Torches

function selectTorch()
  for i = 9, 16 do
    if turtle.getItemCount(i) > 0 then
      turtle.select(i)
      return
    end
  end
  turtle.select(9)
end

function selectCobble()
  for i = 1, 8 do
    if turtle.getItemCount(i) > 0 then
      turtle.select(i)
      return
    end
  end
  turtle.select(1)
end

while true do
  while turtle.detectDown() do
    turtle.digDown()
  end
  turtle.down()
  while turtle.detectDown() do
    turtle.digDown()
  end
  selectCobble()
  turtle.placeDown()
  turtle.up()
  selectTorch()
  turtle.placeDown()

  for i = 1, 9 do
    while not turtle.forward() do
    end
  end
end