feat: compare two page versions (#2385)

apps/client/src/features/page-history/utils/resolve-compare-pair.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, it } from "vitest";
+import { resolveComparePair } from "./resolve-compare-pair";
+
+// list is newest-first, matching usePageHistoryListQuery order
+const items = [{ id: "v3" }, { id: "v2" }, { id: "v1" }];
+
+describe("resolveComparePair", () => {
+  it("orders newer before older regardless of selection order", () => {
+    expect(resolveComparePair(items, ["v1", "v3"])).toEqual({
+      newerId: "v3",
+      olderId: "v1",
+    });
+    expect(resolveComparePair(items, ["v3", "v1"])).toEqual({
+      newerId: "v3",
+      olderId: "v1",
+    });
+  });
+
+  it("returns null unless exactly two versions are selected", () => {
+    expect(resolveComparePair(items, [])).toBeNull();
+    expect(resolveComparePair(items, ["v1"])).toBeNull();
+    expect(resolveComparePair(items, ["v1", "v2", "v3"])).toBeNull();
+  });
+
+  it("returns null when a selected id is not in the list", () => {
+    expect(resolveComparePair(items, ["v1", "missing"])).toBeNull();
+  });
+
+  it("returns null when the same id is selected twice", () => {
+    expect(resolveComparePair(items, ["v2", "v2"])).toBeNull();
+  });
+});