Add array utils

src/utils.js

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+export function nextIndex(list, currentIndex) {
+  return Math.min(list.length - 1, Math.floor(currentIndex + 1));
+}
+
+export function prevIndex(list, currentIndex) {
+  return Math.max(0, Math.ceil(currentIndex - 1));
+}

src/utils.test.js

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+import { nextIndex, prevIndex } from "./utils";
+
+describe("nextIndex", () => {
+  const fiveItems = [1, 2, 3, 4, 5];
+  test("works with middle index", () => {
+    expect(nextIndex(fiveItems, 2)).toBe(3);
+  });
+  test("works with last index", () => {
+    expect(nextIndex(fiveItems, 4)).toBe(4);
+  });
+  test("works with fractions", () => {
+    expect(nextIndex(fiveItems, 1.1)).toBe(2);
+    expect(nextIndex(fiveItems, 1.9)).toBe(2);
+  });
+});
+
+describe("prevIndex", () => {
+  const fiveItems = [1, 2, 3, 4, 5];
+  test("works with middle index", () => {
+    expect(prevIndex(fiveItems, 2)).toBe(1);
+  });
+  test("works with start index", () => {
+    expect(prevIndex(fiveItems, 0)).toBe(0);
+  });
+  test("works with fractions", () => {
+    expect(prevIndex(fiveItems, 1.1)).toBe(1);
+    expect(prevIndex(fiveItems, 1.9)).toBe(1);
+  });
+});