Spiral

--range in blocks
range = 32

function failsafe()
  turtle.turnLeft()
  while turtle.forward()==false do
    turtle.dig()
  end
  while turtle.up()==false do
    turtle.digUp()
  end
  while turtle.up()==false do
    turtle.digUp()
  end
  while turtle.back()==false do
    turtle.turnRight()
    turtle.turnRight()
    turtle.dig()
    turtle.turnRight()
    turtle.turnRight()
  end
  turtle.turnRight()
  os.shutdown()
end

function getitem()
  for z=1,16 do
    turtle.select(z)
    if turtle.getItemCount()>0 then
      break
    end
  end
  if turtle.getItemCount()>=1 then
    return true
  else
    return false
  end
end

--place the turtle down under the ground:
turtle.turnLeft()
while turtle.forward()==false do
  turtle.dig()
end
while turtle.down()==false do
  turtle.digDown()
end
while turtle.down()==false do
  turtle.digDown()
end
while turtle.back()==false do
  turtle.turnRight()
  turtle.turnRight()
  turtle.dig()
  turtle.turnRight()
  turtle.turnRight()
end
turtle.turnRight()

starty=8+0.25
for i=1,range*4 do
  for i=1,starty do
    if turtle.forward()==false then
      turtle.dig()
      if turtle.forward()==false then
        failsafe()
      end
    end
    if turtle.getItemCount()<=0 then
      if getitem()==false then
        failsafe()
      end
    end
    if turtle.detectUp()==false and turtle.placeUp()==false then
      failsafe()
    end
  end
  starty=starty+0.50
  turtle.turnRight()
end

failsafe()