defmodule Game do

  # the field 
  @field_width   700
  @field_height  600

  # a paddle
  @paddle_width 20
  @paddle_height 100  
  @paddle_speed 5

  # a ball
  @ball_width 20
  @dy  0
  @dx 10

  
  
  def player(name) do
    y = trunc( (@field_height - @paddle_height) / 2)
    {name, y, 0}
  end

  def down({name, y, s}) do
    if (y + @paddle_height) < @field_height do
      {:ok, {name, y + @paddle_speed, s}}
    else
      :no
    end
  end

  def up({name, y, s}) do
    if y > 0 do
      {:ok, {name, y - @paddle_speed, s}}
    else
      :no
    end
  end


  def serve(name,{name, y, _}, _) do
    dx = @dx
    dy = @dy
    bx = (2 * @paddle_width) 	       
    by = (y + trunc(@paddle_height/2))
    {{bx, by}, {:ball, bx, by, dx, dy}}
  end
  def serve(name, _, {name, y, _}) do
    dx = -@dx
    dy = @dy
    bx = (@field_width - 2*@paddle_width - @ball_width) 	    
    by = (y + trunc(@paddle_height/2))
    {{bx, by}, {:ball, bx, by, dx, dy}}
  end  
  
  

  def move_ball(player1, player2, ball) do

    {:ball, bx, by, dx, dy}  = ball
      {name1, y1, s1} = player1
      {name2, y2, s2} = player2
      
      bx = bx + dx
      by = by + dy
	  
      cond  do
	dx > 0 and (bx >= (@field_width - 2*@paddle_width - @ball_width)) and (by >= (y2-@ball_width) and by <= (y2 + @paddle_height)) ->
	  #:io.format(" ball hits right paddle at: ~w ~w\n", [bx, by])
	  {dx, dy} = spin(y2, by, dx, dy)

	  {:bounce, {bx, by}, {:ball, bx, by, dx, dy}}

	dx < 0 and (bx <= (2*@paddle_width)) and  (by >= (y1-@ball_width) and by <= (y1 + @paddle_height)) ->
	  #:io.format(" ball hits left paddle\n")
	  {dx, dy} = spin(y1, by, dx, dy)
	  {:bounce, {bx, by}, {:ball, bx, by, dx, dy}}

	bx >= (@field_width - @ball_width) ->
	  s1 = s1+1
	  {:score, name1, s1, {name1, y1, s1}}
	  
	bx <= 0 ->
	  s2 = s2+1
	  {:score, name2, s2, {name2, y2, s2}}

	by <= 0 ->
          dy = -dy
    	  bx = bx + dx
	  by = by + dy	
	  {:moved, {bx, by}, {:ball, bx, by, dx, dy}}

	by >= (@field_height - @ball_width) ->
          dy = -dy
    	  bx = bx + dx
	  by = by + dy	
	  {:moved, {bx, by}, {:ball, bx, by, dx, dy}}

	true ->
    	  bx = bx + dx
	  by = by + dy	
	  {:moved, {bx, by}, {:ball, bx, by, dx, dy}}
      end
  end

  
  def spin(y, by, dx, dy) do
    hit = by - y

    cond do
      hit < trunc((@paddle_height * 0.2)) ->
	{-dx, dy-4}  # change direction
      hit <  trunc((@paddle_height * 0.4)) ->
	{-dx, dy-2}  # change direction	
      hit <  trunc((@paddle_height * 0.6)) ->
	{-dx, dy}    # change direction
      hit >  trunc((@paddle_height * 0.8)) ->
	{-dx, dy+2}  # change direction
      true ->
	{-dx, dy+4}   # 
    end
    
  end
  
  
end