Tree Farm

-- This is currently slightly broken: the loop runs correctly once,
-- but doesn't return home correctly.

local SAPLING = "minecraft:birch_sapling"
local LOG = "minecraft:birch_log"

local function findItem(item_name)
  for slot = 1,16,1 do
    local details = turtle.getItemDetail(slot)

    if details ~= nil then
      if details.name == item_name then
        return slot
      end
    end
  end

  return nil
end

local function ensureFueled(width, height)
  local current_fuel = turtle.getFuelLevel()
  local needed_fuel =
    3 + -- moveToStart()
    1 + -- fellTree()
    width * 3 + -- moveToNextTree()
    1 + height * 3 + -- moveToNextColumn() 1
    height * 3 -- moveToNextColumn() 2

  local has_enough_fuel = current_fuel >= needed_fuel

  if has_enough_fuel then
    return true
  else
    local slot = findItem(LOG)
    turtle.select(slot)

    turtle.refuel()

    return ensureFueled(width, height)
  end
end

local function turnRight()
  turtle.suck()
  turtle.turnRight()
  turtle.suck()
end
local function turnLeft()
  turtle.suck()
  turtle.turnLeft()
  turtle.suck()
end

local function forward()
  turtle.suck()
  turtle.forward()
  turtle.suck()
end

local function returnToChest(width)
  turnRight()
  forward()
  turnLeft()
  for _ = 1,3 * width,1 do
    turtle.forward()
  end
  forward()
  forward()
  turnRight()
  turnRight()
end

local function depositItems()
  local sapling_slot = findItem(SAPLING)
  local log_slot = findItem(LOG)
  for slot = 1,16,1 do
    -- the log slot is for fuel
    -- this works, because findItem
    -- only ever returns the first slot
    -- where an item is found
    if slot ~= sapling_slot and slot ~= log_slot then
      turtle.select(slot)
      turtle.dropDown()
    end
  end
end

local function placeSapling()
  local slot = findItem(SAPLING)
  if slot ~= nil then
    turtle.select(slot)

    turtle.place()
  end
end

-- This assumes that the tree will be felled in one swoop
local function fellTree()
  local blocked, data = turtle.inspect()
  if blocked and data.name == LOG then
    turtle.dig()
    os.sleep(1)
    turtle.suck()
    placeSapling()
  end
  turnLeft()
end

local function moveToStart()
  forward()
  forward()
  turnRight()
  forward()
  turnRight()
end

local function moveToNextColumn(height)
  turnLeft()
  turnLeft()

  for _ = 1,height * 2,1 do
    forward()
  end
  forward()

  turnRight()
  forward()
  moveToStart()
end

local function moveToNextTree()
  forward()
  forward()
  forward()
  turnRight()
end

local function farm(width, height)
  moveToStart()
  for col = 1,width,1 do
    for row = 1,height - 1,1 do
      fellTree()
      moveToNextTree()
    end
    fellTree()
    moveToNextColumn(height)
  end
end

local function sleep(seconds)
  while seconds > 0 do
    term.setCursorPos(1, 2)
    term.clearLine()
    term.write("Sleeping for " .. seconds .. " s")
    os.sleep(1)
    seconds = seconds - 1
  end
end

local function initialMessage()
  term.clear()
  term.setCursorPos(1, 1)
  term.write("farmTrees online!")
end

local function main(width, height)
  initialMessage()
  while true do
    assert(ensureFueled(width, height))
    farm(width, height)
    returnToChest(width)
    depositItems()
    sleep(60)
  end
end

main(3, 3)