flatten.rb

require "rspec"

def flatten(arr)
  return [arr] unless arr.is_a? Array

  arr.reduce([]) { |concat, item| concat + flatten(item) }
end

RSpec.describe "Flattening a nested array" do
  it "is valid for simple array" do
    expect(flatten([[1]])).to eq([1])
  end

  it "is valid for empty array" do
    expect(flatten([[]])).to eq([])
  end

  it "is valid for partitioned array" do
    expect(flatten([[1], [2], [3], [4], [5]])).to eq([1, 2, 3, 4, 5])
  end

  it "is valid for array wrapped in array" do
    expect(flatten([[1, 2, 3, 4, 5]])).to eq([1, 2, 3, 4, 5])
  end

  it "is valid for deeply nested empty array" do
    expect(flatten([[[], [], [], [nil]]])).to eq([nil])
  end

  it "is valid for deeply nested inhomogeneous array" do
    expect(flatten([[[], [{a: [1, [2]]}], ['a'], [nil]]])).to eq([{a: [1, [2]]}, 'a', nil])
  end

  it "is valid for a flat array" do
    expect(flatten([1, 2, 3, 4])).to eq([1, 2, 3, 4])
  end
end