Small code sample to demonstrate the implementation of an array flattening algorithm in Ruby

readme.md

Flattenable

Extend your classes to flatten list-like objects that contain other lists and integers.

Synopsis

require_relative "flattenable"

class MyFlattener
  extends Flattenable
end

MyFlattener.flatten(
  [1, [2], [3, [4]]]
)

#=> [1, 2, 3, 4]

Testing

Run in documentation format:

rspec -fdoc flattenable_spec.rb

flattenable.rb

# Flattens lists containing lists and integers
# Works with list-like objects via duck typing
module Flattenable
  def flatten(list_to_flatten)
    unless self.list_type?(list_to_flatten)
      raise ArgumentError, "Argument must be a list type"
    end

    list_to_flatten.inject([]) do |flattened_list, item|
      if self.list_type?(item)
        flattened_list.concat(self.flatten(item))
      else
        unless item.is_a?(Integer)
          raise ArgumentError, "Argument list may contain only integers"
        end

        flattened_list << item
      end
    end
  end

  protected

  def list_type?(value)
    value.respond_to?(:each) and not value.respond_to?(:keys)
  end
end

flattenable_spec.rb

require "rspec"

require_relative "flattenable"

class Flattener
  extend Flattenable
end

RSpec.describe Flattener do
  it "flattens correctly with the provided example" do
    expect(described_class.flatten( [[1, 2, [3]], 4])).to eq([1, 2, 3, 4])
  end

  it "flattens correctly with deep nesting" do
    expect(described_class.flatten( [0, [1, [2, [3, [4]]]]])).to eq([0, 1, 2, 3, 4])
  end

  it "flattens correctly with simple lists" do
    expect(described_class.flatten([1])).to eq([1])
  end

  it "fails when given a list of characters" do
    expect{ described_class.flatten(["a"]) }.to raise_error(ArgumentError, /contain only integers/)
  end

  it "fails when given a hash type" do
    expect{ described_class.flatten({a: 1}) }.to raise_error(ArgumentError, /must be a list/)
  end

  it "fails when given a hash type inside a list" do
    expect{ described_class.flatten([1, {a: 1}]) }.to raise_error(ArgumentError, /contain only integers/)
  end

  it "fails when given an integer outside of a list" do
    expect{ described_class.flatten(1) }.to raise_error(ArgumentError, /must be a list/)
  end
end