time for a snapshot

workshop/steps/10-move-and-slide/moveAndSlide.ts

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+import {
+	type AABB,
+	add,
+	EPSILON,
+	type Hit,
+	scale,
+	slide,
+	sweptAABB,
+	type Vec,
+} from "./given.ts";
+
+/**
+ * Move `box` by the full-frame displacement `v`, sliding along any `walls` it
+ * hits, up to 4 iterations. Returns the final corner position.
+ *
+ * See the README for the algorithm. You already have everything you need in
+ * given.ts: sweptAABB, slide, add, scale, EPSILON.
+ */
+export function moveAndSlide(box: AABB, v: Vec, walls: AABB[]): Vec {
+	let pos: Vec = { x: box.x, y: box.y };
+	let vel: Vec = { x: v.x, y: v.y };
+
+	let timeLeft = 1;
+	let i = 0;
+	while (i++ < 4 && timeLeft > 0) {
+		const move = scale(vel, timeLeft);
+		let nearest: Hit = {
+			normal: { x: 0, y: 0 },
+			time: 2,
+		};
+
+		for (const wall of walls) {
+			const hit = sweptAABB(
+				{
+					x: pos.x,
+					y: pos.y,
+					w: box.w,
+					h: box.h,
+				},
+				move,
+				wall,
+			);
+			if (hit !== null) {
+				nearest = hit.time < nearest.time ? hit : nearest;
+			}
+		}
+
+		if (nearest.time === 2) {
+			pos = add(pos, move);
+			break;
+		} else {
+			pos = add(pos, scale(move, Math.max(0, nearest.time - EPSILON)));
+			vel = slide(vel, nearest.normal);
+			timeLeft = timeLeft * (1 - nearest.time);
+		}
+	}
+
+	return pos;
+}