From 4fc201d392db75150ac346fbbe1282e43577670e Mon Sep 17 00:00:00 2001
From: cha0s <git@cha0s.io>
Date: Tue, 23 Mar 2021 06:09:47 -0500
Subject: [PATCH] refactor: better physics simulation

---
 packages/physics/src/matter/body.js     |  6 +++++-
 packages/physics/src/matter/world.js    |  6 ++++--
 packages/physics/src/traits/physical.js | 27 +++++++++++++++++++++----
 3 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/packages/physics/src/matter/body.js b/packages/physics/src/matter/body.js
index 427b042..cc79a8d 100644
--- a/packages/physics/src/matter/body.js
+++ b/packages/physics/src/matter/body.js
@@ -11,7 +11,7 @@ import CircleShape from '../shape/circle';
 import RectangleShape from '../shape/rectangle';
 
 // Translate "real" coordinates to physics coordinates.
-const SCALE = 1;
+const SCALE = 1 / 12;
 
 export default class Body extends AbstractBody {
 
@@ -106,6 +106,10 @@ export default class Body extends AbstractBody {
     return this.filterCategories[group];
   }
 
+  set friction(friction) {
+    MatterBody.set(this.matterBody, 'frictionAir', friction);
+  }
+
   static lookupBody(matterBody) {
     return this.bodies
       ? this.bodies.get(matterBody)
diff --git a/packages/physics/src/matter/world.js b/packages/physics/src/matter/world.js
index 7d44c25..764e9b9 100644
--- a/packages/physics/src/matter/world.js
+++ b/packages/physics/src/matter/world.js
@@ -76,8 +76,10 @@ export default class World extends AbstractWorld {
       const it = this.entities.entries();
       for (let value = it.next(); value.done !== true; value = it.next()) {
         const [body] = value.value;
-        const [x, y] = Vector.scale(body.impulse, stepTime);
-        MatterBody.translate(body.matterBody, {x, y});
+        const [x, y] = Vector.scale(body.impulse, stepTime / (1 - body.matterBody.frictionAir));
+        if (x || y) {
+          MatterBody.setVelocity(body.matterBody, {x, y});
+        }
       }
       Engine.update(this.engine, stepTimeInMs);
       // eslint-disable-next-line no-param-reassign
diff --git a/packages/physics/src/traits/physical.js b/packages/physics/src/traits/physical.js
index 74ff8d6..6c3e628 100644
--- a/packages/physics/src/traits/physical.js
+++ b/packages/physics/src/traits/physical.js
@@ -1,9 +1,16 @@
 import {Vector} from '@avocado/math';
-import {Trait} from '@avocado/traits';
+import {StateProperty, Trait} from '@avocado/traits';
+import {compose} from '@latus/core';
 
 import BodyView from '../body-view';
 
-export default () => class Physical extends Trait {
+const decorate = compose(
+  StateProperty('friction', {
+    track: true,
+  }),
+);
+
+export default () => class Physical extends decorate(Trait) {
 
   #body;
 
@@ -15,6 +22,7 @@ export default () => class Physical extends Trait {
     const world = this.#world;
     if (world) {
       const body = world.createBody(this.entity.shape);
+      body.friction = this.entity.friction;
       world.associateBodyWithEntity(body, this.entity);
       body.setCollisionForEntity(this.entity);
       world.addBody(body);
@@ -64,6 +72,12 @@ export default () => class Physical extends Trait {
     };
   }
 
+  static defaultState() {
+    return {
+      friction: 0.5,
+    };
+  }
+
   static dependencies() {
     return [
       'Shaped',
@@ -86,10 +100,15 @@ export default () => class Physical extends Trait {
         this.removeFromWorld();
       },
 
+      frictionChanged: () => {
+        if (this.#body) {
+          this.#body.friction = this.entity.friction;
+        }
+      },
+
       positionChanged: () => {
         if (this.#body) {
-          const {position} = this.entity;
-          this.#body.position = position;
+          this.#body.position = this.entity.position;
         }
       },