time for a snapshot

workshop/steps/03-integration/motion.test.ts

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+import { expect, test } from "bun:test";
+import { vec } from "./given.ts";
+import { integrate } from "./motion.ts";
+
+test("integrate advances position by velocity * delta", () => {
+	// velocity 0.1 units/ms, 100ms => move 10 units
+	expect(integrate(vec(0, 0), vec(0.1, 0), 100)).toEqual({ x: 10, y: 0 });
+});
+
+test("zero delta does not move", () => {
+	expect(integrate(vec(5, 5), vec(0.1, 0.1), 0)).toEqual({ x: 5, y: 5 });
+});
+
+test("framerate independence: one big step == many small steps", () => {
+	const start = vec(0, 0);
+	const vel = vec(0.1, -0.05);
+
+	const oneBigStep = integrate(start, vel, 16);
+
+	let manySmall = start;
+	for (let i = 0; i < 16; i++) {
+		manySmall = integrate(manySmall, vel, 1);
+	}
+
+	expect(manySmall.x).toBeCloseTo(oneBigStep.x);
+	expect(manySmall.y).toBeCloseTo(oneBigStep.y);
+});