flattened.rb

# https://www.codewars.com/kata/521a849a05dd182a09000043

class Hash
  def collapsed parent_key, key
    return key unless parent_key
    val = "#{parent_key}_#{key}"
    return val.to_s   if parent_key.is_a? String
    return val.to_sym if key.is_a? Symbol
    return val
  end

  def flattened_keys parent_key = nil, accumulator = { }, current = self
    current.each do |k, v|
      if current[k].is_a? Hash 
        flattened_keys collapsed(parent_key, k), accumulator, v
      else
        accumulator[collapsed(parent_key, k)] = v
      end
    end

    accumulator
  end
end