CC Turtle program for replacing two items

-- redstone off -> set slot1 item
-- redstone on -> set slot2 item

-- 1. Set the slot1 item into the forward chest.
-- 2. Turtle's inventory: slot1 is empty, and slot2 has an item.
-- 3. Execute this program.

RS_SIDE = 'top'

while true do
  if (not rs.getInput(RS_SIDE)) and (turtle.getItemSpace(1) ~= 64) and (turtle.getItemSpace(2) == 64) then
    turtle.select(2)
    turtle.suck()
    turtle.select(1)
    turtle.drop()
  elseif rs.getInput(RS_SIDE) and (turtle.getItemSpace(2) ~= 64) and (turtle.getItemSpace(1) == 64) then
    turtle.select(1)
    turtle.suck()
    turtle.select(2)
    turtle.drop()
  end
  sleep(1)
end