flatten.rb

# Write some code, that will flatten an array of arbitrarily nested arrays of
# integers into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4].

# Your solution should be a link to a gist on gist.github.com with your
# implementation.

# When writing this code, you can use any language you're comfortable with. The
# code must be well tested and documented if necessary, and in general please
# treat the quality of the code as if it was ready to ship to production.

# Try to avoid using language defined methods like Ruby's Array#flatten.

class Flatten
  attr_reader :values

  # Preserve .initialize for dependency injection
  def call input
    @values = input.dup

    output = []

    values.map do |value|
      if value.is_a? Array
        output += Flatten.new.call(value)
      else
        output << value
      end
    end

    output
  end
end

flatten_spec.rb

require 'rspec'
require './flatten'

describe Flatten do
  describe "#call" do
    it "returns the passed-in array if there are no sub-arrays" do
      expect(Flatten.new.call([1, 2])).to eq [1, 2]
    end

    context "when there are sub-arrays" do
      it "returns the flattened array" do
        expect(Flatten.new.call([1, [2]])).to eq [1, 2]
        expect(Flatten.new.call([1, [2], 3, [3, 2, [4, 5], 6]])).to \
          eq [1, 2, 3, 3, 2, 4, 5, 6]
      end
    end
  end
end

Gemfile

# frozen_string_literal: true
ruby '2.5.1'

source "https://rubygems.org"

git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }

gem "rspec"