Add FourFifths (4/5) BPM scaling control

Complement the FiveFourths (5/4) scaling from mxmilkiib's commit
with its inverse FourFifths (4/5 = 0.8x) scaling.

Added across all layers: enum, beats math, engine control,
controller picker menu, right-click track menu, track properties
dialog, and unit tests.

Closes #14686

Author: Swarnadip-Kar

src/controllers/controlpickermenu.cpp

@@ -292,6 +292,10 @@ ControlPickerMenu::ControlPickerMenu(QWidget* pParent)
             tr("3/4 BPM"),
             tr("Multiply current BPM by 0.75"),
             pBpmMenu);
+    addDeckAndSamplerControl("beats_set_fourfifths",
+            tr("4/5 BPM"),
+            tr("Multiply current BPM by 0.8"),
+            pBpmMenu);
     addDeckAndSamplerControl("beats_set_fivefourths",
             tr("5/4 BPM"),
             tr("Multiply current BPM by 1.25"),

src/engine/controls/bpmcontrol.cpp

@@ -154,6 +154,16 @@ BpmControl::BpmControl(const QString& group,
                     slotScaleBpm(mixxx::Beats::BpmScale::ThreeFourths);
                 }
             });
+    m_pBeatsFourFifths = std::make_unique<ControlPushButton>(
+            ConfigKey(group, "beats_set_fourfifths"), false);
+    connect(m_pBeatsFourFifths.get(),
+            &ControlObject::valueChanged,
+            this,
+            [this](int value) {
+                if (value > 0) {
+                    slotScaleBpm(mixxx::Beats::BpmScale::FourFifths);
+                }
+            });
     m_pBeatsFiveFourths = std::make_unique<ControlPushButton>(
             ConfigKey(group, "beats_set_fivefourths"), false);
     connect(m_pBeatsFiveFourths.get(),

src/engine/controls/bpmcontrol.h

@@ -158,6 +158,7 @@ class BpmControl : public EngineControl {
     std::unique_ptr<ControlPushButton> m_pBeatsHalve;
     std::unique_ptr<ControlPushButton> m_pBeatsTwoThirds;
     std::unique_ptr<ControlPushButton> m_pBeatsThreeFourths;
+    std::unique_ptr<ControlPushButton> m_pBeatsFourFifths;
     std::unique_ptr<ControlPushButton> m_pBeatsFiveFourths;
     std::unique_ptr<ControlPushButton> m_pBeatsFourThirds;
     std::unique_ptr<ControlPushButton> m_pBeatsThreeHalves;

src/library/dlgtrackinfo.cpp

@@ -130,6 +130,9 @@ void DlgTrackInfo::init() {
     connect(bpmThreeFourths, &QPushButton::clicked, this, [this] {
         slotBpmScale(mixxx::Beats::BpmScale::ThreeFourths);
     });
+    connect(bpmFourFifths, &QPushButton::clicked, this, [this] { // NEW
+        slotBpmScale(mixxx::Beats::BpmScale::FourFifths);        // NEW
+    });
     connect(bpmFiveFourths, &QPushButton::clicked, this, [this] {
         slotBpmScale(mixxx::Beats::BpmScale::FiveFourths);
     });

src/library/dlgtrackinfo.ui

@@ -823,6 +823,22 @@ Often results in higher quality beatgrids, but will not do well on tracks that h
             </widget>
            </item>
            <item row="3" column="1">
+            <widget class="QPushButton" name="bpmFourFifths">
+             <property name="minimumSize">
+              <size>
+               <width>125</width>
+               <height>0</height>
+              </size>
+             </property>
+             <property name="toolTip">
+              <string>Sets the BPM to 80% of the current value.</string>
+             </property>
+             <property name="text">
+              <string>4/5 BPM</string>
+             </property>
+            </widget>
+           </item>
+           <item row="4" column="0">
             <widget class="QPushButton" name="bpmFiveFourths">
              <property name="minimumSize">
               <size>
@@ -838,7 +854,7 @@ Often results in higher quality beatgrids, but will not do well on tracks that h
              </property>
             </widget>
            </item>
-           <item row="4" column="0">
+           <item row="4" column="1">
             <widget class="QPushButton" name="bpmFourThirds">
              <property name="minimumSize">
               <size>
@@ -854,7 +870,7 @@ Often results in higher quality beatgrids, but will not do well on tracks that h
              </property>
             </widget>
            </item>
-           <item row="4" column="1">
+           <item row="5" column="0">
             <widget class="QPushButton" name="bpmThreeHalves">
              <property name="minimumSize">
               <size>
@@ -870,7 +886,7 @@ Often results in higher quality beatgrids, but will not do well on tracks that h
              </property>
             </widget>
            </item>
-           <item row="5" column="0">
+           <item row="5" column="1">
             <widget class="QPushButton" name="bpmDouble">
              <property name="minimumSize">
               <size>
@@ -1088,6 +1104,7 @@ Often results in higher quality beatgrids, but will not do well on tracks that h
   <tabstop>bpmHalve</tabstop>
   <tabstop>bpmTwoThirds</tabstop>
   <tabstop>bpmThreeFourths</tabstop>
+  <tabstop>bpmFourFifths</tabstop>
   <tabstop>bpmFiveFourths</tabstop>
   <tabstop>bpmThreeHalves</tabstop>
   <tabstop>bpmFourThirds</tabstop>

src/test/beatgridtest.cpp

@@ -97,6 +97,14 @@ TEST(BeatGridTest, Scale) {
             pGrid->getBpmInRange(audio::kStartFramePos, trackEndPosition)
                     .value());
 
+    pGrid = Beats::fromConstTempo(pTrack->getSampleRate(),
+            mixxx::audio::kStartFramePos,
+            mixxx::Bpm(bpm));
+    pGrid = *pGrid->tryScale(Beats::BpmScale::FourFifths);
+    EXPECT_DOUBLE_EQ(bpm.value() * 4 / 5,
+            pGrid->getBpmInRange(audio::kStartFramePos, trackEndPosition)
+                    .value());
+
     pGrid = Beats::fromConstTempo(pTrack->getSampleRate(),
             mixxx::audio::kStartFramePos,
             mixxx::Bpm(bpm));

src/test/beatmaptest.cpp

@@ -81,6 +81,12 @@ TEST_F(BeatMapTest, Scale) {
             pMap->getBpmInRange(audio::kStartFramePos, trackEndPosition)
                     .value());
 
+    pMap = Beats::fromBeatPositions(m_pTrack->getSampleRate(), beats);
+    pMap = *pMap->tryScale(Beats::BpmScale::FourFifths);
+    EXPECT_DOUBLE_EQ(bpm.value() * 4 / 5,
+            pMap->getBpmInRange(audio::kStartFramePos, trackEndPosition)
+                    .value());
+
     pMap = Beats::fromBeatPositions(m_pTrack->getSampleRate(), beats);
     pMap = *pMap->tryScale(Beats::BpmScale::FiveFourths);
     EXPECT_DOUBLE_EQ(bpm.value() * 5 / 4,

src/track/beats.cpp

@@ -660,6 +660,9 @@ std::optional<BeatsPointer> Beats::tryScale(BpmScale scale) const {
     case BpmScale::ThreeFourths:
         scaleFactor *= 3.0 / 4;
         break;
+    case BpmScale::FourFifths:
+        scaleFactor *= 4.0 / 5;
+        break;
     case BpmScale::FiveFourths:
         scaleFactor *= 5.0 / 4;
         break;

src/track/beats.h

@@ -267,6 +267,7 @@ class Beats : private std::enable_shared_from_this<Beats> {
         Halve,
         TwoThirds,
         ThreeFourths,
+        FourFifths,
         FiveFourths,
         FourThirds,
         ThreeHalves,

src/widget/wtrackmenu.cpp

@@ -495,6 +495,8 @@ void WTrackMenu::createActions() {
         storeActionTextAndScaleInProperties(m_pBpmTwoThirdsAction, 2.0 / 3.0);
         m_pBpmThreeFourthsAction = make_parented<QAction>(tr("3/4 BPM"), m_pBPMMenu);
         storeActionTextAndScaleInProperties(m_pBpmThreeFourthsAction, 3.0 / 4.0);
+        m_pBpmFourFifthsAction = make_parented<QAction>(tr("4/5 BPM"), m_pBPMMenu);
+        storeActionTextAndScaleInProperties(m_pBpmFourFifthsAction, 4.0 / 5.0);
         m_pBpmFiveFourthsAction = make_parented<QAction>(tr("5/4 BPM"), m_pBPMMenu);
         storeActionTextAndScaleInProperties(m_pBpmFiveFourthsAction, 5.0 / 4.0);
         m_pBpmFourThirdsAction = make_parented<QAction>(tr("4/3 BPM"), m_pBPMMenu);
@@ -513,6 +515,9 @@ void WTrackMenu::createActions() {
         connect(m_pBpmThreeFourthsAction, &QAction::triggered, this, [this] {
             slotScaleBpm(mixxx::Beats::BpmScale::ThreeFourths);
         });
+        connect(m_pBpmFourFifthsAction, &QAction::triggered, this, [this] {
+            slotScaleBpm(mixxx::Beats::BpmScale::FourFifths);
+        });
         connect(m_pBpmFiveFourthsAction, &QAction::triggered, this, [this] {
             slotScaleBpm(mixxx::Beats::BpmScale::FiveFourths);
         });
@@ -646,6 +651,7 @@ void WTrackMenu::setupActions() {
         m_pBPMMenu->addAction(m_pBpmHalveAction);
         m_pBPMMenu->addAction(m_pBpmTwoThirdsAction);
         m_pBPMMenu->addAction(m_pBpmThreeFourthsAction);
+        m_pBPMMenu->addAction(m_pBpmFourFifthsAction);
         m_pBPMMenu->addAction(m_pBpmFiveFourthsAction);
         m_pBPMMenu->addAction(m_pBpmFourThirdsAction);
         m_pBPMMenu->addAction(m_pBpmThreeHalvesAction);
@@ -1137,6 +1143,7 @@ void WTrackMenu::updateMenus() {
             m_pBpmHalveAction->setEnabled(!anyBpmLocked);
             m_pBpmTwoThirdsAction->setEnabled(!anyBpmLocked);
             m_pBpmThreeFourthsAction->setEnabled(!anyBpmLocked);
+            m_pBpmFourFifthsAction->setEnabled(!anyBpmLocked);
             m_pBpmFiveFourthsAction->setEnabled(!anyBpmLocked);
             m_pBpmFourThirdsAction->setEnabled(!anyBpmLocked);
             m_pBpmThreeHalvesAction->setEnabled(!anyBpmLocked);
@@ -1157,6 +1164,7 @@ void WTrackMenu::updateMenus() {
                     appendBpmPreviewtoBpmAction(m_pBpmHalveAction, bpm);
                     appendBpmPreviewtoBpmAction(m_pBpmTwoThirdsAction, bpm);
                     appendBpmPreviewtoBpmAction(m_pBpmThreeFourthsAction, bpm);
+                    appendBpmPreviewtoBpmAction(m_pBpmFourFifthsAction, bpm);
                     appendBpmPreviewtoBpmAction(m_pBpmFiveFourthsAction, bpm);
                     appendBpmPreviewtoBpmAction(m_pBpmFourThirdsAction, bpm);
                     appendBpmPreviewtoBpmAction(m_pBpmThreeHalvesAction, bpm);