subgroups exercise using bit ops

subgroups.spec.js

const _ = require("lodash");

function subgroups(elements) {
    const bits = _.range(32);

    const indices = (i) =>
        _(bits)
            .map((b) => (1 + b) * (0x1 & (i >> b)))
            .compact() // only leave the `1`s
            .map((b) => b - 1)
            .value();

    return _(_.range(1 << elements.length))
        .map((i) => _.at(elements, indices(i)))
        .value();
}

describe("subgroups", () => {
    _(_.range(2, 9))
        .map((n) => _.range(1, n))
        .each((inputElements) => {
            describe(`${inputElements}`, () => {
                let res;

                beforeAll(() => (res = subgroups(inputElements)));

                it("should 2^n", () => expect(res.length).toBe(1 << inputElements.length));
                it("should unique", () => expect(_.uniq(res).length).toBe(1 << inputElements.length));
            });
        });
});