feat: Introduce combinations in fp

packages/fp/src/combinations/combinations.js

@@ -0,0 +1,9 @@
+import { reduce } from 'lodash/fp';
+
+const appendTo = values => value => [...values, value];
+
+export default reduce(
+  (accumulated, current) =>
+    accumulated.flatMap(values => current.map(appendTo(values))),
+  [[]],
+);

packages/fp/src/combinations/combinations.test.js

@@ -0,0 +1,50 @@
+import combinations from './combinations';
+
+describe('combinations', () => {
+  it('given arrays of same size, returns all combinations', () => {
+    const actual = combinations([
+      [1, 2],
+      [3, 4],
+      [5, 6],
+    ]);
+
+    const expected = [
+      [1, 3, 5],
+      [1, 3, 6],
+      [1, 4, 5],
+      [1, 4, 6],
+      [2, 3, 5],
+      [2, 3, 6],
+      [2, 4, 5],
+      [2, 4, 6],
+    ];
+
+    expect(actual).toEqual(expected);
+  });
+
+  it('given arrays of different size, returns all combinations', () => {
+    const actual = combinations([
+      [1, 2],
+      [3, 4, 5],
+    ]);
+
+    const expected = [
+      [1, 3],
+      [1, 4],
+      [1, 5],
+      [2, 3],
+      [2, 4],
+      [2, 5],
+    ];
+
+    expect(actual).toEqual(expected);
+  });
+
+  it('given one of the arrays is empty, returns no combinations', () => {
+    const actual = combinations([[1, 2], [], [3, 4, 5]]);
+
+    const expected = [];
+
+    expect(actual).toEqual(expected);
+  });
+});