fix CCD (but introduce another issue >.<)

engine/system/physics.ts

@@ -2,13 +2,7 @@ import type { Circle } from "#/engine/Circle.ts";
 import type { Polygon } from "#/engine/Polygon.ts";
 import type { Rect } from "#/engine/Rect.ts";
 import $ from "#/engine/Scene.ts";
-import {
-	createVector,
-	getDotProduct,
-	invScale,
-	scale,
-	type Vector,
-} from "#/engine/Vector.ts";
+import { createVector, invScale, type Vector, ZERO } from "#/engine/Vector.ts";
 
 export enum Shape {
 	Circle,
@@ -418,6 +412,7 @@ function createCollision(): Collision {
 	};
 }
 
+const relVelocity = createVector();
 export function moveAndCollide(
 	id: string,
 	velocity: Vector,
@@ -425,6 +420,7 @@ export function moveAndCollide(
 ): Array<Collision> {
 	const colliders = $<Collider>("collider")!;
 	const positions = $<Vector>("position")!;
+	const velocities = $<Vector>("velocity");
 	const selfColl = colliders.get(id)!.value;
 	const selfPos = positions.get(id)!.value;
 
@@ -438,13 +434,22 @@ export function moveAndCollide(
 
 		const otherColl = collider.value;
 		const otherPos = positions.get(cid)!.value;
+		const otherVel = velocities?.get(cid)?.value ?? ZERO;
+
+		relVelocity.x = velocity.x - otherVel.x;
+		relVelocity.y = velocity.y - otherVel.y;
+
+		// no relative motion
+		if (relVelocity.x === 0 && relVelocity.y === 0) {
+			continue;
+		}
 
 		const collision = next(selfColl.buffer);
 		const tHit = sweptAABB(
 			collision,
 			selfColl,
 			selfPos,
-			velocity,
+			relVelocity,
 			otherColl,
 			otherPos,
 			delta,
@@ -462,7 +467,7 @@ export function moveAndCollide(
 					collision,
 					selfColl,
 					selfPos,
-					velocity,
+					relVelocity,
 					otherColl,
 					otherPos,
 					delta,
@@ -474,7 +479,7 @@ export function moveAndCollide(
 					collision,
 					selfColl,
 					selfPos,
-					velocity,
+					relVelocity,
 					otherColl,
 					otherPos,
 					delta,
@@ -486,7 +491,7 @@ export function moveAndCollide(
 					collision,
 					selfColl,
 					selfPos,
-					velocity,
+					relVelocity,
 					otherColl,
 					otherPos,
 					delta,
@@ -510,6 +515,7 @@ export function moveAndSlide(
 ): void {
 	let timeLeft = 1;
 	const position = $<Vector>("position").get(id)!.value;
+	const velocities = $<Vector>("velocity");
 
 	let i = 0;
 	while (i++ < 4 && timeLeft > 0) {
@@ -530,19 +536,18 @@ export function moveAndSlide(
 		}
 
 		const time = closest.time;
-		// if the collider overlap already
-		if (time === 0) {
-			break;
-		}
-
-		const dot = getDotProduct(velocity, closest.normal);
-		velocity.x -= closest.normal.x * dot;
-		velocity.y -= closest.normal.y * dot;
-
 		const scalar = Math.max(0, time - EPSILON) * deltaLeft;
 		position.x += velocity.x * scalar;
 		position.y += velocity.y * scalar;
 
+		// a stationary entity might not have a velocity compnent
+		const otherVel = velocities.get(closest.cid)?.value ?? ZERO;
+		const rvx = velocity.x - otherVel.x;
+		const rvy = velocity.y - otherVel.y;
+		const dot = rvx * closest.normal.x + rvy * closest.normal.y;
+		velocity.x -= closest.normal.x * dot;
+		velocity.y -= closest.normal.y * dot;
+
 		timeLeft *= 1 - time;
 	}
 }