gistfile1.md

  graph TD
    N2[5]
    N1[ARRAY] --> |0| N2
    N3["foo"]
    N1[ARRAY] --> |1| N3
    N4[:a]
    N1[ARRAY] --> |2| N4
    N0[Foo] --> |"@a"| N1
    N0[Foo] --> |"@b"| N0
    N5[1]
    N0[Foo] --> |"@c"| N5
    N7[12]
    N6[Hash] --> |0| N7
    N8["aaa"]
    N6[Hash] --> |a| N8
    N9[:b]
    N6[Hash] --> |key| N9
    N0[Foo] --> |"@d"| N6

gp.rb

module GP
  def gp(obj)
    print "```mermaid\n"
    print "  graph TD\n"
    gp_aux(obj, {})
    print "```"
  end

  def gp_aux(obj, occur)
    label = occur[obj.__id__]
    unless label
      label = occur[obj.__id__] = "N#{occur.size}"
      case obj
      when String
        print "    #{label}[\"#{obj}\"]\n"

      when Numeric, Symbol
        print "    #{label}[#{obj.inspect}]\n"

      when Array
        obj.each_with_index do |ivname, i|
          val = obj[i]
          vlabel = gp_aux(val, occur)
          print "    #{label}[ARRAY] --> |#{i}| #{vlabel}\n"
        end

      when Hash
        obj.each do |key, val|
          vlabel = gp_aux(val, occur)
          print "    #{label}[Hash] --> |#{key}| #{vlabel}\n"
        end

      else
        #user defined object
        obj.instance_variables.each do |ivname|
          ivvalue = obj.instance_variable_get(ivname)
          ivlabel = gp_aux(ivvalue, occur)
          print "    #{label}[#{obj.class.to_s}] --> |\"#{ivname}\"| #{ivlabel}\n"
        end
      end
    end
    label
  end
end

if __FILE__ == $0 then
  include GP

  class Foo
    def initialize
      @a = []
      @a[0] = 5
      @a[1] = "foo"
      @a[2] = :a
      @b = self
      @c = 1
      @d = {}
      @d[0] = 12
      @d[:a] = "aaa"
      @d["key"] = :b
    end
  end

  gp(Foo.new)
end