groupByMultiToMap

1/**
2 * Groups items into a Map based on one or more keys returned by a callback.
3 * Each item can be placed into multiple groups.
4 *
5 * @param arr - Array of items to group.
6 * @param fn - Function that returns an array of grouping keys for each item.
7 * @returns A Map where each key maps to an array of items.
8 */
9export function groupByMultiToMap<T, K>(arr: T[], fn: (item: T) => K[]): Map<K, T[]> {
10  const result = new Map<K, T[]>();
11
12  for (const item of arr) {
13    const keys = fn(item);
14    for (const key of keys) {
15      if (!result.has(key)) {
16        result.set(key, []);
17      }
18      result.get(key)!.push(item);
19    }
20  }
21
22  return result;
23}

1test('groupByMultiToMap - basic usage', () => {
2  const animals = [
3    { name: 'dog', tags: ['mammal', 'pet'] },
4    { name: 'cat', tags: ['mammal', 'pet'] },
5    { name: 'crocodile', tags: ['reptile', 'wild'] }
6  ];
7
8  const grouped = groupByMultiToMap(animals, animal => animal.tags);
9
10  expect(grouped.get('mammal')).toEqual([
11    { name: 'dog', tags: ['mammal', 'pet'] },
12    { name: 'cat', tags: ['mammal', 'pet'] }
13  ]);
14
15  expect(grouped.get('pet')).toEqual([
16    { name: 'dog', tags: ['mammal', 'pet'] },
17    { name: 'cat', tags: ['mammal', 'pet'] }
18  ]);
19
20  expect(grouped.get('reptile')).toEqual([
21    { name: 'crocodile', tags: ['reptile', 'wild'] }
22  ]);
23
24  expect(grouped.get('wild')).toEqual([
25    { name: 'crocodile', tags: ['reptile', 'wild'] }
26  ]);
27});
28
29test('groupByMultiToMap - empty array', () => {
30  const grouped = groupByMultiToMap([], () => ['x']);
31  expect(grouped.size).toBe(0);
32});
33
34test('groupByMultiToMap - empty keys array', () => {
35  const grouped = groupByMultiToMap(['a', 'b'], () => []);
36  expect(grouped.size).toBe(0);
37});