From 0f4b8fcb25c054f89620ffaad067538b49a78bb8 Mon Sep 17 00:00:00 2001
From: Paris DOUADY <paris.douady@hotmail.fr>
Date: Wed, 5 Jun 2024 21:05:07 +0200
Subject: [PATCH] add moments of inertia

---
 geom/src/aabb.rs   | 7 +++++++
 geom/src/circle.rs | 6 ++++++
 geom/src/obb.rs    | 9 +++++++++
 3 files changed, 22 insertions(+)

diff --git a/geom/src/aabb.rs b/geom/src/aabb.rs
index a89454b3..c4e4a37d 100644
--- a/geom/src/aabb.rs
+++ b/geom/src/aabb.rs
@@ -88,6 +88,13 @@ impl AABB {
         }
     }
 
+    #[inline]
+    /// Calculates the moment of inertia of the AABB around its center.
+    pub fn moment_of_inertia(&self, mass: f32) -> f32 {
+        let Vec2 { x: w, y: h } = self.size();
+        mass * (w * w + h * h) / 12.0
+    }
+
     #[inline]
     pub fn size(&self) -> Vec2 {
         self.ur - self.ll
diff --git a/geom/src/circle.rs b/geom/src/circle.rs
index c1344cc3..12c2e5a8 100644
--- a/geom/src/circle.rs
+++ b/geom/src/circle.rs
@@ -8,9 +8,15 @@ pub struct Circle {
 }
 
 impl Circle {
+    #[inline]
     pub fn contains(&self, p: Vec2) -> bool {
         self.center.is_close(p, self.radius)
     }
+
+    #[inline]
+    pub fn moment_of_inertia(&self, mass: f32) -> f32 {
+        mass * self.radius * self.radius * 0.5
+    }
 }
 
 impl Circle {
diff --git a/geom/src/obb.rs b/geom/src/obb.rs
index b0fdee3c..10bd87a2 100644
--- a/geom/src/obb.rs
+++ b/geom/src/obb.rs
@@ -55,6 +55,15 @@ impl OBB {
         ]
     }
 
+    #[inline]
+    /// Calculates the moment of inertia of the OBB around its center.
+    pub fn moment_of_inertia(&self, mass: f32) -> f32 {
+        let [a, b] = self.axis();
+        let a = a.mag2();
+        let b = b.mag2();
+        mass * (a + b) / 12.0
+    }
+
     #[inline]
     pub fn center(&self) -> Vec2 {
         (self.corners[2] + self.corners[0]) * 0.5