Fix BeatMap::scale() -- which was totally broken.

Add tests for BeatGrid::scale() and BeatMap::scale(). Potentially a fix
for Bug #1341181.

src/test/beatgridtest.cpp

@@ -18,6 +18,28 @@ class BeatGridTest : public testing::Test {
     }
 };
 
+TEST_F(BeatGridTest, Scale) {
+    TrackPointer pTrack(new TrackInfoObject(), &QObject::deleteLater);
+
+    int sampleRate = 44100;
+    double bpm = 60.0;
+    pTrack->setBpm(bpm);
+    pTrack->setSampleRate(sampleRate);
+
+    BeatGrid* pGrid = new BeatGrid(pTrack.data(), 0);
+    pGrid->setBpm(bpm);
+
+    EXPECT_DOUBLE_EQ(bpm, pGrid->getBpm());
+    pGrid->scale(2);
+    EXPECT_DOUBLE_EQ(2 * bpm, pGrid->getBpm());
+
+    pGrid->scale(0.5);
+    EXPECT_DOUBLE_EQ(bpm, pGrid->getBpm());
+
+    pGrid->scale(0.25);
+    EXPECT_DOUBLE_EQ(0.25 * bpm, pGrid->getBpm());
+}
+
 TEST_F(BeatGridTest, TestNthBeatWhenOnBeat) {
     TrackPointer pTrack(new TrackInfoObject(), &QObject::deleteLater);
 

src/test/beatmaptest.cpp

@@ -44,6 +44,27 @@ class BeatMapTest : public testing::Test {
     int m_iFrameSize;
 };
 
+TEST_F(BeatMapTest, Scale) {
+    const double bpm = 60.0;
+    m_pTrack->setBpm(bpm);
+    m_pTrack->setSampleRate(m_iSampleRate);
+    double beatLengthFrames = getBeatLengthFrames(bpm);
+    double startOffsetFrames = 7;
+    const int numBeats = 100;
+    // Note beats must be in frames, not samples.
+    QVector<double> beats = createBeatVector(startOffsetFrames, numBeats, beatLengthFrames);
+    BeatMap* pMap = new BeatMap(m_pTrack, 0, beats);
+
+    // Check edge cases
+
+    EXPECT_DOUBLE_EQ(bpm, pMap->getBpm());
+    pMap->scale(2);
+    EXPECT_DOUBLE_EQ(2 * bpm, pMap->getBpm());
+
+    pMap->scale(0.5);
+    EXPECT_DOUBLE_EQ(bpm, pMap->getBpm());
+}
+
 TEST_F(BeatMapTest, TestNthBeat) {
     const double bpm = 60.0;
     m_pTrack->setBpm(bpm);

src/track/beatmap.cpp

@@ -407,17 +407,20 @@ void BeatMap::translate(double dNumSamples) {
 
 void BeatMap::scale(double dScalePercentage) {
     QMutexLocker locker(&m_mutex);
-    if (!isValid() || dScalePercentage <= 0.0) {
+    if (!isValid() || dScalePercentage <= 0.0 || m_beats.isEmpty()) {
         return;
     }
-    // Scale every beat relative to the first one.
-    Beat firstBeat = m_beats.first();
-    for (BeatList::iterator it = m_beats.begin();
-         it != m_beats.end(); ++it) {
+
+    // Scale the distance between every beat by 1/dScalePercentage to scale the
+    // BPM by dScalePercentage.
+    const double kScaleBeatDistance = 1.0 / dScalePercentage;
+    Beat prevBeat = m_beats.first();
+    BeatList::iterator it = m_beats.begin() + 1;
+    for (; it != m_beats.end(); ++it) {
         // Need to not accrue fractional frames.
         double newFrame = floor(
-            (1 - dScalePercentage) * firstBeat.frame_position() +
-            dScalePercentage * it->frame_position());
+                (1 - kScaleBeatDistance) * prevBeat.frame_position() +
+                kScaleBeatDistance * it->frame_position());
         it->set_frame_position(newFrame);
     }
     onBeatlistChanged();