Generates combinations from an array

combinations_spec.rb

require "rspec"

def combinations(array, length)
  if length.zero?
    [[]]
  else
    array.flat_map.with_index do |element, index|
      tail = array[(index + 1)..-1]

      combinations(tail, length - 1).map do |combination|
        [element] + combination
      end
    end
  end
end

RSpec.describe "combinations" do
  it "works for combinations of 0" do
    expect(combinations([], 0)).to eq [[]]
    expect(combinations([1], 0)).to eq [[]]
    expect(combinations([1, 2], 0)).to eq [[]]
    expect(combinations([1, 2, 3], 0)).to eq [[]]
  end

  it "works for combinations of 1" do
    expect(combinations([], 1)).to eq []
    expect(combinations([1], 1)).to eq [[1]]
    expect(combinations([1, 2], 1)).to eq [[1], [2]]
    expect(combinations([1, 2, 3], 1)).to eq [[1], [2], [3]]
  end

  it "works for combinations of 2" do
    expect(combinations([], 2)).to eq []
    expect(combinations([1], 2)).to eq []
    expect(combinations([1, 2], 2)).to eq [[1, 2]]
    expect(combinations([1, 2, 3], 2)).to eq [[1, 2], [1, 3], [2, 3]]
  end

  it "works for combinations of 3" do
    expect(combinations([], 3)).to eq []
    expect(combinations([1], 3)).to eq []
    expect(combinations([1, 2], 3)).to eq []
    expect(combinations([1, 2, 3], 3)).to eq [[1, 2, 3]]
  end
end