pub fn acceleration(&self, t: f32) -> V {
+        let n = self.degree as f32;
+
+        let mut dd_pts: Vec<V> = self
+            .points
+            .windows(3)
+            .map(|w| (w[2] - w[1] * 2.0 + w[0]) * (n * (n - 1.0)))
+            .collect();
+
+        for i in 0..(self.degree - 2) {
+            for j in 0..(self.degree - 2 - i) {
+                dd_pts[j] = dd_pts[j].lerp(&dd_pts[j + 1], t);
+            }
+        }
+
+        dd_pts[0].clone()
+    }
+
+    /// Returns the curvature at the position `t` along the curve.
+    pub fn curvature(&self, t: f32) -> f32 {
+        let v = self.velocity(t);
+        let a = self.acceleration(t);
+
+        let s2 = v.dot(&v);
+        if s2 == 0.0 {
+            return 0.0;
+        }
+
+        let a_para = v.clone() * (a.dot(&v) / s2);
+        let a_perp = a - a_para;
+
+        a_perp.length() / s2.sqrt().powi(2)
+    }
+
+    /// Returns the arclength of the curve.
+    pub fn arclength(&self) -> f32 {
+        let mut length = 0.0;
+        let dt = 1.0 / 1000.0;
+        let mut prev = self.evaluate(0.0);
+        for i in 1..=1000 {
+            let t = i as f32 * dt;
+            let curr = self.evaluate(t);
+            length += (curr - prev).length();
+            prev = curr;
+        }
+        length
     }
 }