Add Margolus block offset utility with tests

src/__tests__/margolus.test.ts

@@ -0,0 +1,28 @@
+import { describe, expect, test } from "bun:test";
+import { getBlockOffset } from "../sand/margolus";
+
+describe("getBlockOffset", () => {
+    test("frame 0 returns even offset", () => {
+        expect(getBlockOffset(0)).toEqual({ x: 0, y: 0 });
+    });
+
+    test("frame 1 returns odd offset", () => {
+        expect(getBlockOffset(1)).toEqual({ x: 1, y: 1 });
+    });
+
+    test("frame 2 returns even offset", () => {
+        expect(getBlockOffset(2)).toEqual({ x: 0, y: 0 });
+    });
+
+    test("frame 3 returns odd offset", () => {
+        expect(getBlockOffset(3)).toEqual({ x: 1, y: 1 });
+    });
+
+    test("large even number returns even offset", () => {
+        expect(getBlockOffset(1000)).toEqual({ x: 0, y: 0 });
+    });
+
+    test("large odd number returns odd offset", () => {
+        expect(getBlockOffset(1001)).toEqual({ x: 1, y: 1 });
+    });
+});

src/sand/margolus.ts

@@ -0,0 +1,4 @@
+export function getBlockOffset(frame: number): { x: number; y: number } {
+    const parity = frame & 1;
+    return { x: parity, y: parity };
+}