TreeFarmer

state = "charging"
position = 0

while true do
    -- print("state = " .. state .. ", position = " .. position .. ", fuel = " .. turtle.getFuelLevel())

    if state == "nextposition" then
        position = position + 1
        if position > 35 then
           position = 0
        end

        if position >= 0 and position <= 8 then
            turtle.turnLeft()
            if position > 0 then
                turtle.forward()
                turtle.turnRight()
            end
        end

        if position >= 9 and position <= 17 then
            turtle.turnLeft()
            if position > 9 then
                turtle.forward()
                turtle.turnRight()
            end
        end

        if position >= 18 and position <= 26 then
            turtle.turnLeft()
            if position > 18 then
                turtle.forward()
                turtle.turnRight()
            end
        end

        if position >= 27 and position <= 35 then
            turtle.turnLeft()
            if position > 27 then
                turtle.forward()
                turtle.turnRight()
            end
        end

        -- there might be items on the ground, pick them up

        turtle.select(1)
        turtle.suck()

        if turtle.getItemCount(2) > 0 then  -- we have excess items, drop them
            turtle.turnRight()
            turtle.turnRight()
            for n=2,4 do
                turtle.select(n)
                turtle.drop()
            end
            turtle.turnRight()
            turtle.turnRight()
        end

        state = "logging"
    end

    if state == "logging" then
        turtle.select(1)

        if turtle.getItemCount(1) > 0 and turtle.compareTo(16) == true then

            turtle.place()
            turtle.select(15)

            if turtle.compare() == true then
                turtle.select(2)
                turtle.dig()
                turtle.forward()
                while turtle.compareUp() == true do
                    turtle.digUp()
                    turtle.up()
                end

                while turtle.detectDown() == false do
                    turtle.down()
                end

                turtle.back()
                turtle.turnRight()
                turtle.turnRight()
                turtle.drop()
                turtle.turnRight()
                turtle.turnRight()
                turtle.select(1)
                turtle.place()
            end

            state = "nextposition"
        else
            state = "reload"
        end
    end

    if state == "charging" then
        if turtle.getFuelLevel() > 1000 then
            turtle.forward()
            turtle.forward()
            turtle.forward()
            turtle.forward()
            turtle.turnLeft()
            turtle.forward()
            turtle.forward()
            turtle.forward()
            turtle.forward()
            state = "logging"
            position = 0
            print("switching to logging state")
        end
    end

    if state == "reload" then

        -- prepare turtle to return to center of farm

        turtle.down()
        turtle.down()
        turtle.back()
        turtle.back()
        turtle.back()
        turtle.back()
        turtle.turnRight()

        -- now get it to the center block

        if position >= 0 and position <= 8 then
            while position < 4 do
                turtle.back()
                position = position + 1
            end

            while position > 4 do
                turtle.forward()
                position = position - 1
            end
        end

        if position >= 9 and position <= 17 then
            while position < 13 do
                turtle.back()
                position = position + 1
            end

            while position > 13 do
                turtle.forward()
                position = position - 1
            end
            turtle.turnRight()
        end

        if position >= 18 and position <= 26 then
            while position < 22 do
                turtle.back()
                position = position + 1
            end

            while position > 22 do
                turtle.forward()
                position = position - 1
            end
            turtle.turnRight()
            turtle.turnRight()
        end

        if position >= 27 and position <= 35 then
            while position < 31 do
                turtle.back()
                position = position + 1
            end

            while position > 31 do
                turtle.forward()
                position = position - 1
            end
            turtle.turnLeft()
        end


        -- now get it to the chest with the saplings

        turtle.turnRight()
        turtle.turnRight()

        for n=1,8 do
            turtle.forward()
        end
        turtle.up()
        turtle.up()
        turtle.up()
        turtle.forward()
        turtle.forward()

        -- we should be on top of the chest, get some saplings

        turtle.select(1)
        turtle.suckDown()

        -- and get turtle back to center block

        turtle.turnLeft()
        turtle.turnLeft()
        turtle.forward()
        turtle.forward()
        turtle.down()
        turtle.down()
        turtle.down()

        for n=1,8 do
            turtle.forward()
        end

        turtle.up()
        turtle.up()

        state = "charging"
    end

    if turtle.getFuelLevel() < 100 then
        state = "reload"
    end

    os.queueEvent("loop")
    os.pullEvent()
end