vertical

function findJunkToPlace ()
    for slot = 1, 12, 1 do
        -- compare this slot to each slot in the bottom row
        -- if it matches, leave it selected
        turtle.select(slot)
        for compare_slot = 13, 16, 1 do
            if (turtle.compareTo(compare_slot)) then
                -- found a match, leave "slot" selected
                return true
            end
        end
    end
    -- no matches found
    return false
end

turtle.refuel()
local max_depth = tonumber(...)
local current_depth = 0

-- dig downwards
while (current_depth < max_depth) do
    print("main loop")
    if (turtle.detectDown()) then
        print("dig down")
        turtle.digDown()
    end
    print("move down");
    turtle.down()
    current_depth = current_depth + 1
end

print("main loop end")

-- travel back up the shaft, placing garbage below as we rise
while (current_depth > 0) do
    print("up loop")
    print("moving up")
    turtle.up()
    current_depth = current_depth - 1

    -- any junk left in selected slot?
    if (turtle.getItemCount() < 1) then
        print("nothing in slot")
        -- nope. Let's find another slot
        if (findJunkToPlace() == false) then
            print("no junk found")
            -- ran out of junk to place
            break
        end
    end
    print("placing down")
    turtle.placeDown()
end
print("end run")