From 37f4f56cd66e762bba0b470ba3591d5f18fca15c Mon Sep 17 00:00:00 2001
From: Syntriax <syntriax@gmail.com>
Date: Sat, 4 Apr 2026 19:38:24 +0300
Subject: [PATCH] feat(physics): IRigidbody Intertia and Inverse Mass &
 Intertia fields added

---
 Engine.Physics2D/Abstract/IRigidBody2D.cs | 15 +++++
 Engine.Physics2D/RigidBody2D.cs           | 67 +++++++++++++++++++++--
 2 files changed, 78 insertions(+), 4 deletions(-)

diff --git a/Engine.Physics2D/Abstract/IRigidBody2D.cs b/Engine.Physics2D/Abstract/IRigidBody2D.cs
index 871fc0e..5c0fa27 100644
--- a/Engine.Physics2D/Abstract/IRigidBody2D.cs
+++ b/Engine.Physics2D/Abstract/IRigidBody2D.cs
@@ -27,6 +27,21 @@ public interface IRigidBody2D : IBehaviour2D
     /// </summary>
     float Mass { get; set; }
 
+    /// <summary>
+    /// The inverse mass (1 / Mass) of the <see cref="IRigidBody2D"/>.
+    /// </summary>
+    float InverseMass { get; }
+
+    /// <summary>
+    /// The Invertia of the <see cref="IRigidBody2D"/>.
+    /// </summary>
+    float Inertia { get; }
+
+    /// <summary>
+    /// The inverse Invertia (1 / Invertia) of the <see cref="IRigidBody2D"/>.
+    /// </summary>
+    float InverseInertia { get; }
+
     /// <summary>
     /// The value indicating whether the <see cref="IRigidBody2D"/> is static/immovable.
     /// </summary>
diff --git a/Engine.Physics2D/RigidBody2D.cs b/Engine.Physics2D/RigidBody2D.cs
index dbf97be..92d54d2 100644
--- a/Engine.Physics2D/RigidBody2D.cs
+++ b/Engine.Physics2D/RigidBody2D.cs
@@ -1,16 +1,75 @@
+using System.Collections.Generic;
+
 using Engine.Core;
 
 namespace Engine.Physics2D;
 
-public class RigidBody2D : Behaviour2D, IRigidBody2D
+public class RigidBody2D : Behaviour2D, IRigidBody2D, IFirstFrameUpdate, ILastFrameUpdate
 {
     private const float LOWEST_ALLOWED_MASS = 0.00001f;
 
-    public IPhysicsMaterial2D Material { get; set; } = ReadOnlyPhysicsMaterial2D.Default;
+    public IPhysicsMaterial2D Material { get; set; } = new PhysicsMaterial2D(0f, 0f);
 
-    public Vector2D Velocity { get; set; } = Vector2D.Zero;
+    public Vector2D Velocity { get => IsStatic ? Vector2D.Zero : field; set => field = IsStatic ? Vector2D.Zero : value; } = Vector2D.Zero;
     public float AngularVelocity { get; set; } = 0f;
     public bool IsStatic { get; set; } = false;
 
-    public float Mass { get; set => field = Math.Max(value, LOWEST_ALLOWED_MASS); } = 1f;
+    public float Mass { get; set { field = Math.Max(value, LOWEST_ALLOWED_MASS); } } = 1f;
+    public float InverseMass { get; protected set; } = 1f;
+
+    public float Inertia { get; protected set; } = 1f;
+    public float InverseInertia { get; protected set; } = 1f;
+
+    private readonly List<ICollider2D> childColliders = [];
+
+    public void LastActiveFrame() => DisconnectColliders();
+    public void FirstActiveFrame()
+    {
+        ReconnectColliders();
+        UpdateValues();
+    }
+
+    private void ReconnectColliders()
+    {
+        DisconnectColliders();
+
+        BehaviourController.GetBehavioursInChildren(childColliders);
+
+        foreach (ICollider2D collider in childColliders)
+            collider.OnRecalculated.AddListener(RecalculateCallback);
+    }
+
+    private void DisconnectColliders()
+    {
+        foreach (ICollider2D collider in childColliders)
+            collider.OnRecalculated.RemoveListener(RecalculateCallback);
+    }
+
+    private void RecalculateCallback(ICollider2D _) => UpdateValues();
+    private void UpdateValues()
+    {
+        InverseMass = Mass.OneOver();
+
+        Vector2D center = Transform.Position;
+        Inertia = 0f;
+
+        float totalColliderArea = 0f;
+
+        foreach (ICollider2D collider in childColliders)
+            totalColliderArea += collider.Area;
+
+        foreach (ICollider2D collider in childColliders)
+        {
+            float colliderMass = Mass * (collider.Area / totalColliderArea);
+            float colliderInertia = collider.GeometricInertia * colliderMass;
+            float distanceSquared = center.FromTo(collider.Transform.Position).MagnitudeSquared;
+
+            Inertia += colliderInertia + colliderMass * distanceSquared;
+        }
+
+        if (childColliders.Count == 0)
+            Inertia = 1f;
+
+        InverseInertia = Inertia.OneOver();
+    }
 }