fillSquare.lua

turtle.select(2)
while (not turtle.detectDown()) do
    if (turtle.getFuelLevel() < 10) then
        local last = turtle.getSelectedSlot()
        turtle.select(1)
        turtle.refuel(1)
        turtle.select(last)
    end

    if (turtle.getItemCount() == 0) then
        turtle.select(turtle.getSelectedSlot()+1)
    end
    turtle.placeDown()
    turtle.forward()

    if (turtle.detectDown()) then
        turtle.turnLeft()
        turtle.forward()
        turtle.turnLeft()
        turtle.forward()
    elseif (turtle.detect()) then
        turtle.placeDown()
        turtle.turnRight()
        turtle.forward()
        turtle.turnRight()
    end
end