require_relative "block"

module Virtual
  # static description of a method
  # name
  # args (with defaults)
  # code
  # return arg (usually mystery, but for coded ones can be more specific)
  # known local variable names
  # temp variables (numbered)
  #
  # Methods are similar to Blocks. Where Blocks can be jumped to, Methods can be called.

  # Methods also have arguments and a return. These are Value subclass instances, ie specify
  #   type (by class type) and register by instance

  # They also have local variables. Args take up the first n regs, then locals the rest. No 
  #  direct manipulating of registers (ie specifying the number) should be done.

  # Code-wise Methods are made up from a list of Blocks, in a similar way blocks are made up of codes
  # The function starts with one block, and that has a start and end (return)
  
  # Blocks can be linked in two ways:
  # -linear:  flow continues from one to the next as they are sequential both logically and "physically"
  #           use the block set_next for this. 
  #           This "the straight line", there must be a continuous sequence from body to return
  #           Linear blocks may be created from an existing block with new_block
  # - branched: You create new blocks using function.new_block which gets added "after" return
  #            These (eg if/while) blocks may themselves have linear blocks ,but the last of these 
  #            MUST have an uncoditional branch. And remember, all roads lead to return.
  
  class MethodDefinition < Virtual::Object
    #return the main function (the top level) into which code is compiled
    def MethodDefinition.main
      MethodDefinition.new(:main , [] )
    end
    def initialize name , args , receiver = Virtual::Self.new , return_type = Virtual::Mystery , start = MethodEnter.new()
      @name = name.to_sym
      @args = args
      @locals = []
      @tmps = []
      @receiver = receiver
      @return_type = return_type
      @blocks = []
      # first block we have to create with .new , as new_block assumes a current
      enter = Block.new( name  , self ).add_code(start)
      @blocks << enter
      @current = enter
      new_block("return").add_code(MethodReturn.new)
    end
    attr_reader :name , :args , :receiver , :blocks
    attr_accessor :return_type , :current

    # add an instruction after the current (insertion point)
    # the added instruction will become the new insertion point
    def add_code instruction
      raise instruction.inspect unless instruction.is_a? Instruction
      @current.add_code(instruction) #insert after current
      self
    end

    # return a list of registers that are still in use after the given block
    # a call_site uses pushes and pops these to make them available for code after a call
    def locals_at l_block
      used =[]
      # call assigns the return register, but as it is in l_block, it is not asked.
      assigned = [ RegisterReference.new(Vm::RegisterMachine.instance.return_register) ]
      l_block.reachable.each do |b|
        b.uses.each {|u|
          (used << u) unless assigned.include?(u) 
        }
        assigned += b.assigns
      end
      used.uniq
    end

    # control structures need to see blocks as a graph, but they are stored as a list with implict branches
    # So when creating a new block (with new_block), it is only added to the list, but instructions
    #   still go to the current one
    # With this function one can change the current block, to actually code it.
    # This juggling is (unfortunately) neccessary, as all compile functions just keep puring their code into the  
    # method and don't care what other compiles (like if's) do.

    # Example: while, needs  2 extra blocks
    #          1 condition code, must be its own blockas we jump back to it
    #           -       the body, can actually be after the condition as we don't need to jump there
    #          2 after while block. Condition jumps here 
    # After block 2, the function is linear again and the calling code does not need to know what happened
    
    # But subsequent statements are still using the original block (self) to add code to
    # So the while expression creates the extra blocks, adds them and the code and then "moves" the insertion point along
    def current block
      @current = block
      self
    end

    # create a new linear block after the current insertion block. 
    # Linear means there is no brach needed from that one to the new one. 
    # Usually the new one just serves as jump address for a control statement
    # In code generation , the new_block is written after this one, ie zero runtime cost
    # This does _not_ change the insertion point, that has do be done with insert_at(block)
    def new_block new_name
      block_name = "#{@current.name}_#{new_name}"
      new_b = Block.new( block_name , self )
      index = @blocks.index( @current )
      @blocks.insert( index + 1 , new_b ) # + one because we want the ne after the insert_at
      return new_b
    end



    # determine whether this method has a variable by the given name
    # variables are locals and and arguments
    # used to determine if a send must be issued 
    def has_var name
      name = name.to_sym
      var = @args.find {|a| a.name == name }
      var = @locals.find {|a| a.name == name } unless var
      var = @tmps.find {|a| a.name == name } unless var
      var
    end

    # determine whether this method has an argument by the name
    def has_arg name
      name = name.to_sym
      var = @args.find {|a| a.name == name }
      var
    end

    def set_var name , var
      v = has_var name
      if( v )
        v.type = var
      else
        v = Local.new(name , var)
        @locals << v
      end
      v
    end

    def get_var name
      var = has_var name
      raise "no var #{name} in method #{self.name} , #{@locals} #{@args}" unless var
      var
    end

    def get_tmp
      name = "__tmp__#{@tmps.length}"
      @tmps << name
      Ast::NameExpression.new(name)
    end
  end
end