Turtle Harvest Cactus

LENGTH = 8
-- for harvesting from a square cactus farm
-- turtle should be on top of a corner cactus, on "left" edge of square, facing the inside of the square

function checkFuel()
    if turtle.getFuelLevel() < (LENGTH * LENGTH) then -- basic check, not precise
        shell.run("refuel")
    end
end

function seesCactus()
    not_air, block = turtle.inspectDown()

    if not_air then
        if block.name == "minecraft:cactus" then
            return true
        end
    end

    return false

end

function selectItem(query)
    for slot = 1, 16, 1 do
        turtle.select(slot)
        if turtle.getItemDetail() then
            if turtle.getItemDetail().name == query then
                return true
            end

            if slot == 16 then
                return false
            end
        end
    end
end

function breakCactus()
    turtle.digDown()
    turtle.down()
    turtle.digDown()
    turtle.up()
end

function turnSwitch(t)
    if t % 2 == 0 then
        turtle.turnLeft()
    else
        turtle.turnRight()
    end
end

function homeFromEnd()
    turtle.turnRight()

    for k = 1, LENGTH - 1, 1 do
        turtle.forward()
    end

    turtle.turnRight()
end

function dropNoFuel()
    item = turtle.getItemDetail()

    if item then
        if item.name ~= "minecraft:coal" then
            turtle.dropDown()
        end
    end
end

function dumpInventory()
    turtle.back()
    turtle.back()
    for k = 1, 16, 1 do
        turtle.select(k)
        dropNoFuel()
    end

    turtle.forward()
    turtle.forward()
end

-------------------------------------------------------------

for i = 1, LENGTH, 1 do
    checkFuel()

    for j = 1, LENGTH, 1 do
        if seesCactus() == true then
            breakCactus()
        end

        if j < LENGTH then
            turtle.forward()
        end
    end

    if i < LENGTH then
        turnSwitch(i)
        turtle.forward()
        turnSwitch(i)
    else
        homeFromEnd()
        dumpInventory()
    end
end

-- enderlein