Flatland

flatten.rb

def flatten(input, output=[])
  input.each do |e|
    if e.is_a? Array
      output += flatten(e)
    else
      output.push e
    end
  end
  output
end

flatten_spec.rb

require_relative 'flatten'

describe 'flatten' do
  subject { flatten(input) }

  context 'empty array' do
    let(:input) { [] }
    it { should eq [] }
  end

  context 'flat with multiple elements' do
    let(:input) { [1, 2] }
    it { should eq [1, 2] }
  end

  context 'nested arrays 1 level' do
    let(:input) { [[1], [2]] }
    it { should eq [1, 2] }
  end

  context 'mixed nested 1 level' do
    let(:input) { [[1], 2, [3]] }
    it { should eq [1, 2, 3] }
  end

  context 'multi level nesting' do
    let(:input) { [1, [2], [[3]]] }
    it { should eq [1, 2, 3] }
  end

  context 'task data' do
    let(:input) { [[1, 2, [3]], 4] }
    it { should eq [1, 2, 3, 4] }
  end
end