Wallbuilder (ComputerCraft)

local curslot, currow = 1,1

while true do
    if currow == 1 then
        while turtle.up() do end

        for i=1,16 do if turtle.getItemCount(i) > 0 then
            turtle.select(i)
            turtle.dropUp()
        end end

        curslot = 1
        turtle.select(curslot)

        for i=1,16 do turtle.suckUp() end

        while not (turtle.detect() or turtle.detectDown()) do
            while not turtle.down() do turtle.attackDown() end
        end

        if turtle.detect() then if not turtle.up() then error() end end
    end

    for j=1,48 do
        if turtle.getItemCount(curslot) == 0 then
            curslot = curslot + 1
            turtle.select(curslot)
        end

        turtle.place()

        if j<48 then
            if bit.band(currow,1)==1 then
                turtle.turnRight()
                turtle.forward()
                turtle.turnLeft()
            else
                turtle.turnLeft()
                turtle.forward()
                turtle.turnRight()
            end
        end
    end

    currow = currow + 1
    if currow > 20 then currow = 1 end

    if not turtle.up() then error() end
end