Local GPS for turtles

local args = {...}
local path="bin/LGPS/"
local data=path.."data"
local current={x=nil,y=nil,z=nil,f=nil}
local init=false
local save=function()
  local file=fs.open(data,"w")
  file.write(current.x.."\n"..current.y.."\n"..current.z.."\n"..current.f.."\n")
  file.close()
  return true
end
start = function(x,y,z,f)
  x=tonumber(x)
  y=tonumber(y)
  z=tonumber(z)
  f=tonumber(f)
  if x~=nil and y~=nil and z~=nil and f~=nil then
    if f<0 or f>3 then return nil end
    local file=fs.open(data,"w")
    current.x=x
    current.y=y
    current.z=z
    current.f=f
    save()
    init=true
    return current
  end
  if gps.locate() then
    local lx,ly,lz = gps.locate()
    for i=1,4,1 do
      if turtle.detect() then
        turtle.turnLeft()
      else
        turtle.forward()
        local x,y,z = gps.locate()
        if lx==x and lz==z then return nil end
        if lx==x then
          if lz>z then
            current.f=0
          else
            current.f=2
          end
        else
          if lx>x then
            current.f=3
          else
            current.f=1
          end
        end
        local file=fs.open(data,"w")
        current.x=x
        current.y=y
        current.z=z
        save()
        init=true
        return current
      end
    end
  end
  if fs.exists(data) and not fs.isDir(data) then
    local file = fs.open(data,"r")
    for i=1,4,1 do
      local l=tonumber(file.readLine())
      if l==nil then break end
      if i==1 then
        current.x=l
      elseif i==2 then
        current.y=l
      elseif i==3 then
        current.z=l
      elseif i==4 then
        current.f=l
        init=true
        file.close()
        return current
      end
    end
    file.close()
  end
  return nil
end
getPos = function()
  if init then return current
  else return nil end
end
forward = function(i)
  if not init then return nil end
  if i==nil then i=1 end
  i=tonumber(i)
  if i==0 then return true,0
  elseif i==nil then return false,0 end
  for j=1,i,1 do
    if turtle.forward() then
      if current.f==0 then
        current.z=current.z-1
      elseif current.f==1 then
        current.x=current.x+1
      elseif current.f==2 then
        current.z=current.z+1
      else
        current.x=current.x-1
      end
      save()
    else
      return false,j
    end
  end
  return true,i
end
back = function(i)
  if not init then return nil end
  if i==nil then i=1 end
  i=tonumber(i)
  if i==0 then return true,0
  elseif i==nil then return false,0 end
  for j=1,i,1 do
    if turtle.back() then
      if current.f==0 then
        current.z=current.z+1
      elseif current.f==1 then
        current.x=current.x-1
      elseif current.f==2 then
        current.z=current.z-1
      else
        current.x=current.x+1
      end
      save()
    else
      return false,j
    end
  end
  return true,i
end
up = function(i)
  if not init then return nil end
  if i==nil then i=1 end
  i=tonumber(i)
  if i==0 then return true,0
  elseif i==nil then return false,0 end
  for j=1,i,1 do
    if turtle.up() then
      current.y=current.y+1
      save()
    else
      return false,j
    end
  end
  return true,i
end
down = function(i)
  if not init then return nil end
  if i==nil then i=1 end
  i=tonumber(i)
  if i==0 then return true,0
  elseif i==nil then return false,0 end
  for j=1,i,1 do
    if turtle.down() then
      current.y=current.y-1
      save()
    else
      return false,j
    end
  end
  return true,i
end
turnLeft = function(i)
  if not init then return nil end
  if i==nil then i=1 end
  i=tonumber(i)
  if i==0 then return true,0
  elseif i==nil then return false,0 end
  for j=1,i,1 do
    if turtle.turnLeft() then
      if current.f==0 then
        current.f=3
      else
        current.f=current.f-1
      end
      save()
    else
      return false,j
    end
  end
  return true,i
end
turnRight = function(i)
  if not init then return nil end
  if i==nil then i=1 end
  i=tonumber(i)
  if i==0 then return true,0
  elseif i==nil then return false,0 end
  for j=1,i,1 do
    if turtle.turnRight() then
      if current.f==3 then
        current.f=0
      else
        current.f=current.f+1
      end
      save()
    else
      return false,j
    end
  end
  return true,i
end

local movements={["forward"]=forward,["back"]=back,["up"]=up,["down"]=down,["turnLeft"]=turnLeft,["turnRight"]=turnRight}
if args[1]=="set" then
  if tonumber(args[2])~=nil and tonumber(args[3])~=nil and tonumber(args[4])~=nil and tonumber(args[5])~=nil then
    if tonumber(args[5])<0 or tonumber(args[5])>3 then
      print("Invalid facing value!")
      return 1
    else
      start(args[2],args[3],args[4],args[5])
      return 0
    end
  else
    print("Insuficient number of arguments or wrong types")
    return 1
  end
elseif args[1]=="get" then
  if start() then
    local facing={"North","East","South","West"}
    print("X: "..current.x.."\nY: "..current.y.."\nZ: "..current.z.."\nF: "..current.f.." "..facing[current.f+1])
    return 0
  else
    print("No data available!")
    return 1
  end
elseif movements[args[1]] then
  if start() then
    local i=1
    if args[2]~=nil then
      if tonumber(args[2]) then
        i=args[2]
      else
        print("Invalid arguments!")
        return 1
      end
    end
    if movements[args[1]](i) then
      return 0
    else
      print("Movement blocked!")
      return 1
    end
  else
    print("No data available!")
    return 1
  end
end