fix: deduplicate distractors against each other, guard thin distractor pool

apps/api/src/services/gameService.test.ts

@@ -32,7 +32,14 @@ const fakeTerms = [
 beforeEach(() => {
   vi.clearAllMocks();
   mockGetGameTerms.mockResolvedValue(fakeTerms);
-  mockGetDistractors.mockResolvedValue(["wrong1", "wrong2", "wrong3"]);
+  mockGetDistractors.mockResolvedValue([
+    "wrong1",
+    "wrong2",
+    "wrong3",
+    "wrong4",
+    "wrong5",
+    "wrong6",
+  ]);
 });
 
 describe("createGameSession", () => {
@@ -151,6 +158,39 @@ describe("createGameSession", () => {
       createGameSession(validRequest, store, "user-1"),
     ).rejects.toThrow("db timeout");
   });
+
+  it("throws when fewer than 3 unique distractors remain after deduplication", async () => {
+    mockGetDistractors.mockResolvedValueOnce([
+      "cane",
+      "cane",
+      "cane",
+      "cane",
+      "cane",
+      "cane",
+    ]);
+
+    await expect(
+      createGameSession(validRequest, store, "user-1"),
+    ).rejects.toThrow("Not enough unique distractors");
+  });
+
+  it("duplicate distractors are deduplicated against each other", async () => {
+    mockGetDistractors.mockResolvedValueOnce([
+      "wrong1",
+      "wrong1",
+      "wrong1",
+      "wrong2",
+      "wrong3",
+      "wrong4",
+    ]);
+
+    const session = await createGameSession(validRequest, store, "user-1");
+    const question = session.questions[0]!;
+    const optionTexts = question.options.map((o) => o.text);
+
+    expect(new Set(optionTexts).size).toBe(4);
+    expect(question.options).toHaveLength(4);
+  });
 });
 
 describe("evaluateAnswer", () => {

apps/api/src/services/gameService.ts

@@ -42,9 +42,16 @@ export const createGameSession = async (
         6,
       );
 
-      const uniqueDistractors = distractorTexts.filter(
-        (t) => t !== term.targetText,
-      );
+      const uniqueDistractors = [
+        ...new Set(distractorTexts.filter((t) => t !== term.targetText)),
+      ];
+
+      if (uniqueDistractors.length < 3) {
+        throw new Error(
+          `Not enough unique distractors for term: ${term.targetText}`,
+        );
+      }
+
       const optionTexts = [term.targetText, ...uniqueDistractors.slice(0, 3)];
       const shuffledTexts = shuffleArray(optionTexts);
       const correctOptionId = shuffledTexts.indexOf(term.targetText);