add shortestPitchDiff function

src/test/keyutilstest.cpp

@@ -301,3 +301,17 @@ TEST_F(KeyUtilsTest, GetCompatibleKeys) {
                     mixxx::track::io::key::A_MINOR,
                     mixxx::track::io::key::G_MINOR));
 }
+
+TEST_F(KeyUtilsTest, ShortestPitchDiff) {
+    // test no pitch difference in octaves
+    double diff = KeyUtils::shortestPitchDiff(12.0, 24.0);
+    EXPECT_EQ(0.0, diff);
+    // test for symmetry
+    diff = KeyUtils::shortestPitchDiff(6.0, 19.0);
+    EXPECT_EQ(1.0, diff);
+    diff = KeyUtils::shortestPitchDiff(19.0, 6.0);
+    EXPECT_EQ(1.0, diff);
+    // test that the difference is wrapped around boundaries
+    diff = KeyUtils::shortestPitchDiff(1.0, 23.0);
+    EXPECT_EQ(2.0, diff);
+}

src/track/keyutils.cpp

@@ -716,11 +716,11 @@ double KeyUtils::trackSyncPitchDifference(
     const double delta2 = powerOf2ToSemitoneChange(100 / bpm2.value());
 
     // get the resulting key for each track at 100BPM
-    const double resPitch1 = normalizePitch(keyToNumericValue(key1) - 1 + delta1);
-    const double resPitch2 = normalizePitch(keyToNumericValue(key2) - 1 + delta2);
+    const double resPitch1 = keyToNumericValue(key1) - 1 + delta1;
+    const double resPitch2 = keyToNumericValue(key2) - 1 + delta2;
 
     // return the pitch difference when both tracks are played at the same tempo
-    return normalizePitch(resPitch1 - resPitch2);
+    return shortestPitchDiff(resPitch1, resPitch2);
 }
 
 // Calculates Similarity (0 to 1, -1 if invalid) between two tracks based on their key and BPM.

src/track/keyutils.h

@@ -72,6 +72,12 @@ class KeyUtils {
         return std::min(CWSteps, 12 - CWSteps);
     }
 
+    // returns the shortest pitch difference up or down, handling octaves
+    static inline double shortestPitchDiff(double pitch1, double pitch2) {
+        const double normPitchDiff = normalizePitch(pitch1 - pitch2);
+        return std::min(normPitchDiff, 12.0 - normPitchDiff);
+    }
+
     static QString keyToString(mixxx::track::io::key::ChromaticKey key,
                                KeyNotation notation = KeyNotation::Custom);