feat(tarot): useTarotShuffle hook (Fisher-Yates + reversed 플래그) (T9)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/pages/tarot/hooks/useTarotShuffle.js

@@ -0,0 +1,25 @@
+import { useCallback, useState } from 'react';
+
+export function fisherYates(input) {
+  const a = [...input];
+  for (let i = a.length - 1; i > 0; i--) {
+    const j = Math.floor(Math.random() * (i + 1));
+    [a[i], a[j]] = [a[j], a[i]];
+  }
+  return a;
+}
+
+function buildShuffle(deck, size) {
+  return fisherYates(deck).slice(0, size).map((c) => ({
+    ...c,
+    reversed: Math.random() < 0.5,
+  }));
+}
+
+export function useTarotShuffle(deck, size = 16) {
+  const [slice, setSlice] = useState(() => buildShuffle(deck, size));
+  const reshuffle = useCallback(() => {
+    setSlice(buildShuffle(deck, size));
+  }, [deck, size]);
+  return { slice, reshuffle };
+}

src/pages/tarot/hooks/useTarotShuffle.test.js

@@ -0,0 +1,47 @@
+import { describe, it, expect } from 'vitest';
+import { renderHook, act } from '@testing-library/react';
+import { useTarotShuffle, fisherYates } from './useTarotShuffle';
+
+describe('fisherYates', () => {
+  it('원본 길이를 유지하고 모든 원소를 포함한다', () => {
+    const src = [1, 2, 3, 4, 5, 6, 7, 8];
+    const out = fisherYates(src);
+    expect(out).toHaveLength(8);
+    expect([...out].sort()).toEqual(src);
+  });
+
+  it('새 배열을 반환한다 (mutation 없음)', () => {
+    const src = [1, 2, 3];
+    const out = fisherYates(src);
+    expect(src).toEqual([1, 2, 3]);
+    expect(out).not.toBe(src);
+  });
+});
+
+describe('useTarotShuffle', () => {
+  it('16장 슬라이스를 반환한다', () => {
+    const deck = Array.from({ length: 78 }, (_, i) => ({ id: i, slug: `c${i}` }));
+    const { result } = renderHook(() => useTarotShuffle(deck, 16));
+    expect(result.current.slice).toHaveLength(16);
+  });
+
+  it('reshuffle 시 새 슬라이스를 만든다 (대부분 다름)', () => {
+    const deck = Array.from({ length: 78 }, (_, i) => ({ id: i, slug: `c${i}` }));
+    const { result } = renderHook(() => useTarotShuffle(deck, 16));
+    const first = result.current.slice.map((c) => c.id);
+    act(() => result.current.reshuffle());
+    const second = result.current.slice.map((c) => c.id);
+    // 무작위지만 16장 모두 같을 확률은 사실상 0
+    const overlap = first.filter((id) => second.includes(id)).length;
+    expect(overlap).toBeLessThan(16);
+  });
+
+  it('reversed 플래그를 카드에 부여한다', () => {
+    const deck = Array.from({ length: 78 }, (_, i) => ({ id: i, slug: `c${i}` }));
+    const { result } = renderHook(() => useTarotShuffle(deck, 16));
+    for (const card of result.current.slice) {
+      expect(card).toHaveProperty('reversed');
+      expect(typeof card.reversed).toBe('boolean');
+    }
+  });
+});