list of arrays to nested hash tree

nest_array_test.rb

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'
  gem 'minitest', require: false
end

require 'minitest/autorun'

module DeepMerge
  refine Hash do
    def deep_merge(other_hash, &block)
      dup.deep_merge!(other_hash, &block)
    end

    def deep_merge!(other_hash, &block)
      other_hash.each_pair do |current_key, other_value|
        this_value = self[current_key]

        self[current_key] = if this_value.is_a?(Hash) && other_value.is_a?(Hash)
                              this_value.deep_merge(other_value, &block)
                            else
                              if block_given? && key?(current_key)
                                yield(current_key, this_value, other_value)
                              else
                                other_value
                              end
                            end
      end

      self
    end
  end
end

class NestArrayFooTest < Minitest::Test
  using DeepMerge

  def work(arrays)
    arrays
      .map { |e| e.reverse.reduce { |inner, key| inner.nil? ? key : { key => inner } } }
      .reduce({}) { |a, e| a.deep_merge(e) }
  end

  def test_it_does_something_useful1
    assert_equal({ a: { b: { c: :d } } }, work([%i[a b c d]]))
  end

  def test_it_does_something_useful
    assert_equal({ a: { b: { c: :d, e: :f } } }, work([%i[a b c d], %i[a b e f]]))
  end
end

deep_merge stuff is copied from active_support