api2

function forward(num)
  num = num or 1
  for i = 1, num do
    if turtle.forward() then
      if f == 0 then
        z = z - 1
        update("z",z)
      elseif f == 1 then
        x = x + 1
        update("x",x)
      elseif f == 2 then
        z = z + 1
        update("z",z)
      elseif f == 3 then
        x = x - 1
        update("x",x)
      end
      checkFuel(returning)
    else
      return false
    end
  end
  return true
end

function back(num)
  num = num or 1
  for i = 1, num do
    if turtle.back() then
      if f == 0 then
        z = z + 1
        update("z",z)
      elseif f == 1 then
        x = x - 1
        update("x",x)
      elseif f == 2 then
        z = z - 1
        update("z",z)
      elseif f == 3 then
        x = x + 1
        update("x",x)
      end
      checkFuel(returning)
    else
      return false
    end
  end
  return true
end

function up(num)
  num = num or 1
  for i = 1, num do
    if turtle.up() then
      y = y + 1
      update("y",y)
      checkFuel(returning)
    else
      return false
    end
  end
  return true
end

function down(num)
  num = num or 1
  for i = 1, num do
    if turtle.down() then
      y = y - 1
      update("y",y)
      checkFuel(returning)
      return true
    else
      return false
    end
  end
  return true
end

function left(num)
  num = num or 1
  for i = 1, num do
    turtle.turnLeft()
    f = (f+3)%4
    update("f",f)
  end
end

function right(num)
  num = num or 1
  for i = 1, num do
    turtle.turnRight()
    f = (f+1)%4
    update("f",f)
  end
end

function home()
  returning = true
  update("returning",true)
  while y ~= 0 do
    if y < 0 then
      turtle.digUp()
      up()
    elseif y > 0 then
      turtle.digDown()
      down()
    end
  end
  if z > 0 then
    while f ~= 0 do
      left()
    end
  elseif z < 0 then
    while f ~= 2 do
      left()
    end
  end
  while z ~= 0 do
    forward()
  end
  if x > 0 then
    while f ~= 3 do
      left()
    end
  elseif x < 0 then
    while f ~= 1 do
      left()
    end
  end
  while x ~= 0 do
    forward()
  end
end


function checkFuel(flag)
  if not flag then
    local fuel = turtle.getFuelLevel()
    local distance = math.abs(x) + math.abs(y) + math.abs(z)
    if fuel-1 <= distance then
      home()
      error("Alert: Fuel level critical. Returning to home point...")
    end
  end
end

function update(var,value)
    if not tostring(var) then
        error("var needs to be a string")
    end
    value = tostring(value)
    local str = fs.open("info","r").readAll()
    if str:find(var) then
        str = str:gsub(var.."%=.-%;",var.."="..value..";")
    else
        str = str .."\n"..var.."="..value..";"
    end
    local h = fs.open("info","w")
    h.write(str)
    h.close()
end

function log(data)
  local h = fs.open("log", "a")
  if type(data) == "string" then
    h.writeLine(data)
  elseif type(data) == "table" then
    for i = 1, #data do
      h.writeLine(data[i])
    end
  end
  h.close()
end

if not fs.exists("info") then
    local h = fs.open("info","w")
    x,y,z,f = 0,0,0,0
    h.writeLine("x=0;")
    h.writeLine("y=0;")
    h.writeLine("z=0;")
    h.writeLine("f=0;")
    h.writeLine("returning=false;")
    h.close()
else
    shell.run("info")
end