Portal Deconstructor

-- determine movement direction
local move, detect = turtle.detectDown() and turtle.up
                  or turtle.detectUp() and turtle.down
                  or error("Place facing one of the corners.", 0),
                     turtle.detectDown() and turtle.detectUp
                  or turtle.detectDown
local dir = move == turtle.up and true or false -- 1 if moving up, 0 otherwise
if not turtle.detect() then error("Place facing one of the corners.", 0) end

local _print = print
local function print(...)
  _print("["..(os.clock() or "X").."]", ...)
end

-- moves and digs
local function bottom()
  print("BOTTOM - DIG")
  turtle.dig()
  turtle.digDown()
  if dir then
    print("BOTTOM - MOVE")
    move()
    return
  end
  print("BOTTOM - NO MOVE")
end
local function sides()
  print("SIDE - DIG")
  turtle.dig()
  print("SIDE - MOVE")
  move()
end
local function top()
  print("TOP - DIG")
  turtle.dig()
  turtle.digUp()
  if not dir then
    print("TOP - MOVE")
    move()
    return
  end
  print("TOP - NO MOVE")
end
local t1, t2 = dir and bottom or top, dir and top or bottom
local function switch()
  print("SWITCH - BACK")
  turtle.back()
  print("SWITCH - TURN")
  turtle.turnLeft()
  turtle.turnLeft()

  -- toggle the movement direction to the other direction
  move = move == turtle.down and turtle.up or turtle.down
  detect = detect == turtle.detectDown and turtle.detectUp or turtle.detectDown
  dir = not dir
end


-- actually move. If starting from the top, go down then up, otherwise go up then down.
print("Turtle is positioned at", dir and "bottom" or "top", "corner of portal.")
t1()
while not detect() do
  sides()
end

local count = 0
repeat
  count = count + 1
  t2()
until not turtle.back()

switch()

t2()
while not detect() do
  sides()
end
for i = 1, count do
  t1()
  if i ~= count then
    turtle.back()
  end
end
print("Done.")