flatten_array.rb

require 'rspec/autorun'

class FlattenArray
  def self.call(array)
    new(array).call
  end

  def initialize(array)
    @array = array
    @flat_array = []
  end

  def call(array = @array)
    array.each do |element|
      if element.is_a?(Array)
        call(element)
      else
        @flat_array.push(element)
      end
    end
    @flat_array
  end
end

RSpec.describe FlattenArray do
  describe '.call' do
    def call(array)
      FlattenArray.call(array)
    end

    context 'given an array with zero elements' do
      it 'returns an empty array' do
        expect(call([])).to eq([])
      end
    end

    context 'given an array with no nested arrays' do
      it 'returns the array as is' do
        expect(call([1, 2, 3])).to eq([1, 2, 3])
      end
    end

    context 'given an array with nested arrays' do
      it 'returns a single array with all elements' do
        expect(call([1, [2, [3]]])).to eq([1, 2, 3])
      end
    end
  end
end