Flattens arbitrarily nested arrays

flatten.rb

class Flatten
  def self.flatten(array)
    array.reduce([]) do |acc, value|
      if value.is_a?(Array)
        acc += Flatten.flatten(value)
      else
        acc << value
      end
    end
  end
end

flatten_spec.rb

require_relative 'flatten'

RSpec.describe Flatten do
  describe ".flatten" do
    context "when given an empty array as an input" do
      it "returns an empty array" do
        expect(Flatten.flatten([])).to eq []
      end
    end

    context "when given a flat array as an input" do
      it "returns the same array" do
        initial = [1, 2, 3, 4]
        expected = [1, 2, 3, 4]

        expect(Flatten.flatten(initial)).to eq expected
      end
    end

    context "when given a singly nested array" do
      it "returns a new flattened array" do
        initial = [1, [2], 3, 4]
        expected = [1, 2, 3, 4]

        result = Flatten.flatten(initial)

        expect(result).to eq expected
      end

      it "does not modify the original array" do
        initial = [1, [2], 3, 4]
        result = Flatten.flatten(initial)
        expect(initial).to eq [1, [2], 3, 4]
      end
    end

    context "when given a deeply nested array" do
      it "returns a new flattened array" do
        initial = [1, [2, [3, 4, [5, 6, [7]], 8]], 9]
        expected = [1, 2, 3, 4, 5, 6, 7, 8, 9]

        result = Flatten.flatten(initial)

        expect(result).to eq expected
      end
    end
  end
end

Gemfile

source "https://rubygems.org"

gem 'rspec'

Gemfile.lock

GEM
  remote: https://rubygems.org/
  specs:
    diff-lcs (1.2.5)
    rspec (3.4.0)
      rspec-core (~> 3.4.0)
      rspec-expectations (~> 3.4.0)
      rspec-mocks (~> 3.4.0)
    rspec-core (3.4.2)
      rspec-support (~> 3.4.0)
    rspec-expectations (3.4.0)
      diff-lcs (>= 1.2.0, < 2.0)
      rspec-support (~> 3.4.0)
    rspec-mocks (3.4.1)
      diff-lcs (>= 1.2.0, < 2.0)
      rspec-support (~> 3.4.0)
    rspec-support (3.4.1)

PLATFORMS
  ruby

DEPENDENCIES
  rspec

BUNDLED WITH
   1.10.6