replace

function replacedown()
  turtle.select(1)
  turtle.digDown()
  turtle.placeDown()
  turtle.forward()
end

x = 2

function leftcontinue()
  turtle.turnLeft()
  turtle.forward()
  turtle.turnLeft()
  turtle.forward()
end

function refill()
  if turtle.getItemCount(1) < 1 then
  turtle.select(3)
  turtle.place()
  turtle.select(1)
  turtle.suck()
  turtle.select(3)
  turtle.dig()
  turtle.select(1)
end
end

function rightcontinue()
  turtle.turnRight()
  turtle.forward()
  turtle.turnRight()
  turtle.forward()
end

function continue()
  if x%2 then leftcontinue() x=x+1 return "x"
  else rightcontinue() x=x+1 return "x"
end
end

function main()
  turtle.select(2)
  if turtle.compareDown() == true then continue()
  else replacedown()
  refill()
 end
end

while true do
main()

end