Flatten array of arbitrarily nested arrays

array_tools.rb

=begin

Ruby code that will flatten an array of nested arrays of integers (or whatever)
into a flat array of integers. e.g. [[1,2,[3]],4] -> [1,2,3,4].
A tail recursion was adopted so the minimal amount of variables is added to the stack.

Usage examples:

ArrayTools.flatten([])

ArrayTools.flatten([1,2,3,4,[5,6]])

ArrayTools.flatten([[1],[2],[3],[4],[5,6]])

=end

class ArrayTools
  def self.flatten(array)
    # abort if it isn;t an array and return nil
    return nil if !(array.is_a? Array)
    
    return self.flatten_pass(array, [])
  end
    
  def self.flatten_pass(array, flat_array)
    # for each item  in array
    array.each { |x| 
      # if item is not array
      if !(x.is_a? Array)
        # push it to flattened array
        flat_array.push(x)
      else
        # else use tail recursion call to flatten
        flatten_pass(x, flat_array)
      end
    }
    return flat_array
  end
end

=begin

Since this is single gist, there really wasn't a place to show test coverage so I put it here as a comment

require 'rails_helper'

RSpec.describe ArrayTools, type: :model do
  describe ".flatten" do 
    
    # it should return the same array if nothing needs to be flattened
    it "should return nil when not given an array" do
      expect(ArrayTools.flatten([1,2,3])).to eq [1,2,3]
    end
    
    # it should return nil when it does not have an array
    it "should return [1,2,3] for [1,2,3]" do
      expect(ArrayTools.flatten(1)).to eq nil
    end
    
    # it should flatten an array
    it "should return [1,2,3,4,5] for [[1],[2,3],[4],5]" do
      expect(ArrayTools.flatten([[1],[2,3],[4],5])).to eq [1,2,3,4,5]
    end
    
  end
end

Example of test output

rspec spec/models/array_tools_spec.rb -f d
#<Gem::Specification name=sidekiq version=4.2.10>

ArrayTools
  .flatten
    should return nil when not given an array
    should return [1,2,3] for [1,2,3]
    should return [1,2,3,4,5] for [[1],[2,3],[4],5]

Finished in 0.04425 seconds (files took 3.42 seconds to load)
3 examples, 0 failures


=end