From 3787dbbc3a69bc6bc2a627c439efc137f499fb37 Mon Sep 17 00:00:00 2001
From: Jared Miller <jared@smell.flowers>
Date: Wed, 11 Mar 2026 14:05:17 -0400
Subject: [PATCH] Add Margolus block offset utility with tests

---
 src/__tests__/margolus.test.ts | 28 ++++++++++++++++++++++++++++
 src/sand/margolus.ts           |  4 ++++
 2 files changed, 32 insertions(+)
 create mode 100644 src/__tests__/margolus.test.ts
 create mode 100644 src/sand/margolus.ts

diff --git a/src/__tests__/margolus.test.ts b/src/__tests__/margolus.test.ts
new file mode 100644
index 0000000..f6705e7
--- /dev/null
+++ b/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 });
+    });
+});
diff --git a/src/sand/margolus.ts b/src/sand/margolus.ts
new file mode 100644
index 0000000..0345a40
--- /dev/null
+++ b/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 };
+}