enumerable_flattener.rb

### IMPLEMENTATION ###
# Flattens an arbitrarily nested enumerable of enumerables.
# Works for Arrays of Integers, and more.
class EnumerableFlattener
  attr_reader :array

  def initialize(array)
    @array = array
  end

  def call
    array.inject([]) do |output, array_or_item|
      if array_or_item.respond_to?(:each)
        flattened = self.class.new(array_or_item).call
        output.push(*flattened)
      else
        output.push(array_or_item)
      end
    end
  end
end

# Optional core extension, depending on your feelings about 🐒-patching
class Array
  def deep_flatten
    EnumerableFlattener.new(self).call
  end
end

### SPECS ###
# Usage:
#   ruby -Ilib:test enumerable_flattener.rb

require "minitest/autorun"
require "set"

describe EnumerableFlattener do
  describe "call" do
    describe "with an array of arbitrarily nested empty arrays" do
      it "flattens to an empty array" do
        flatten([[],[[],[[]],[]]]).must_equal []
      end
    end
    describe "with an array of arbitrarily nested arrays of integers" do
      it "flattens into an array of integers" do
        flatten([[[[[1]]]]]).must_equal [1]
        flatten([[1,2,[3]],4]).must_equal [1,2,3,4]
        flatten([[1,2,[3, [4, 5], 6]],7]).must_equal [1,2,3,4,5,6,7]
      end
    end
    describe "with an array of arbitrarily nested enumerables of non-enumerables" do
      it "flattens into an array of non-enumerables" do
        input = Set.new([1, [2, "buckle my shoe"], Set.new([3, 4]), Set.new([5]), 6])
        flatten(input).must_equal [1, 2, "buckle my shoe", 3, 4, 5, 6]
      end
    end
  end

  describe "core extension Array#deep_flatten" do
    describe "with an array of arbitrarily nested arrays of integers" do
      it "flattens into an array of integers" do
        [[1,2,[3]],4].deep_flatten.must_equal [1,2,3,4]
      end
    end
  end

  def flatten(array)
    EnumerableFlattener.new(array).call
  end
end