class Hash

  # Confirms that a key is set in the Hash
  def assert_required_keys(*required_keys)
    missing_keys = []
    [required_keys].flatten.each { |k| missing_keys << k unless keys.has_key?(k) }
    raise(ArgumentError, "Missing required key(s): #{missing_keys.join(", ")}") unless missing_keys.empty?
  end

  # Confirms that a key is present and that is has value
  def assert_required_keys!(*required_keys)
    assert_required_keys(required_keys)

    empty_keys, required_keys = [], [required_keys].flatten
    required_keys.each { |k| empty_keys << k if keys[k].nil? }
    raise(ArgumentError, "Required key has no value: #{empty_keys.join(", ")}") unless empty_keys.empty?
  end

  # assert_dependent_keys( :scope => [:modifier, :object], :person => [:site, :group] )
  # If parent is not present it is bypassed.  Only dependencies are required.
  def assert_presence_of_dependent_keys(*dependents)
    dependents.each { |set| set.each { |k,v| v.each { |required_key| assert_required_keys(required_key) } }
  end

  # requires that the parent is also provided as a key
  def assert_presence_of_dependent_keys!(*dependents)
    dependents.each { |set| set.each { |k,v| assert_required_keys(k); v.each { |r| assert_required_keys(r) } } }
  end

end