Nether highway

local args = {...}

local dist = tonumber(args[1])

local movementCount = 0
local maxSlots = 16
local curSlot = turtle.getSelectedSlot()

function dig()
    turtle.digUp()
    turtle.dig()
end

function nextSlot()
    if curSlot == maxSlots then
        curSlot = 1
    else
        curSlot = curSlot + 1
    end

    return curSlot
end

function bridge()
    if not turtle.detectDown() then
        local ok, err = turtle.placeDown()

        if not ok then
            turtle.select(nextSlot())

            bridge()
        end
    end
end

function moveForward()
    turtle.forward()
    bridge()
    movementCount = movementCount + 1
end


while movementCount < dist do

    if turtle.detect() or turtle.detectUp() then
        dig()
    else
        moveForward()
    end
end