MC lay blocks

-- lay_perp_line.lua
-- Place blocks perpendicular to travel while moving forward.
-- Usage:
--   lay_perp_line <down|left|right> [steps]
-- Examples:
--   lay_perp_line down        -- place below each step until blocked
--   lay_perp_line left 25     -- 25 steps, place to the left each step

local args = {...}
local DIR = args[1]
local STEPS = tonumber(args[2] or "0")  -- 0 = run until blocked

local function die(msg) print(msg) error(msg, 0) end
if not DIR or not ({down=true,left=true,right=true})[DIR] then
  die("Usage: lay_perp_line <down|left|right> [steps]")
end

-- Select a slot with blocks; keep current slot if it still has items
local function ensureBlockSelected()
  if turtle.getItemCount() > 0 then return true end
  for i=1,16 do
    if turtle.getItemCount(i) > 0 then turtle.select(i); return true end
  end
  return false
end

local function placeDownIfAir()
  if turtle.detectDown() then return true end
  if not ensureBlockSelected() then return false end
  return turtle.placeDown()
end

local function placeLeftIfAir()
  turtle.turnLeft()
  local ok = true
  if not turtle.detect() then
    ok = ensureBlockSelected() and turtle.place() or false
  end
  turtle.turnRight()
  return ok
end

local function placeRightIfAir()
  turtle.turnRight()
  local ok = true
  if not turtle.detect() then
    ok = ensureBlockSelected() and turtle.place() or false
  end
  turtle.turnLeft()
  return ok
end

local function placePerp()
  if DIR == "down"  then return placeDownIfAir()
  elseif DIR == "left"  then return placeLeftIfAir()
  else return placeRightIfAir() end
end

local stepsDone = 0
while true do
  -- Place in the requested perpendicular direction if there is air there.
  if not placePerp() then
    if not ensureBlockSelected() then print("Out of blocks."); break end
    -- If placement failed for another reason, continue. It might already be filled.
  end

  -- Stop if something is immediately ahead.
  if turtle.detect() then
    print("Blocked ahead. Stopping.")
    break
  end

  -- Move forward one block.
  if not turtle.forward() then
    print("Movement failed. Stopping.")
    break
  end

  stepsDone = stepsDone + 1
  if STEPS > 0 and stepsDone >= STEPS then break end
end

print(("Done. Steps: %d"):format(stepsDone))