transfer (in-place)

-- turtle stack transfer
-- moves blocks from one side to the other (up, forward, down)

local source   = "up"
local target   = "forward"
local overflow = ""
local slot     = 1

function getSuck(dir)
  if     dir == "up"      then return turtle.suckUp
  elseif dir == "forward" then return turtle.suck
  elseif dir == "down"    then return turtle.suckDown
  else
    print("Unknown Direction: " .. dir)
    exit()
  end
end
function getDrop(dir)
  if     dir == "up"      then return turtle.dropUp
  elseif dir == "forward" then return turtle.drop
  elseif dir == "down"    then return turtle.dropDown
  else
    print("Unknown Direction: " .. dir)
    exit()
  end
end

sourceSuck = getSuck(source)
targetDrop = getDrop(target)
if overflow ~= "" then
  overflowSuck = getSuck(overflow)
  overflowDrop = getDrop(overflow)
  overflow = true
else
  overflow = false
end

turtle.select(slot)
while true do
  if turtle.getItemCount(slot) > 0 then targetDrop() end
  if overflow and turtle.getItemCount(slot) > 0 then overflowDrop() end
  if turtle.getItemCount(slot) == 0 then sourceSuck() end
  if overflow and turtle.getItemCount(slot) == 0 then overflowSuck() end
  sleep(0)
end