Playing a few frames of a movie in reverse but stopping at set points

Question

I'm very new to flash and even though i'm capable of using the search function and finding posts which look relevant i don't really understand them, most notably i am only familiar with controlling a movie clip with buttons within the movie clip symbol and i get confused by actionscript that seems to do otherwise.
Within a frame on my main stage i have a movie clip symbol of around 125 frames. To control this movie clip symbol i have 4 button symbols within the movie clip symbol. The actionscript code that runs throughout this movie looks like this:
forward_btn.addEventListener(MouseEvent.CLICK,nudgeforward)
function nudgeforward(event:MouseEvent):void
    nextFrame();
backward_btn.addEventListener(MouseEvent.CLICK,nudgebackward)
function nudgebackward(event:MouseEvent):void
    prevFrame();
fastforward_btn.addEventListener(MouseEvent.CLICK,fastforward)
function fastforward(event:MouseEvent):void
    nextFrame();
    play();
On another layer i have stop(); commands on a few select frames. (25,50,75 etc)
This leads to free frame by frame navigation using the buttons forward_btn and backward_btn and if you'd like to move to set points in the animation you can use the fastforward_btn which plays the movie clip forwards until it hits a stop command on one of my chosen frames.
What i'd like to do is have a button that plays the frames backwards until it hits these frames. Is there a combination of while loops and prevFrame() i can use or anything straight forward?

Answer 1

The only thing that comes to mind is to use some modulus math to determine when you are in a frame that should be a stopping point. This would work as long as you have spacing such as you explained...25, 50, 75....  If the currentFrame%25 == 0, then that is a frame to stop in.  As far as the act of keeping things moving, you want to employ an ENTER_FRAME event listener so that it continually processes the prevFrame() call.  As soon as you hit a stopping frame you want to remove that listener.
If the modulus approach wouldn't cut it for all cases, then you could also try to work out some form variable implementation that is assigned the last stopping frame number as you move forward beyond it, using that variable to check against the currentFrame value in the ENTER_FRAME event handler function.