Turtle Program: Ender Quarry

args = { ... }

function usage()
  usages = {
    "fence",
    "destroy",
    "compute",
    "mark"
  }
  for i = 1, #usages do
    print("Usage: " .. shell.getRunningProgram() .. " " .. usages[i] .. " <length> <width>")
  end
end

function distance(dist, exec)
  for i = 1, dist do
    while not turtle.forward() do turtle.dig() turtle.attack() end
    exec(i == dist)
  end
end

function goSize(length, width, exec)
  distance(length, exec)
  turtle.turnLeft()
  distance(width, exec)
  turtle.turnLeft()
  distance(length, exec)
  turtle.turnLeft()
  distance(width, exec)
  turtle.turnLeft()
end

if #args < 3 then
  usage()
  return
end

local execute = function() end
local length = args[2] - 1
local width = args[3] - 1

if args[1] == "fence" then
  execute = function()
    turtle.digDown()
    slot = 2
    while turtle.getItemCount(1) <= 1 do
      if slot > 16 then error("Could not find more fences!") os.exit(1) end
      turtle.select(slot)
      if turtle.compareTo(1) then
        turtle.transferTo(1)
      end
      slot = slot + 1
    end
    turtle.select(1)
    turtle.placeDown()
  end
elseif args[1] == "destroy" then
  execute = function()
    turtle.digDown()
  end
elseif args[1] == "mark" then
  execute = function(atEnd)
    if atEnd then
      while not turtle.down() do turtle.digDown() turtle.attackDown() end
      turtle.select(2)
      turtle.placeDown()
      while not turtle.up() do turtle.digUp() turtle.attackUp() end
      turtle.select(1)
      turtle.placeDown()
    end
  end
elseif args[1] == "compute" then
  print("Fence needed: " .. tostring(2 * length + 2 * width))
  return
else
  return
end

goSize(length, width, execute)