Keyboard scrubbing: faster change of direction

If one of the keyboard scrubbing keys is being held down, and the other keyboard scrubbing key is pressed:
1. With current behaviour, scrubbing in the other direction only starts when the original key is released - scrubbing stops and then starts in the other direction.
2. With the new behaviour, scrubbing immediately changes direction, and does not stop when the original key is released - scrubbing does not stop and then start again.

New behaviour:
If one of the keyboard scrubbing keys is being held down,

src/AudioIO.cpp

@@ -2827,14 +2827,13 @@ void AudioIO::FillBuffers()
                      if (!mSilentScrub)
                      {
                         for (i = 0; i < mPlaybackTracks.size(); i++) {
-                           if (mPlaybackSchedule.mPlayMode == PlaybackSchedule::PLAY_AT_SPEED ||
-                              mPlaybackSchedule.mPlayMode == PlaybackSchedule::PLAY_KEYBOARD_SCRUB) {
-                              mPlaybackMixers[i]->SetSpeed(mScrubSpeed);
-                           }
-                           else {
+                           if (mPlaybackSchedule.mPlayMode == PlaybackSchedule::PLAY_AT_SPEED)
+                              mPlaybackMixers[i]->SetSpeedForPlayAtSpeed(mScrubSpeed);
+                           else if (mPlaybackSchedule.mPlayMode == PlaybackSchedule::PLAY_KEYBOARD_SCRUB)
+                              mPlaybackMixers[i]->SetSpeedForKeyboardScrubbing(mScrubSpeed, startTime);
+                           else
                               mPlaybackMixers[i]->SetTimesAndSpeed(
                                  startTime, endTime, fabs( mScrubSpeed ));
-                           }
                         }
                      }
                      mTimeQueue.mLastTime = startTime;

src/Mix.cpp

@@ -762,12 +762,35 @@ void Mixer::SetTimesAndSpeed(double t0, double t1, double speed)
    Reposition(t0);
 }
 
-void Mixer::SetSpeed(double speed)
+void Mixer::SetSpeedForPlayAtSpeed(double speed)
 {
    wxASSERT(std::isfinite(speed));
    mSpeed = fabs(speed);
 }
 
+void Mixer::SetSpeedForKeyboardScrubbing(double speed, double startTime)
+{
+   wxASSERT(std::isfinite(speed));
+
+   // Check if the direction has changed
+   if ((speed > 0.0 && mT1 < mT0) || (speed < 0.0 && mT1 > mT0)) {
+      // It's safe to use 0 and DBL_MAX, because Mixer::MixVariableRates()
+      // doesn't sample past the start or end of the audio in a track.
+      if (speed > 0.0 && mT1 < mT0) {
+         mT0 = 0;
+         mT1 = DBL_MAX;
+      }
+      else {
+         mT0 = DBL_MAX;
+         mT1 = 0;
+      }
+
+      Reposition(startTime, true);
+   }
+
+   mSpeed = fabs(speed);
+}
+
 MixerSpec::MixerSpec( unsigned numTracks, unsigned maxNumChannels )
 {
    mNumTracks = mNumChannels = numTracks;

src/Mix.h

@@ -134,7 +134,8 @@ class AUDACITY_DLL_API Mixer {
 
    // Used in scrubbing.
    void SetTimesAndSpeed(double t0, double t1, double speed);
-   void SetSpeed(double speed);
+   void SetSpeedForPlayAtSpeed(double speed);
+   void SetSpeedForKeyboardScrubbing(double speed, double startTime);
 
    /// Current time in seconds (unwarped, i.e. always between startTime and stopTime)
    /// This value is not accurate, it's useful for progress bars and indicators, but nothing else.

src/menus/TransportMenus.cpp

@@ -198,7 +198,7 @@ void DoKeyboardScrub(AudacityProject& project, bool backwards, bool keyUp)
 
    if (keyUp) {
       auto &scrubber = Scrubber::Get(project);
-      if (scrubber.IsKeyboardScrubbing()) {
+      if (scrubber.IsKeyboardScrubbing() && scrubber.IsBackwards() == backwards) {
          auto gAudioIO = AudioIOBase::Get();
          auto time = gAudioIO->GetStreamTime();
          auto &viewInfo = ViewInfo::Get(project);
@@ -221,7 +221,11 @@ void DoKeyboardScrub(AudacityProject& project, bool backwards, bool keyUp)
    else {      // KeyDown
       auto gAudioIO = AudioIOBase::Get();
       auto &scrubber = Scrubber::Get(project);
-      if (!gAudioIO->IsBusy() && !scrubber.HasMark()) {
+      if (scrubber.IsKeyboardScrubbing() && scrubber.IsBackwards() != backwards) {
+         // change direction
+         scrubber.SetBackwards(backwards);
+      }
+      else if (!gAudioIO->IsBusy() && !scrubber.HasMark()) {
          auto &viewInfo = ViewInfo::Get(project);
          auto &selection = viewInfo.selectedRegion;
          double endTime = TrackList::Get(project).GetEndTime();

src/tracks/ui/Scrubbing.h

@@ -82,6 +82,10 @@ public:
    { return mKeyboardScrubbing; }
    bool IsKeyboardScrubbing() const
    { return IsScrubbing() && mKeyboardScrubbing; }
+   void SetBackwards(bool backwards)
+   { mBackwards = backwards;}
+   bool IsBackwards() const
+   { return mBackwards;}
    // True iff the user has clicked to start scrub and not yet stopped,
    // but IsScrubbing() may yet be false
    bool HasMark() const