Advanced GPS goto program with position file

local tArgs = {...}
local x, y, z, f
local xCur, yCur, zCur, fCur
local xDif, yDif, zDif
local posIn, posOut, pos

local function syncFile()
  posOut = io.open("data/pos.txt", "w")
  posOut:write(textutils.serialize({xCur, yCur, zCur, fCur}))
  posOut:close()
end

local function abort(msg)
  syncFile()
  print("ERROR: "..msg)
  do return end
end

local function up(n)
  for i = 1, n do
	while not turtle.up() do
	  turtle.digUp()
	end
	yCur = yCur + 1
	syncFile()
  end
end

local function down(n)
  for i = 1, n do
	while not turtle.down() do
	  turtle.digDown()
	end
	yCur = yCur - 1
	syncFile()
  end
end

local function moveF(dir, n)
  while fCur ~= dir do
	turtle.turnLeft()
	if dir < 0 or dir > 3 then
	  do return end
	end
	if fCur ~= 0 then
	  fCur = fCur - 1
	else
	  fCur = 3
	end
	syncFile()
  end
  for i = 1, n do
	while not turtle.forward() do
	  up(1)
	end
	if fCur == 0 then
	  zCur = zCur + 1
	elseif fCur == 1 then
	  xCur = xCur - 1
	elseif fCur == 2 then
	  zCur = zCur - 1
	elseif fCur == 3 then
	  xCur = xCur + 1
	else
	  abort("invalid state for fCur")
	end
	syncFile()
  end
end

if not tArgs[1] or not tArgs[2] or not tArgs[3] then
  print("Usage: goto <x> <y> <z> [<f>]")
  do return end
end

x = tonumber(tArgs[1])
y = tonumber(tArgs[2])
z = tonumber(tArgs[3])

if tArgs[4] then
  f = tonumber(tArgs[4])
end

posIn = io.open("data/pos.txt", "r")
pos = posIn:read()
posIn:close()

pos = textutils.unserialize(pos)

xCur = tonumber(pos[1])
yCur = tonumber(pos[2])
zCur = tonumber(pos[3])
fCur = tonumber(pos[4])

xDif = x - xCur
zDif = z - zCur

if xDif &amp;lt; 0 then
  xDif = xDif * -1
  moveF(1, xDif)
else
  moveF(3, xDif)
end

if zDif &amp;lt; 0 then
  zDif = zDif * -1
  moveF(2, zDif)
else
  moveF(0, zDif)
end

yDif = y - yCur

if yDif &amp;lt; 0 then
  yDif = yDif * -1
  down(yDif)
else
  up(yDif)
end

if f then
  moveF(f, 0)
end

syncFile()

local xIni, yIni, zIni
local xCur, yCur, zCur
local fCur
local posOut

xIni, yIni, zIni = gps.locate(5)

if not xIni or not yIni or not zIni then
  print("No or insufficient GPS hosts found")
  do return end
end

while not turtle.forward() do
  while not turtle.up() do
	turtle.digUp()
  end
end

xCur, yCur, zCur = gps.locate(5)
if zCur > zIni then
  fCur = 0
elseif xCur < xIni then
  fCur = 1
elseif zCur < zIni then
  fCur = 2
elseif xCur > xIni then
  fCur = 3
else
  exit()
end

while not turtle.back() do
  while not turtle.up() do
	turtle.digUp()
  end
end

xCur, yCur, zCur = gps.locate(5)