linearAlgorithm: add test, fix selection order

app/soapbox/features/timeline-insertion/__tests__/linear.test.ts

@@ -0,0 +1,19 @@
+import { linearAlgorithm } from '../linear';
+
+const DATA = Object.freeze(['a', 'b', 'c', 'd']);
+
+test('linearAlgorithm', () => {
+  const result = Array(50).fill('').map((_, i) => {
+    return linearAlgorithm(DATA, i, { interval: 5 });
+  });
+
+  // console.log(result);
+  expect(result[0]).toBe(undefined);
+  expect(result[4]).toBe('a');
+  expect(result[8]).toBe(undefined);
+  expect(result[9]).toBe('b');
+  expect(result[10]).toBe(undefined);
+  expect(result[14]).toBe('c');
+  expect(result[15]).toBe(undefined);
+  expect(result[19]).toBe('d');
+});

app/soapbox/features/timeline-insertion/linear.ts

@@ -8,7 +8,7 @@ type Opts = {
 /** Picks the next item every iteration. */
 const linearAlgorithm: PickAlgorithm = (items, iteration, rawOpts) => {
   const opts = normalizeOpts(rawOpts);
-  const itemIndex = items ? Math.floor((iteration + 1) / opts.interval) % items.length : 0;
+  const itemIndex = items ? Math.floor(iteration / opts.interval) % items.length : 0;
   const item = items ? items[itemIndex] : undefined;
   const showItem = (iteration + 1) % opts.interval === 0;