feat: add leader rating song recommendations

src/lib/engine/song-rating.test.ts

@@ -0,0 +1,33 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { leaderRating, leaderRatingCharts, rankSongCharts, rankSongs, songRating } from './song-rating.ts';
+
+test('레이팅은 5000점 단위로 올림하며 잘못된 점수를 거부한다', () => {
+  assert.deepEqual([0, 1, 5000, 5001, 1234567].map(songRating), [0, 1, 1, 2, 247]);
+  for (const score of [-1, NaN, Infinity]) assert.throws(() => songRating(score));
+});
+
+test('리더 가창곡의 공개된 채보만 선택한다', () => {
+  const charts = [
+    { songId: 'solo', title: 'Solo', difficulty: 'expert', singerCharacterIds: ['leader'] },
+    { songId: 'solo', title: 'Solo', difficulty: 'hard', singerCharacterIds: ['leader'] },
+    { songId: 'group', title: 'Group', difficulty: 'expert', singerCharacterIds: ['other', 'leader'], releaseTime: 200 },
+    { songId: 'other', title: 'Other', difficulty: 'expert', singerCharacterIds: ['other'] },
+    { songId: 'unknown', title: 'Unknown', difficulty: 'expert', singerCharacterIds: [] }
+  ];
+  assert.deepEqual(leaderRatingCharts(charts, 'leader', 'expert', 199), [charts[0]]);
+  assert.deepEqual(leaderRatingCharts(charts, 'leader', 'both', 200), charts.slice(0, 3));
+  assert.deepEqual(leaderRatingCharts(charts, '', 'both', 200), []);
+});
+
+test('고점 순위와 상위 3곡 합계는 동일 곡의 난이도를 중복 집계하지 않는다', () => {
+  const row = (songId: string, score: number, difficulty = 'expert') => ({
+    songId, title: songId, difficulty, theoreticalMaxScore: score, expectedScore: score / 2
+  });
+  const results = [row('a', 5001), row('b', 10001), row('a', 20001, 'hard'), row('c', 9999), row('d', 5000)];
+  assert.deepEqual(rankSongCharts(results).map((item) => [item.songId, item.difficulty]), [['a', 'hard'], ['b', 'expert'], ['c', 'expert'], ['a', 'expert'], ['d', 'expert']]);
+  assert.deepEqual(rankSongs(results).map((item) => [item.songId, item.difficulty]), [['a', 'hard'], ['b', 'expert'], ['c', 'expert'], ['d', 'expert']]);
+  assert.equal(leaderRating(results), 10);
+  assert.equal(leaderRating([]), 0);
+  assert.equal(results[0].theoreticalMaxScore, 5001);
+});