Implementation

flatten.rb

#!/usr/bin/env ruby

class Flatten
  def self.flatten(array)
    result = []
    array.each do |x|
      if x.kind_of? Array # flatten again and add the elements individually
        self.flatten(x).each { |y| result.push(y) }
      else
        result.push(x) #add the item to the result array
      end
    end
    return result
  end
end

flatten_spec.rb

require 'minitest/autorun'
require 'minitest/spec'
require './flatten'

#place this file on the same folder as flatten.rb and run rake test to run these tests

describe Flatten do
  before (:each) do
    @cases = [ [1,2,3,4,5], [3,4,6,[3,4] ], [4,[3,4],[2,[1]]],[1,[3,4,[5,6],7,9 ],10] ]
  end

  it "returns an array" do
    @cases.each { |x|  Flatten.flatten(x).must_be_instance_of(Array) }
  end

  it "returns a non empty array for non empty entries" do
    @cases.each { |x|  Flatten.flatten(x).wont_be_empty }
  end

  it "returns an empty array when given empty entry" do
    Flatten.flatten([]).must_be_empty
  end

  it "returns a non nested array" do
    @cases.each do |x|
      Flatten.flatten(x).each do |y|
        y.wont_be_instance_of(Array)
      end
    end
  end

  it "has the same amount of elements than an array flattened with native method" do
    @cases.each do |x|
      Flatten.flatten(x).length.must_equal x.flatten.length
    end
  end

end

Gemfile

source 'https://rubygems.org/'
gem 'minitest'

Rakefile

require 'rake/testtask'
  
Rake::TestTask.new do |t|
  t.libs << "spec"
  t.pattern = "*_spec.rb"
  t.verbose = true
end