module Parfait
  # A factory has the one job of handing out new instances
  #
  # A factory is for a specific type (currently, may change by size at some point)
  #
  # get_next_object is the main entry point, all other functions help to get more
  # memory and objects as needed
  #
  # A factory keeps a reserve, and in case the freelist is empty, switches that in _immediately
  # This is especially useful for messages, that can then be used even they run out.
  #
  # The idea (especially for messages) is to call out from the MessageSetup to the
  # factory when the next (not current) is nil.
  # This is btw just as easy a check, as the next needs to be gotten to swap the list.
  class Factory < Object
    attr :type , :for_type , :next_object , :reserve , :attribute_name

    PAGE_SIZE = 1024
    RESERVE = 10

    # initialize for a given type (for_type). The attribute that is used to create the
    # list is the first that starts with next_ . "next" itself would have been nice and general
    # but is a keyword, so no go.
    def initialize(type)
      self.for_type = type
      self.attribute_name = type.names.find {|name| name.to_s.start_with?("next")}
      raise "No next found for #{type.class_name}" unless attribute_name
    end

    # get the next free object, advancing the list.
    # Calls out to get_more if the list is empty.
    # This function is not realy used, as it is hard-coded in risc, but the get_more is
    # used, as it get's called from risc (or will)
    def get_next_object
      unless( next_object )
        self.next_object = reserve
        get_more
      end
      get_head
    end

    # this gets the head of the freelist, swaps it out agains the next and returns it
    def get_head
      nekst = next_object
      self.next_object = get_next_for(nekst)
      return nekst
    end

    # get more from system
    # and rebuilt the reserve (get_next already instantiates the reserve)
    #
    def get_more
      first_object = get_chain
      link = first_object
      count = RESERVE
      while(count > 0)
        link = get_next_for(link)
        count -= 1
      end
      self.next_object = get_next_for(link)
      set_next_for( link , nil )
      self.reserve = first_object
      self
    end

    # this initiates the syscall to get more memory.
    # it creates objects from the mem and link them into a chain
    def get_chain
      raise "type is nil" unless self.for_type
      first = sys_mem( for_type , PAGE_SIZE)
      chain = first
      counter = PAGE_SIZE
      while( counter > 0)
        nekst = get_next_raw( chain )
        set_next_for(chain,  nekst)
        chain = nekst
        counter -= 1
      end
      first
    end

    # get the next_* attribute from the given object.
    # the attribute name is determined in initialize
    def get_next_for(object)
      object.send(attribute_name)
    end

    # set the next_* attribute of the given object, with the value.
    # the attribute name is determined in initialize
    def set_next_for(object , value)
      object.send("#{attribute_name}=".to_sym , value)
    end

    # Return the object _after the given one. In memory terms the next object starts
    # after the object ends. So this is in fact pointer arithmetic (once done)
    # This implementation will be moved to the adapter, as the real thing needs to be coded
    # in risc
    # This adapter version just return a new object
    def get_next_raw( object )
      sys_mem( object.get_type , 1)
    end

    # return more memory from the system.
    # Or to be more precise (as that is not really possible), allocate memory
    # for PAGE_SIZE objects, and return the first object.
    # ( the object has a type as first member, that type will be the for_type of this factory)
    # This implementation will be moved to the adapter, as the real thing needs to be coded
    # in risc
    # This adapter version just return a new object (by establishing the ruby class
    # and using ruby's allocate and set_type)
    def sys_mem( type , amount)
      r_class = eval( "Parfait::#{type.object_class.name}" )
      obj = r_class.allocate
      obj.set_type(type)
      #puts "Factory #{type.object_class.name} at 0x#{obj.object_id.to_s(16)}"
      obj
    end
  end
end