Tunnel

-- Last pos: -7944
SAFE_SLOTS = 3
LAST_FILLER_SLOT = 8
SLOT_COUNT = 16
CHUNK_LOADER_PERIOD=16

function selectFiller()
  for slot = SAFE_SLOTS + 1, SLOT_COUNT do
    turtle.select(slot)
    if turtle.compareTo(2) and turtle.getItemCount() > 1 then
      return
    end
  end
end

function selectFillerDestination()
  turtle.select(SAFE_SLOTS + 1)
end

function placeFiller(direction)
  if direction == nil then
    direction = ''
  end

  while turtle['detect' .. direction]() do
    selectFillerDestination()
    turtle['dig' .. direction]()
  end
  selectFiller()
  turtle['place' .. direction]()
end

function clear(direction)
  if direction == nil then
    direction = ''
  end

  while turtle['detect' .. direction]() do
    selectFiller()
    turtle['place' .. direction]()
    selectFillerDestination()
    turtle['dig' .. direction]()
  end
end

function move(direction)
  clear(direction)

  if direction == nil or direction == '' then
    turtle.forward()
  elseif direction == 'Up' then
    turtle.up()
  else
    turtle.down()
  end
end

function dropNonFuel()
  for slot = LAST_FILLER_SLOT + 1, SLOT_COUNT do
    turtle.select(slot)
    if not turtle.refuel() then
      turtle.drop()
    end
  end
end

function buildLayer(width, height)
  local startFuel = turtle.getFuelLevel()
  local left = math.floor(width / 2)
  local right = left

  move()
  turtle.turnLeft()
  for i = 1, left + 1 do
    placeFiller('Down')
    clear('')
    move()
  end

  placeFiller('Down')

  turtle.turnLeft()
  turtle.turnLeft()
  for i = 1, height - 1 do
    move('Up')
    clear()
    placeFiller('Down')
  end

  placeFiller('Up')
  turtle.forward()
  turtle.turnLeft()
  turtle.turnLeft()
  placeFiller()
  turtle.turnLeft()
  turtle.turnLeft()

  for i = 1, width do
    placeFiller('Up')
    move()
  end

  placeFiller('Up')

  turtle.turnLeft()
  turtle.turnLeft()
  for i = 1, height - 1 do
    move('Down')
    clear()
    placeFiller('Up')
  end

  placeFiller('Down')
  turtle.forward()
  turtle.turnLeft()
  turtle.turnLeft()
  placeFiller()
  turtle.turnLeft()
  turtle.turnLeft()

  for i = 1, right - 1 do
    placeFiller('Down')
    move()
  end

  placeFiller('Down')

  local currentDirection = 'Up'
  move('Up')
  for x = 1, width - 2 do
    for y = 1, height - 3 do
        move(currentDirection)
    end

    move()
    if currentDirection == 'Up' then
      currentDirection = 'Down'
    else
      currentDirection = 'Up'
    end
  end

  if currentDirection == 'Down' then
    for y = 1, height - 2 do
      move('Down')
    end
  else
    move('Down')
  end

  turtle.turnLeft()
  turtle.turnLeft()
  for i = 1, left do
    move()
  end
  turtle.turnLeft()

  local consumedFuel = startFuel - turtle.getFuelLevel()

  dropNonFuel()

  return consumedFuel
end

function moveChunkLoader()
  turtle.turnLeft()
  turtle.turnLeft()

  turtle.select(3)
  turtle.place()

  turtle.up()
  for i = 1, CHUNK_LOADER_PERIOD do
    move()
  end
  turtle.down()

  turtle.dig()

  turtle.up()
  for i = 1, CHUNK_LOADER_PERIOD do
    turtle.back()
  end
  turtle.down()

  turtle.turnLeft()
  turtle.turnLeft()
end

args = {...}
tunnelWidth = tonumber(args[1])
tunnelHeight = tonumber(args[2])

if turtle.getFuelLevel() < tunnelWidth * tunnelHeight then
  error('Need some fuel to get started!')
end

turtle.turnLeft()
turtle.turnLeft()

turtle.select(3)
turtle.place()

turtle.turnLeft()
turtle.turnLeft()

fuelUsage = buildLayer(tunnelWidth, tunnelHeight)
averageFuelUsage = fuelUsage
layers = 1

while turtle.getFuelLevel() > 2 * fuelUsage and (#args < 3 or layers < tonumber(args[3])) do
  fuelUsage = buildLayer(tunnelWidth, tunnelHeight)
  layers = layers + 1
  averageFuelUsage = averageFuelUsage + (fuelUsage - averageFuelUsage) / layers

  if layers % CHUNK_LOADER_PERIOD == 0 then
    moveChunkLoader()
  end

  if turtle.getFuelLevel() <= 2 * fuelUsage then
    turtle.select(1)
    turtle.refuel(1)
  end
end

print('Ran out of fuel or terminated after ' .. tostring(layers) .. ' layers!')
print('Average fuel usage was: ' .. tostring(averageFuelUsage))