bridge

local function doFuel()
  while turtle.getFuelLevel() < 10 do
    for i = 1,16 do
      if turtle.getItemCount(i) > 0 then
        turtle.select(i)
        if turtle.refuel(4) then
          break
        end
      end
    end
    if turtle.getFuelLevel() >= 10 then
      break
    end
    print("Please insert more fuel")
    os.pullEvent("turtle_inventory")
  end
end

local function putDown()
  if not turtle.detectDown() and not turtle.placeDown() then
    while not turtle.detectDown() do
      for i = 1,16 do
        if turtle.getItemCount(i) > 0 then
          turtle.select(i)
          if turtle.placeDown() then
            break
          end
        end
      end
      if turtle.detectDown() then
        break
      end
      print("Please insert building material.")
      os.pullEvent("turtle_inventory")
    end
  end
end

local args = {...}
if #args ~= 1 or not tonumber(args[1]) then
  print("Usage: bridge <length>")
  return
end

local length = tonumber(args[1])
maxHeight = 0
minHeight = 0
height = 0

for i = 1,length-1 do
  doFuel()
  while turtle.detect() do
    height = height + 1
    while not turtle.up() do
      turtle.back()
      i = i-1
    end
  end
  turtle.forward()
end
maxHeight = height+1

while not turtle.detectDown() do
  doFuel()
  height = height - 1
  turtle.down()
end
minHeight = height

turtle.turnLeft()
turtle.turnLeft()

for i = 1, length do
  if i < length then
    doFuel()
    if length-i <= maxHeight-1 then
      if height > 1 then
        turtle.down()
        height = height - 1
      end
    else
      if height < maxHeight then
        turtle.up()
        height = height + 1
      end
    end
    putDown()
    turtle.forward()
  end
end