class Hash

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)
        block.call(current_key, this_value, other_value)
      else
        other_value
      end
    end
  end
  self
end