feat: Add crud for getMergedTasksMap (#2748)

Author: KATO-Hiro

docs/dev-notes/2025-10-25/refactor-getMergedTasksMap/lesson.md

@@ -0,0 +1,61 @@
+# getMergedTasksMap のリファクタリング教訓
+
+## 学習内容
+
+### 1. **参照 vs コピー**
+
+TypeScript（JavaScript）の `const newTask = task;` は参照をコピーするため、`newTask` を変更すると元の `task` も変更されます。
+
+- **浅いコピー**: `const newTask = { ...task };`
+- **深いコピー**: `const newTask = JSON.parse(JSON.stringify(task));`
+
+### 2. **TypeScript らしいコード書き方**
+
+- `map()` で初期化: `new Map(tasks.map(task => [task.task_id, task]))`
+- ループではなく関数型メソッド (`filter()`, `map()`, `flatMap()`)
+- スプレッド演算子で Map をマージ: `new Map([...map1, ...map2])`
+
+### 3. **`flatMap()` vs `map()`**
+
+`flatMap()` は返した配列を1段階フラット化するため、条件付きの可変長結果に最適：
+
+```typescript
+// flatMap で条件分岐を自然に表現
+.flatMap((pair) => {
+  if (!task || !contestType) return [];
+  return [createMergedTask(...)];
+});
+// 結果: 該当する要素だけが含まれる
+```
+
+### 4. **読みやすさ > 1行でまとめる**
+
+無理やり `return` や1行で書く必要はない：
+
+- 複雑な条件は `if` 文で早期リターン
+- オブジェクト生成は `key` と `value` を分けて記述
+- 難しいロジックはヘルパー関数に抽出
+
+### 5. **計算量の分析**
+
+- 全体: **O(N + M)** （N: タスク数、M: ペア数）
+- `Map.has()`, `Map.get()` は **O(1)** なのでループ内で複数回呼んでもOK
+
+### 6. **ドキュメント化のポイント**
+
+- 関数の目的と副作用を明確に
+- **計算量と根拠** を記載
+- **使用例** を `@example` で示す
+- 戻り値の構造を詳しく説明
+
+## コード例（改善版）
+
+src/lib/services/tasks.ts を参照
+
+## キーポイント
+
+- ✅ 非破壊的な変更（スプレッド演算子）
+- ✅ 関数型パラダイム（`filter()`, `flatMap()` 使用）
+- ✅ 早期リターンで複雑さを減らす
+- ✅ ヘルパー関数で責任分離
+- ✅ 明確なドキュメント化

src/lib/services/tasks.ts

@@ -1,7 +1,17 @@
 import { default as db } from '$lib/server/database';
+
+import { getContestTaskPairs } from '$lib/services/contest_task_pairs';
+
+import { ContestType } from '$lib/types/contest';
+import type { Task, TaskGrade } from '$lib/types/task';
+import type {
+  ContestTaskPair,
+  ContestTaskPairKey,
+  TaskMapByContestTaskPair,
+} from '$lib/types/contest_task_pair';
+
 import { classifyContest } from '$lib/utils/contest';
-import type { TaskGrade } from '$lib/types/task';
-import type { Task, Tasks } from '$lib/types/task';
+import { createContestTaskPairKey } from '$lib/utils/contest_task_pair';
 
 // See:
 // https://www.prisma.io/docs/concepts/components/prisma-client/filtering-and-sorting
@@ -11,6 +21,77 @@ export async function getTasks(): Promise<Task[]> {
   return tasks;
 }
 
+/**
+ * Fetches and merges tasks based on contest-task pairs.
+ *
+ * @returns A promise that resolves to a map of merged tasks keyed by contest-task pair.
+ *
+ * @note This function merges tasks with the same task_id but different contest_id
+ *       from the contest-task pairs table. It enriches existing tasks with
+ *       contest-specific information (contest_type, task_table_index, etc.).
+ * @note Time Complexity: O(N + M)
+ *       - N: number of tasks from the database
+ *       - M: number of contest-task pairs
+ *       - Map operations (has, get, set) are O(1)
+ * @example
+ *       const mergedTasksMap = await getMergedTasksMap();
+ *       const task = mergedTasksMap.get(createContestTaskPairKey('tessoku-book', 'typical90_s'));
+ */
+export async function getMergedTasksMap(): Promise<TaskMapByContestTaskPair> {
+  const tasks = await getTasks();
+  const contestTaskPairs = await getContestTaskPairs();
+
+  const baseTaskMap = new Map<ContestTaskPairKey, Task>(
+    tasks.map((task) => [createContestTaskPairKey(task.contest_id, task.task_id), task]),
+  );
+  // Unique task_id in database
+  const taskMap = new Map(tasks.map((task) => [task.task_id, task]));
+
+  // Filter task(s) only the same task_id but different contest_id
+  const additionalTaskMap = contestTaskPairs
+    .filter((pair) => !baseTaskMap.has(createContestTaskPairKey(pair.contestId, pair.taskId)))
+    .flatMap((pair) => {
+      const task = taskMap.get(pair.taskId);
+      const contestType = classifyContest(pair.contestId);
+
+      if (!task || !contestType || !pair.taskTableIndex) {
+        return [];
+      }
+
+      return [createMergedTask(task, pair, contestType)];
+    });
+
+  return new Map([...baseTaskMap, ...additionalTaskMap]);
+}
+
+/**
+ * Creates a merged task from the original task and contest-task pair.
+ *
+ * @param task The original task to be enriched with contest-specific information.
+ * @param pair The contest-task pair containing contestId, taskTableIndex and taskId.
+ * @param contestType The type of contest (e.g., ABC, ARC) derived from contest_id.
+ * @returns A tuple [key, mergedTask] where:
+ *          - key: the unique identifier for the contestId:taskId pair
+ *          - mergedTask: the task with contest-specific fields updated
+ */
+function createMergedTask(
+  task: Task,
+  pair: ContestTaskPair,
+  contestType: ContestType,
+): [ContestTaskPairKey, Task] {
+  const key = createContestTaskPairKey(pair.contestId, pair.taskId);
+
+  const mergedTask: Task = {
+    ...task,
+    contest_type: contestType,
+    contest_id: pair.contestId,
+    task_table_index: pair.taskTableIndex,
+    title: task.title.replace(task.task_table_index, pair.taskTableIndex),
+  };
+
+  return [key, mergedTask];
+}
+
 /**
  * Fetches tasks with the specified task IDs.
  * @param selectedTaskIds - An array of task IDs to filter the tasks.

src/lib/types/contest_task_pair.ts

@@ -1,6 +1,6 @@
 import type { ContestTaskPair as ContestTaskPairOrigin } from '@prisma/client';
 
-import type { TaskResult } from '$lib/types/task';
+import type { Task, TaskResult } from '$lib/types/task';
 
 export type ContestTaskPair = ContestTaskPairOrigin;
 
@@ -20,4 +20,6 @@ export type ContestTaskPairUpdate = ContestTaskPairCreate;
 // For mapping and identification
 export type ContestTaskPairKey = `${string}:${string}`; // "contest_id:task_id"
 
+export type TaskMapByContestTaskPair = Map<ContestTaskPairKey, Task>;
+
 export type TaskResultMapByContestTaskPair = Map<ContestTaskPairKey, TaskResult>;

src/lib/types/task.ts

@@ -1,7 +1,9 @@
 // Import original enum as type.
 import type { TaskGrade as TaskGradeOrigin } from '@prisma/client';
+import type { ContestType } from '$lib/types/contest';
 
 export interface Task {
+  contest_type?: ContestType;
   contest_id: string;
   task_table_index: string;
   task_id: string;

src/lib/utils/contest_task_pair.ts

@@ -0,0 +1,23 @@
+import type { ContestTaskPairKey } from '$lib/types/contest_task_pair';
+
+/**
+ * Creates a unique key for a ContestTaskPair using contestId and taskId.
+ * Throws an error if either argument is an empty string.
+ *
+ * @param contestId - The ID of the contest.
+ * @param taskId - The ID of the task.
+ *
+ * @returns A string in the format "contestId:taskId".
+ *
+ * @throws Will throw an error if contestId or taskId is empty.
+ */
+export function createContestTaskPairKey(contestId: string, taskId: string): ContestTaskPairKey {
+  if (!contestId || contestId.trim() === '') {
+    throw new Error('contestId must be a non-empty string');
+  }
+  if (!taskId || taskId.trim() === '') {
+    throw new Error('taskId must be a non-empty string');
+  }
+
+  return `${contestId}:${taskId}`;
+}