Memory warning while trying to merge multiple video files in swift

Im trying to merge 2 videos together using swift. However i am getting a memory warning and sometimes a crash when i try to run this code.

My hunch is that I exit the dispatch_group early for some reason and finish the writter.

However i have also noticed that sometimes I don't get that far.

I have also noticed that my samples.count is massive at times, which seems strange as the videos are no more than 30secs long each.

Im a stuck at where to start to solve this issue tbh. Any pointers appreciated.

 dispatch_group_enter(self.videoProcessingGroup)               
                asset.requestContentEditingInputWithOptions(options, completionHandler: {(contentEditingInput: PHContentEditingInput?, info: [NSObject : AnyObject]) -> Void in

                    let avAsset = contentEditingInput?.audiovisualAsset


                    let reader = try! AVAssetReader.init(asset: avAsset!)
                    let videoTrack = avAsset?.tracksWithMediaType(AVMediaTypeVideo).first

                    let readerOutputSettings: [String:Int] = [kCVPixelBufferPixelFormatTypeKey as String: Int(kCVPixelFormatType_32BGRA)]

                    let readerOutput = AVAssetReaderTrackOutput(track: videoTrack!, outputSettings: readerOutputSettings)
                    reader.addOutput(readerOutput)
                    reader.startReading()

                    //Create the samples
                    var samples:[CMSampleBuffer] = []

                    var sample: CMSampleBufferRef?

                    sample = readerOutput.copyNextSampleBuffer()

                    while (sample != nil)
                    {
                        autoreleasepool {
                            samples.append(sample!)
                            sample = readerOutput.copyNextSampleBuffer()
                        }
                    }

                    for i in 0...samples.count - 1 {
                        // Get the presentation time for the frame

                        var append_ok:Bool = false

                        autoreleasepool {
                            if  let pixelBufferPool = adaptor.pixelBufferPool {
                                let pixelBufferPointer = UnsafeMutablePointer<CVPixelBuffer?>.alloc(1)
                                let status: CVReturn = CVPixelBufferPoolCreatePixelBuffer(
                                    kCFAllocatorDefault,
                                    pixelBufferPool,
                                    pixelBufferPointer
                                )

                                let frameTime = CMTimeMake(Int64(frameCount), 30)

                                if var buffer = pixelBufferPointer.memory where status == 0 {
                                    buffer = CMSampleBufferGetImageBuffer(samples[i])!
                                    append_ok = adaptor.appendPixelBuffer(buffer, withPresentationTime: frameTime)
                                    pixelBufferPointer.destroy()
                                } else {
                                    NSLog("Error: Failed to allocate pixel buffer from pool")
                                }

                                pixelBufferPointer.dealloc(1)
                                dispatch_group_leave(self.videoProcessingGroup)
                            }
                        }

                    }
                })




        //Finish the session:
        dispatch_group_notify(videoProcessingGroup, dispatch_get_main_queue(), {
            videoWriterInput.markAsFinished()
            videoWriter.finishWritingWithCompletionHandler({
                print("Write Ended")

                // Return writer
                print("Created asset writer for (size.width)x(size.height) video")
            })
        })