From 410a072c3862481f729c293402ddd49c5ae98769 Mon Sep 17 00:00:00 2001
From: Matthew Kosarek <mattkae@protonmail.com>
Date: Thu, 24 Jun 2021 13:49:23 -0400
Subject: (mkosarek) Decent enough discussion of 2d rotational forces

---
 2d/rigidbody/rigidbody_2/snippet3.cpp | 88 +++++++++++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)
 create mode 100644 2d/rigidbody/rigidbody_2/snippet3.cpp

(limited to '2d/rigidbody/rigidbody_2/snippet3.cpp')

diff --git a/2d/rigidbody/rigidbody_2/snippet3.cpp b/2d/rigidbody/rigidbody_2/snippet3.cpp
new file mode 100644
index 0000000..7b7130c
--- /dev/null
+++ b/2d/rigidbody/rigidbody_2/snippet3.cpp
@@ -0,0 +1,88 @@
+
+struct Impulse {
+    Vector2 force = { 0, 0 };
+    Vector2 pointOfApplication = { 0, 0 };
+    float32 timeOfApplicationSeconds = 0.25f;
+    float32 timeAppliedSeconds = 0.f;
+    bool isDead = false;
+};
+
+const int32 NUM_IMPULSES = 4;
+
+struct Rigidbody {
+    int32 numImpulses = 0;
+    Impulse activeImpulses[NUM_IMPULSES];
+    Vector2 velocity = { 0, 0 };
+    Vector2 position = { 0, 0 };
+    float32 mass = 1.f;
+
+	float32 rotationalVelocity = 0.f;
+	float32 rotation = 0.f;
+	float32 momentOfInertia = 1.f;
+
+    void reset() {
+        numImpulses = 0;
+        velocity = { 0, 0 };
+        rotationalVelocity = 0.f;
+        rotation = 0.f;
+    }
+
+    void applyImpulse(Impulse i) {
+        if (numImpulses > NUM_IMPULSES) {
+            printf("Unable to apply impulse. Buffer full.\n");
+            return;
+        }
+
+        activeImpulses[numImpulses] = i;
+        numImpulses++;
+    }
+
+    void applyGravity(float32 deltaTimeSeconds) {
+        velocity += (Vector2 { 0.f, -9.8f } * deltaTimeSeconds);
+    }
+
+    void update(float32 deltaTimeSeconds) {
+        applyGravity(deltaTimeSeconds);
+
+        Vector2 force;
+        float32 torque = 0.f;
+        for (int32 idx = 0; idx < numImpulses; idx++) {
+            Impulse& i = activeImpulses[idx];
+
+            float32 nextTimeAppliedSeconds = i.timeAppliedSeconds + deltaTimeSeconds;
+            if (nextTimeAppliedSeconds >= i.timeOfApplicationSeconds) {
+                nextTimeAppliedSeconds = i.timeOfApplicationSeconds;
+                i.isDead = true;
+            }
+            
+            float32 impulseDtSeconds = nextTimeAppliedSeconds - i.timeAppliedSeconds;
+            Vector2 forceToApply = i.force * (impulseDtSeconds / i.timeOfApplicationSeconds);
+            force += forceToApply * impulseDtSeconds;
+
+            // New! Increment the torque for each force
+            torque += i.pointOfApplication.getPerp().dot(forceToApply);
+
+            i.timeAppliedSeconds = nextTimeAppliedSeconds;
+        }
+        
+        Vector2 acceleration = force / mass;
+        velocity += (acceleration * deltaTimeSeconds);
+        position += (velocity * deltaTimeSeconds);
+
+        // New! Update the rotational velocity as well
+        float32 rotationalAcceleration = torque / momentOfInertia;
+        rotationalVelocity += (rotationalAcceleration * deltaTimeSeconds);
+        rotation += (rotationalVelocity * deltaTimeSeconds);
+
+        for (int32 idx = 0; idx < numImpulses; idx++) {
+            if (activeImpulses[idx].isDead) {
+                for (int j = idx + 1; j < numImpulses; j++) {
+                    activeImpulses[j - 1] = activeImpulses[j];
+                }
+
+                idx = idx - 1;
+                numImpulses--;
+            }
+        }
+    }
+};
-- 
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