module Risc

  # MethodCompiler (old name) is used to generate risc instructions for methods
  # and to instantiate the methods correctly. Most of the init is typed layer stuff,
  # but there is some logic too.

  class MethodCompiler

    def initialize( method )
      @regs = []
      if method == :main
        @type = Parfait.object_space.get_type()
        @method = @type.get_method( :main )
        @method = @type.create_method( :main ,{}) unless @method
      else
        @method = method
        @type = method.for_type
      end
      @current = @method.instructions
    end
    attr_reader :type , :method

    # create the method, do some checks and set it as the current method to be added to
    # class_name and method_name are pretty clear, args are given as a ruby array
    def self.create_method( class_name , method_name , args = {})
      raise "create_method #{class_name}.#{class_name.class}" unless class_name.is_a? Symbol
      clazz = Parfait.object_space.get_class_by_name! class_name
      create_method_for( clazz.instance_type , method_name , args)
    end

    # create a method for the given type ( Parfait type object)
    # method_name is a Symbol
    # args a hash that will be converted to a type
    # the created method is set as the current and the given type too
    # return the compiler (for chaining)
    def self.create_method_for( type , method_name , args )
      raise "create_method #{type.inspect} is not a Type" unless type.is_a? Parfait::Type
      raise "Args must be Hash #{args}" unless args.is_a?(Hash)
      raise "create_method #{method_name}.#{method_name.class}" unless method_name.is_a? Symbol
      method = type.create_method( method_name , args)
      self.new(method)
    end

    # add method entry and exit code. Mainly save_return for the enter and
    # message shuffle and FunctionReturn for the return
    # return self for chaining
    def init_method
      source = "_init_method"
      name = "#{method.for_type.name}.#{method.name}"
      @current = @method.set_instructions( Risc.label(source, name))

      # add the type of the locals to the existing NamedList instance
      frame_reg = use_reg(:Type , method.frame )
      list_reg = use_reg(:NamedList )
      add_load_constant("#{name} load frame type", method.frame , frame_reg)
      add_slot_to_reg( "#{name} get frame from method" , :message , :frame , list_reg )
      add_reg_to_slot( "#{name} store frame type in frame" , frame_reg , list_reg , 1  )

      enter = @current # this is where method body goes
      add_label( source, "return #{name}")
      #load the return address into pc, affecting return. (other cpus have commands for this, but not arm)
      add_function_return( source , Risc.message_reg , Risc.resolve_to_index(:message , :return_address) )
      @current = enter
      self
    end

    def add_known(name)
      case name
      when :receiver
        ret = use_reg @type
        add_slot_to_reg(" load self" , :message , :receiver , ret )
        return ret
      when :space
        space = Parfait.object_space
        reg = use_reg :Space , space
        add_load_constant( "load space", space , reg )
        return reg
      when :message
        reg = use_reg :Message
        add_transfer( "load message", Risc.message_reg , reg )
        return reg
      else
        raise "Unknow expression #{name}"
      end
    end

    # set the insertion point (where code is added with add_code)
    def set_current c
      @current = c
    end

    # convert the given mom instruction to_risc and then add it (see add_code)
    # continue down the instruction chain unti depleted
    # (adding moves the insertion point so the whole mom chain is added as a risc chain)
    def add_mom( instruction )
      raise "whats this a #{instruction}" unless instruction.is_a?(Mom::Instruction)
      return
      while( instruction )
        risc = instruction.to_risc( self )
        add_code(risc)
        instruction = instruction.next
      end
    end
    # add a risc instruction after the current (insertion point)
    # the added instruction will become the new insertion point
    def add_code( instruction )
      raise instruction.to_s unless  instruction.is_a?(Risc::Instruction)
      raise instruction.to_s if( instruction.class.name.split("::").first == "Arm")
      @current.insert(instruction) #insert after current
      @current = instruction
      self
    end

    # for computationally building code (ie writing assembler) these short cuts
    # help to instantiate risc instructions and add them immediately
    [:label, :reg_to_slot , :slot_to_reg , :load_constant, :function_return ,
      :transfer , :reg_to_slot , :byte_to_reg , :reg_to_byte].each do |method|
      define_method("add_#{method}".to_sym) do |*args|
        add_code Risc.send( method , *args )
      end
    end

    # require a (temporary) register. code must give this back with release_reg
    def use_reg( type , value = nil )
      raise "Not type #{type.inspect}" unless type.is_a?(Symbol) or type.is_a?(Parfait::Type)
      if @regs.empty?
        reg = Risc.tmp_reg(type , value)
      else
        reg = @regs.last.next_reg_use(type , value)
      end
      @regs << reg
      return reg
    end

    def copy( reg , source )
      copied = use_reg reg.type
      add_code Register.transfer( source , reg , copied )
      copied
    end

    # releasing a register (accuired by use_reg) makes it available for use again
    # thus avoiding possibly using too many registers
    def release_reg( reg )
      last = @regs.pop
      raise "released register in wrong order, expect #{last} but was #{reg}" if reg != last
    end

    # reset the registers to be used. Start at r4 for next usage.
    # Every statement starts with this, meaning each statement may use all registers, but none
    # get saved. Statements have affect on objects.
    def reset_regs
      @regs.clear
    end

  end
end