class Backports::Data

def self.define(*members, &block)
  members.each do |m|
    raise TypeError, "#{m} is not a Symbol" unless m.is_a?(Symbol) || m.is_a?(String)
    raise ArgumentError, "invalid data member: #{m}" if m.end_with?("=")
  end
  members = members.map(&:to_sym)
  raise ArgumentError, "duplicate members" if members.uniq!
  klass = instance_eval <<-"end_define", __FILE__, __LINE__ + 1
    Class.new(::Backports::Data) do     # Class.new(::Data) do
      def self.members       #   def self.members
        #{members.inspect}   #     [:a_member, :another_member]
      end                    #   end
    end                      # end
  end_define
  members.each do |m|
    klass.define_method(m) { @__members__[m]}
  end
  class << klass
    def new(*values, **named_values)
      if named_values.empty?
        if values.size > members.size
          raise ArgumentError, "wrong number of arguments (given #{values.size}, expected 0..#{members.size})"
        end
        super(**members.first(values.size).zip(values).to_h)
      else
        unless values.empty?
          raise ArgumentError, "wrong number of arguments (given #{values.size}, expected 0)"
        end
        super(**named_values)
      end
    end
    undef :define
  end
  klass.class_eval(&block) if block
  klass
end

def ==(other)
  return false unless other.instance_of?(self.class)
  @__members__ == other.to_h
end

def deconstruct
  @__members__.values
end

def deconstruct_keys(keys_or_nil)
  return @__members__ unless keys_or_nil
  raise TypeError, "Expected symbols" unless keys_or_nil.is_a?(Array) && keys_or_nil.all? {|s| s.is_a?(Symbol)}
  @__members__.slice(*keys_or_nil)
end

def eql?(other)
  return false unless other.instance_of?(self.class)
  @__members__.eql?(other.to_h)
end

def hash
  @__members__.hash
end

def initialize(**named_values)
  given = named_values.keys
  missing = members - given
  unless missing.empty?
    missing = missing.map(&:inspect).join(", ")
    raise ArgumentError, "missing keywords: #{missing}"
  end
  if members.size < given.size
    extra = (given - members).map(&:inspect).join(", ")
    raise ArgumentError, "unknown keywords: #{extra}"
  end
  @__members__ = named_values.freeze
  freeze
end

def initialize_copy(other)
  @__members__ = other.to_h
  freeze
end

def inspect
  data = @__members__.map {|k, v| "#{k}=#{v.inspect}"}.join(", ")
  space = data != "" && self.class.name ? " " : ""
  "#<data #{self.class.name}#{space}#{data}>"
end

def marshal_dump
  @__members__
end

def marshal_load(members)
  @__members__ = members
  freeze
end

def members
  self.class.members
end

def new(*values, **named_values)
  if named_values.empty?
    if values.size > members.size
      raise ArgumentError, "wrong number of arguments (given #{values.size}, expected 0..#{members.size})"
    end
    super(**members.first(values.size).zip(values).to_h)
  else
    unless values.empty?
      raise ArgumentError, "wrong number of arguments (given #{values.size}, expected 0)"
    end
    super(**named_values)
  end
end

def to_h(&block)
  @__members__.to_h(&block)
end

def with(**update)
  return self if update.empty?
  self.class.new(**@__members__.merge(update))
end