add time-based spacing and variable scroll speed (commented out because it wasn't as helpful as I hoped)

stepmania/src/Song.cpp

@@ -1292,3 +1292,27 @@ void Song::RemoveNotes( Notes* pNotes )
 
 	AddAutoGenNotes();
 }
+
+float Song::GetDominantBPM()
+{
+	// Get the average BPM.
+	// TODO:  Make this return the most-used BPM in the song.
+	// This is tricky though because many songs with BPM changes
+	// have tweaked BPMs (e.g. 160BPM at first, then 160.05BPM later).
+	float fSumBPM = 0;
+	float fSumWeights = 0;
+	for( unsigned i=0; i<m_BPMSegments.size(); i++ )
+	{
+		float fBPM = m_BPMSegments[i].m_fBPM;
+		bool bIsLast = i==m_BPMSegments.size()-1;
+		float fLength;
+		if( bIsLast )
+			fLength = m_fLastBeat - m_BPMSegments[i].m_fStartBeat;
+		else
+			fLength = m_BPMSegments[i+1].m_fStartBeat - m_BPMSegments[i].m_fStartBeat;
+		ASSERT( fLength > 0 );
+		fSumBPM += fBPM * fLength;
+		fSumWeights += fLength;
+	}
+	return fSumBPM / fSumWeights;
+}