Keyboard scrubbing and Play-at-speed: remove initial silence

Currently AudioIoCallback::ScrubState::Get(), inserts a period of silence the first time it's called because at this time Scrubber::ContinueScrubbingPoll() has not been called, and so message.end has not been set to an appropriate value.
In the case of keyboard scrubbing and play-at-speed, the initial speed is already known, so message.end can be set to this value, removing the need for an initial silence.
The start of keyboard scrubbing and play-at-speed are now faster (the latter very much faster).

src/AudioIO.cpp

@@ -543,7 +543,7 @@ struct AudioIoCallback::ScrubState
       : mRate(rate)
       , mStartTime( t0 )
    {
-      const double t1 = options.bySpeed ? 1.0 : t0;
+      const double t1 = options.bySpeed ? options.initSpeed : t0;
       Update( t1, options );
    }
 
@@ -558,25 +558,33 @@ struct AudioIoCallback::ScrubState
    {
       // Called by the thread that calls AudioIO::FillBuffers
       startSample = endSample = duration = -1LL;
+      sampleCount s0Init;
 
       Message message( mMessage.Read() );
       if ( !mStarted ) {
-         // Make some initial silence
-         const sampleCount s0 { llrint( mRate *
+         s0Init = llrint( mRate *
             std::max( message.options.minTime,
-               std::min( message.options.maxTime, mStartTime ) ) ) };
-         mData.mS0 = mData.mS1 = s0;
-         mData.mGoal = -1;
-         mData.mDuration = duration = inDuration;
-         mData.mSilence = 0;
-         mStarted = true;
+               std::min( message.options.maxTime, mStartTime ) ) );
+
+         // Make some initial silence. This is not needed in the case of
+         // keyboard scrubbing or play-at-speed, because the initial speed
+         // is known when this function is called the first time.
+         if ( !(message.options.isKeyboardScrubbing ||
+            message.options.isPlayingAtSpeed) ) {
+            mData.mS0 = mData.mS1 = s0Init;
+            mData.mGoal = -1;
+            mData.mDuration = duration = inDuration;
+            mData.mSilence = 0;
+         }
       }
-      else {
+
+      if (mStarted || message.options.isKeyboardScrubbing ||
+         message.options.isPlayingAtSpeed) {
          Data newData;
          inDuration += mAccumulatedSeekDuration;
 
-         // Use the previous end as NEW start.
-         const auto s0 = mData.mS1;
+         // If already started, use the previous end as NEW start.
+         const auto s0 = mStarted ? mData.mS1 : s0Init;
          const sampleCount s1 ( message.options.bySpeed
             ? s0.as_double() +
                lrint(inDuration.as_double() * message.end) // end is a speed
@@ -593,6 +601,8 @@ struct AudioIoCallback::ScrubState
          mData = newData;
       };
 
+      mStarted = true;
+
       Data &entry = mData;
       if (  mStopped.load( std::memory_order_relaxed ) ) {
          // We got the shut-down signal, or we discarded all the work.

src/AudioIOBase.h

@@ -53,7 +53,8 @@ struct ScrubbingOptions {
 
    double delay {};
 
-   // Limiting values for the speed of a scrub interval:
+   // Initial and limiting values for the speed of a scrub interval:
+   double initSpeed { 1.0 };
    double minSpeed { 0.0 };
    double maxSpeed { 1.0 };
 

src/tracks/ui/Scrubbing.cpp

@@ -527,6 +527,7 @@ bool Scrubber::StartSpeedPlay(double speed, double time0, double time1)
    options.pScrubbingOptions = &mOptions;
    options.envelope = nullptr;
    mOptions.delay = (ScrubPollInterval_ms / 1000.0);
+   mOptions.initSpeed = speed;
    mOptions.minSpeed = speed -0.01;
    mOptions.maxSpeed = speed +0.01;
 
@@ -602,12 +603,11 @@ bool Scrubber::StartKeyboardScrubbing(double time0, bool backwards)
    options.envelope = nullptr;
 
    // delay and minStutterTime are used in AudioIO::AllocateBuffers() for setting the
-   // values of mPlaybackQueueMinimum and mPlaybackSamplesToCopy respectively. minStutterTime
-   // is set lower here than in mouse scrubbing to ensure that there is not a long
-   // delay before the start of the playback of the audio.
+   // values of mPlaybackQueueMinimum and mPlaybackSamplesToCopy respectively.
    mOptions.delay = (ScrubPollInterval_ms / 1000.0);
    mOptions.minStutterTime = mOptions.delay;
 
+   mOptions.initSpeed = GetKeyboardScrubbingSpeed();
    mOptions.minSpeed = ScrubbingOptions::MinAllowedScrubSpeed();
    mOptions.maxSpeed = ScrubbingOptions::MaxAllowedScrubSpeed();
    mOptions.minTime = 0;