StairsDown-v1.2

--Stairs Down Simple Script v1
--Turtle Program Computer Craft
--by SemlerPDX(July2016)

--COMMENT SYNTAX EXPLAINATION
--torches must be in slot 1 (one for every 3rd layer dug)
--stairs can be in slot 2 (equal to or greater than number of stairs requested)

--StairsDown 20 (number of stairs down -- will use stairs from slot 2)
--example: StairsDown 20 (makes stairs 20 deep w/ torches and stair blocks)


-- Variables --
local args = {...}
local d = args[1] or 0
local r = 0 + d
local t = turtle
local lit = 2

if d == 0 then
  print("<error421: missing length>")
  print("<ex: Stairs 20 (starts digging stairs 20 blocks deep with stair blocks in slot 2)>")
  return
end

-- Functions --
local function ForceForward()
  while not t.forward() do
    t.dig()
  end
end

--dig down, decrement lit variable
local function ForceDown()
  t.digDown()
  t.down()
  lit = lit - 1
end

--shave 4 blocks and place torch if needed
local function Shave()
  for v=1,4 do
    if v == 3 then
        if lit == 0 then
            t.placeUp()
            lit = lit + 2
            ForceForward()
         else
            ForceForward()
         end
    else
         ForceForward()
    end
  end
end

--move to next shaving position
local function ResetRun()
  for v=1,3 do
    t.back()
  end
end

--place stairs while returning home
local function StairsHome()
  t.turnLeft()
  t.turnLeft()
  t.select(2)
  while r ~=0 do
    t.place()
    t.up()
    t.forward()
    r = r -1
  end
end

--return home
local function ForceHome()
  while r ~=0 do
    t.up()
    t.back()
    r = r -1
  end
end

--clear display screen
local function ClearScr()
  term.clear()
  term.setCursorPos(1,1)
end

-- Pre-fire scripts --
ClearScr()
print("Building staircase "..tostring(d).." blocks down...")
t.digDown()
t.down()
sleep(2)

---- Main Loop ----
while d ~= 0 do

  local function ForceCancel()
    local event, key = os.pullEvent("key")
  end

  local function Main()
    Shave()
    ResetRun()
    ForceDown()
    d = d - 1
  end

  FunctionEnabled = parallel.waitForAny(ForceCancel,Main)

  if FunctionEnabled == 1 then
    textutils.slowPrint("Cancelling Build...", 15)
    sleep(1.65)
    break
  end

end

-- Evaluate Ending --
if FunctionEnabled == 1 then    --(1 = functions cancelled, 2 = standard function looped)
  ClearScr()
  print("Build Cancelled by user")
  print("...returning to start in 5 seconds - move out of path")
  sleep(6)
  ForceHome()
  t.select(1)
else
  ClearScr()
  print("Build Completed")
  print("Building stairs and returning home...")
  t.up()
  StairsHome()
  t.select(1)
  --ClearScr()
  print("...program complete")
end