ComputerCraft Tutorial: ex_turtle_select 0_1

-- #######################################
-- ex_turtle_select
-- (extend Turtle API program)
-- version 0.1
-- http://hevohevo.hatenablog.com/

-- #######################################
-- add new functions and a value in Turtle API
-- turtle.org_select(n)
-- turtle.select(n)
-- turtle.getSelectedSlot()
-- turtle.selectNext()
-- turtle.selectPrev()
-- turtle._selected_slot  <- this is a private value. don't change directly.

if turtle and (turtle.org_select==nil) then
  turtle.org_select = turtle.select
  turtle.org_select(1)
  turtle._selected_slot = 1

  turtle.getSelectedSlot = function()
    return turtle._selected_slot
  end

  turtle.select = function(slot)
    if turtle.org_select(slot) then
      turtle._selected_slot = slot
      return true
    else
      return false
    end
  end

  local ring1_16 = function(num) -- 1=>1, 16=>16, 17=>1, 0=>16
    if (num % 16) == 0 then
      return 16
    else
      return (num % 16)
    end
  end

  turtle.selectNext = function()
    turtle.select(ring1_16(turtle.getSelectedSlot() + 1))
  end

  turtle.selectPrev = function()
    turtle.select(ring1_16(turtle.getSelectedSlot() - 1))
  end
  print("completed: extend turtle.select")
end