1 files changed, 91 insertions, 0 deletions
diff --git a/engine/systems/Physics.ts b/engine/systems/Physics.ts
new file mode 100644
index 0000000..7edeb41
--- /dev/null
+++ b/engine/systems/Physics.ts
@@ -0,0 +1,91 @@
+import { System, SystemNames } from ".";
+import {
+  Acceleration,
+  BoundingBox,
+  ComponentNames,
+  Forces,
+  Gravity,
+  Velocity,
+  Mass,
+  Jump,
+} from "../components";
+import { PhysicsConstants } from "../config";
+import type { Entity } from "../entities";
+import type { Force2D } from "../interfaces";
+import { Game } from "../Game";
+
+export class Physics extends System {
+  constructor() {
+    super(SystemNames.Physics);
+  }
+
+  public update(dt: number, game: Game): void {
+    game.componentEntities.get(ComponentNames.Forces)?.forEach((entityId) => {
+      const entity = game.entities.get(entityId);
+
+      const mass = entity.getComponent<Mass>(ComponentNames.Mass).mass;
+      const forces = entity.getComponent<Forces>(ComponentNames.Forces).forces;
+      const velocity = entity.getComponent<Velocity>(ComponentNames.Velocity);
+      const inertia = entity.getComponent<Moment>(
+        ComponentNames.Moment
+      ).inertia;
+
+      // F_g = mg, applied only until terminal velocity is reached
+      if (entity.hasComponent(ComponentNames.Gravity)) {
+        const gravity = entity.getComponent<Gravity>(ComponentNames.Gravity);
+        if (velocity.dCartesian.dy <= gravity.terminalVelocity) {
+          forces.push({
+            fCartesian: {
+              fy: mass * PhysicsConstants.GRAVITY,
+            },
+          });
+        }
+      }
+
+      // ma = Σ(F), Iα = Σ(T)
+      const sumOfForces = forces.reduce(
+        (accum: Force2D, { fCartesian, torque }: Force2D) => ({
+          fCartesian: {
+            fx: accum.fCartesian.fx + (fCartesian?.fx ?? 0),
+            fy: accum.fCartesian.fy + (fCartesian?.fy ?? 0),
+          },
+          torque: accum.torque + (torque ?? 0),
+        }),
+        { fCartesian: { fx: 0, fy: 0 }, torque: 0 }
+      );
+
+      // integrate accelerations
+      const [ddy, ddx] = [
+        sumOfForces.fCartesian.fy,
+        sumOfForces.fCartesian.fx,
+      ].map((x) => x / mass);
+      velocity.dCartesian.dx += ddx * dt;
+      velocity.dCartesian.dy += ddy * dt;
+      velocity.dTheta += (sumOfForces.torque * dt) / inertia;
+      // clear the forces
+      entity.getComponent<Forces>(ComponentNames.Forces).forces = [];
+
+      // maybe we fell off the floor
+      if (ddy > 0 && entity.hasComponent(ComponentNames.Jump)) {
+        entity.getComponent<Jump>(ComponentNames.Jump).canJump = false;
+      }
+    });
+
+    game.componentEntities.get(ComponentNames.Velocity)?.forEach((entityId) => {
+      const entity = game.entities.get(entityId);
+      const velocity = entity.getComponent<Velocity>(ComponentNames.Velocity);
+      const boundingBox = entity.getComponent<BoundingBox>(
+        ComponentNames.BoundingBox
+      );
+
+      // integrate velocity
+      boundingBox.center.x += velocity.dCartesian.dx * dt;
+      boundingBox.center.y += velocity.dCartesian.dy * dt;
+      boundingBox.rotation += velocity.dTheta * dt;
+      boundingBox.rotation =
+        (boundingBox.rotation < 0
+          ? 360 + boundingBox.rotation
+          : boundingBox.rotation) % 360;
+    });
+  }
+}