Add test for duplex sort

src/tests/duplex-collate.test.ts

@@ -0,0 +1,38 @@
+import { describe, expect, it } from 'vitest';
+import { buildDuplexOrder } from '@/js/logic/duplex-collate-page';
+
+describe('duplex collate order builder', () => {
+  it('interleaves fronts with reversed backs for even page counts', () => {
+    const result = buildDuplexOrder(8, 4, 'reverse');
+
+    expect(result.frontCount).toBe(4);
+    expect(result.backCount).toBe(4);
+    expect(result.order).toEqual([0, 7, 1, 6, 2, 5, 3, 4]);
+  });
+
+  it('interleaves fronts with back block kept as-is', () => {
+    const result = buildDuplexOrder(8, 4, 'keep');
+
+    expect(result.frontCount).toBe(4);
+    expect(result.backCount).toBe(4);
+    expect(result.order).toEqual([0, 4, 1, 5, 2, 6, 3, 7]);
+  });
+
+  it('keeps all pages when front and back counts are uneven (odd total)', () => {
+    const result = buildDuplexOrder(7, 4, 'reverse');
+
+    expect(result.frontCount).toBe(4);
+    expect(result.backCount).toBe(3);
+    expect(result.order).toEqual([0, 6, 1, 5, 2, 4, 3]);
+    expect(result.order).toHaveLength(7);
+  });
+
+  it('appends extra pages from longer back block after paired pages', () => {
+    const result = buildDuplexOrder(8, 3, 'reverse');
+
+    expect(result.frontCount).toBe(3);
+    expect(result.backCount).toBe(5);
+    expect(result.order).toEqual([0, 7, 1, 6, 2, 5, 4, 3]);
+    expect(result.order).toHaveLength(8);
+  });
+});