mytunnel

function updateLocation()
  if dir == "north" then
    z = z + 1
  elseif dir == "south" then
    z = z - 1
  elseif dir == "east" then
    x = x + 1
  elseif dir == "west" then
    x = x - 1
  end
end -- updateLocation

function right()
  turtle.turnRight()
  if dir == "north" then
    dir = "east"
  elseif dir == "east" then
    dir = "south"
  elseif dir == "south" then
    dir = "west"
  elseif dir == "west" then
    dir = "north"
  end
end -- right

function left()
  turtle.turnLeft()
  if dir == "north" then
    dir = "west"
  elseif dir == "west" then
    dir = "south"
  elseif dir == "south" then
    dir = "east"
  elseif dir == "east" then
    dir = "north"
  end
end -- right

function forward()
  while not turtle.forward() do
    turtle.dig()
  end
  updateLocation()
end -- forward

function up()
  while not turtle.up() do
    turtle.digUp()
  end
  y = y + 1
end -- up

function down()
  while not turtle.down() do
    turtle.digDown()
  end
  y = y - 1
end -- down

function mineRow()
  if width > 1 then
    if x == 1 then
      right()
      while x < width do
        forward()
      end
      left()
    else
      left()
      while x > 1 do
        forward()
      end
      right()
    end
  end
end -- row

function mineCrossSection()
  mineRow()
  if y == 1 then
    while y < height do
      up()
      mineRow()
    end
  else
    while y > 1 do
      down()
      mineRow()
    end
  end
end -- mineCrossSection()

function main(args)

  if #args ~= 3 then
    print( "Usage: mytunnel <width> <height> <length>" )
    error()
  end
  
  width  = tonumber(args[1])
  height = tonumber(args[2])
  length = tonumber(args[3])
  
  x = 1
  y = 1
  z = 0
  dir = "north"

  while z < length do
    forward()
    mineCrossSection()
  end

  while y > 1 do
    down()
  end

end -- main

args = { ... }
main(args)