Computercraft Netherbridge

-- Minecraft Computercraft Netherbridge Program
-- Creates a 2 x 2 roofed tunnel with 1 block railing
fuelLimit = 500
args = {...}

function forceDig(detectFunc, digFunc)
    while detectFunc() do
        digFunc()
    end
end

function fDig()
    forceDig(turtle.detect, turtle.dig)
end

function fDigUp()
    forceDig(turtle.detectUp, turtle.digUp)
end

function fDigDown()
    forceDig(turtle.detectDown, turtle.digDown)
end

function forcePlacement(detectFunc, digFunc, placeFunc)
    forceDig(detectFunc, digFunc)

    if turtle.getItemCount(turtle.getSelectedSlot()) > 0 and placeFunc() then
        return
    end

    for i = 1, 16, 1 do
        turtle.select(i)
        if turtle.getItemCount(i) > 0 then
            if placeFunc() then
                return
            end
        end
    end
    error("No more placeable blocks")
end

function fPlace()
    forcePlacement(turtle.detect, turtle.dig, turtle.place)
end

function fPlaceUp()
    forcePlacement(turtle.detectUp, turtle.digUp, turtle.placeUp)
end

function fPlaceDown()
    forcePlacement(turtle.detectDown, turtle.digDown, turtle.placeDown)
end

function fuel()
    local selectedSlot = turtle.getSelectedSlot()
    if turtle.getFuelLevel() <= fuelLimit then
        for i = 1, 16, 1 do
            turtle.select(i)
            turtle.refuel(64)
            if turtle.getFuelLevel() > fuelLimit then
                break
            end
        end
    end
    turtle.select(selectedSlot)
end

function main()
    -- Mine left corridor
    turtle.turnLeft()
    fDig()
    turtle.forward()

    fPlaceDown()
    fPlace()

    fDigUp()
    turtle.up()
    fPlaceUp()

    -- Back To starting position
    turtle.down()
    turtle.back()
    turtle.turnRight()

    -- Mine main block
    fPlaceDown()
    turtle.turnRight()
    fPlace()
    turtle.turnLeft()
    fDigUp()
    turtle.up()
    fPlaceUp()
    turtle.down()
    fDig()
    turtle.forward()
    fuel()
end

-- Arg 1 can be used to set tunnel length
if #(args) < 1 then
    while true do
        main()
    end
else
    for i=0,args[1],1 do
        main()
    end
end