Improve natural sort algorithm

1. Use proper case folding function instead of `toLower()`.
2. Use locale aware comparison instead of comparing unicode code points.
   Now `a` comes before `A` which is the same as the result from QCollator. A nice side effect
   is now it properly compares locale specific characters (for example `C`, `Č`).
3. Improve testing. Now the test is runnable and stable on all platforms.

PR  #20208.

src/base/utils/compare.cpp

@@ -31,7 +31,6 @@
 #include <QChar>
 #include <QString>
 
-#ifndef QBT_USE_QCOLLATOR
 int Utils::Compare::naturalCompare(const QString &left, const QString &right, const Qt::CaseSensitivity caseSensitivity)
 {
     // Return value <0: `left` is smaller than `right`
@@ -45,8 +44,8 @@ int Utils::Compare::naturalCompare(const QString &left, const QString &right, co
         if ((posL == left.size()) || (posR == right.size()))
             return (left.size() - right.size());  // when a shorter string is another string's prefix, shorter string place before longer string
 
-        const QChar leftChar = (caseSensitivity == Qt::CaseSensitive) ? left[posL] : left[posL].toLower();
+        const QChar leftChar = (caseSensitivity == Qt::CaseSensitive) ? left[posL] : left[posL].toCaseFolded();
-        const QChar rightChar = (caseSensitivity == Qt::CaseSensitive) ? right[posR] : right[posR].toLower();
+        const QChar rightChar = (caseSensitivity == Qt::CaseSensitive) ? right[posR] : right[posR].toCaseFolded();
         // Compare only non-digits.
         // Numbers should be compared as a whole
         // otherwise the string->int conversion can yield a wrong value
@@ -89,8 +88,7 @@ int Utils::Compare::naturalCompare(const QString &left, const QString &right, co
         }
         else
         {
-            return (leftChar.unicode() - rightChar.unicode());
+            return QString::localeAwareCompare(leftChar, rightChar);
         }
     }
 }
-#endif

src/base/utils/compare.h

@@ -34,24 +34,35 @@
 // for QT_FEATURE_xxx, see: https://wiki.qt.io/Qt5_Build_System#How_to
 #include <QtCore/private/qtcore-config_p.h>
 
+#ifndef QBT_USE_QCOLLATOR
 // macOS and Windows support 'case sensitivity' and 'numeric mode' natively
 // https://github.com/qt/qtbase/blob/6.0/src/corelib/CMakeLists.txt#L777-L793
 // https://github.com/qt/qtbase/blob/6.0/src/corelib/text/qcollator_macx.cpp#L74-L77
 // https://github.com/qt/qtbase/blob/6.0/src/corelib/text/qcollator_win.cpp#L72-L78
 #if ((QT_FEATURE_icu == 1) || defined(Q_OS_MACOS) || defined(Q_OS_WIN))
-#define QBT_USE_QCOLLATOR
+#define QBT_USE_QCOLLATOR 1
 #include <QCollator>
+#else
+#define QBT_USE_QCOLLATOR 0
+#endif
 #endif
 
 class QString;
 
 namespace Utils::Compare
 {
-#ifdef QBT_USE_QCOLLATOR
+    int naturalCompare(const QString &left, const QString &right, Qt::CaseSensitivity caseSensitivity);
+
     template <Qt::CaseSensitivity caseSensitivity>
     class NaturalCompare
     {
     public:
+#if (QBT_USE_QCOLLATOR == 0)
+        int operator()(const QString &left, const QString &right) const
+        {
+            return naturalCompare(left, right, caseSensitivity);
+        }
+#else
         NaturalCompare()
         {
             m_collator.setNumericMode(true);
@@ -65,20 +76,8 @@ namespace Utils::Compare
 
     private:
         QCollator m_collator;
-    };
-#else
-    int naturalCompare(const QString &left, const QString &right, Qt::CaseSensitivity caseSensitivity);
-
-    template <Qt::CaseSensitivity caseSensitivity>
-    class NaturalCompare
-    {
-    public:
-        int operator()(const QString &left, const QString &right) const
-        {
-            return naturalCompare(left, right, caseSensitivity);
-        }
-    };
 #endif
+    };
 
     template <Qt::CaseSensitivity caseSensitivity>
     class NaturalLessThan

test/testutilscompare.cpp

@@ -26,15 +26,16 @@
  * exception statement from your version.
  */
 
-#include <tuple>
+#include <QLocale>
-
 #include <QObject>
 #include <QTest>
 
 #include "base/global.h"
+
+// only test qbt own implementation, not QCollator
+#define QBT_USE_QCOLLATOR 0
 #include "base/utils/compare.h"
 
-#ifndef QBT_USE_QCOLLATOR  // only test qbt own implementation, not QCollator
 namespace
 {
     enum class CompareResult
@@ -59,8 +60,8 @@ namespace
         {u"a"_s, u""_s, CompareResult::Greater, CompareResult::Greater},
 
         {u"a"_s, u"a"_s, CompareResult::Equal, CompareResult::Equal},
-        {u"A"_s, u"a"_s, CompareResult::Equal, CompareResult::Less},  // ascii code of 'A' is smaller than 'a'
+        {u"A"_s, u"a"_s, CompareResult::Equal, CompareResult::Greater},
-        {u"a"_s, u"A"_s, CompareResult::Equal, CompareResult::Greater},
+        {u"a"_s, u"A"_s, CompareResult::Equal, CompareResult::Less},
 
         {u"0"_s, u"0"_s, CompareResult::Equal, CompareResult::Equal},
         {u"1"_s, u"0"_s, CompareResult::Greater, CompareResult::Greater},
@@ -71,17 +72,17 @@ namespace
         {u"😁"_s, u"😀"_s, CompareResult::Greater, CompareResult::Greater},
 
         {u"a1"_s, u"a1"_s, CompareResult::Equal, CompareResult::Equal},
-        {u"A1"_s, u"a1"_s, CompareResult::Equal, CompareResult::Less},
+        {u"A1"_s, u"a1"_s, CompareResult::Equal, CompareResult::Greater},
-        {u"a1"_s, u"A1"_s, CompareResult::Equal, CompareResult::Greater},
+        {u"a1"_s, u"A1"_s, CompareResult::Equal, CompareResult::Less},
 
         {u"a1"_s, u"a2"_s, CompareResult::Less, CompareResult::Less},
-        {u"A1"_s, u"a2"_s, CompareResult::Less, CompareResult::Less},
+        {u"A1"_s, u"a2"_s, CompareResult::Less, CompareResult::Greater},
-        {u"a1"_s, u"A2"_s, CompareResult::Less, CompareResult::Greater},
+        {u"a1"_s, u"A2"_s, CompareResult::Less, CompareResult::Less},
         {u"A1"_s, u"A2"_s, CompareResult::Less, CompareResult::Less},
 
         {u"abc100"_s, u"abc99"_s, CompareResult::Greater, CompareResult::Greater},
-        {u"ABC100"_s, u"abc99"_s, CompareResult::Greater, CompareResult::Less},
+        {u"ABC100"_s, u"abc99"_s, CompareResult::Greater, CompareResult::Greater},
-        {u"abc100"_s, u"ABC99"_s, CompareResult::Greater, CompareResult::Greater},
+        {u"abc100"_s, u"ABC99"_s, CompareResult::Greater, CompareResult::Less},
         {u"ABC100"_s, u"ABC99"_s, CompareResult::Greater, CompareResult::Greater},
 
         {u"100abc"_s, u"99abc"_s, CompareResult::Greater, CompareResult::Greater},
@@ -135,7 +136,6 @@ namespace
         }
     }
 }
-#endif
 
 class TestUtilsCompare final : public QObject
 {
@@ -145,8 +145,20 @@ class TestUtilsCompare final : public QObject
 public:
     TestUtilsCompare() = default;
 
-#ifndef QBT_USE_QCOLLATOR  // only test qbt own implementation, not QCollator
 private slots:
+    void initTestCase() const
+    {
+        // Test will fail if ran with `C` locale. This is because `C` locale compare chars by code points
+        // and doesn't take account of human expectations
+        QLocale::setDefault(QLocale::English);
+    }
+
+    void cleanupTestCase() const
+    {
+        // restore global state
+        QLocale::setDefault(QLocale::system());
+    }
+
     void testNaturalCompareCaseInsensitive() const
     {
         const Utils::Compare::NaturalCompare<Qt::CaseInsensitive> cmp;
@@ -178,7 +190,6 @@ private slots:
         for (const TestData &data : testData)
             testLessThan(data, cmp(data.lhs, data.rhs), data.caseSensitiveResult);
     }
-#endif
 };
 
 QTEST_APPLESS_MAIN(TestUtilsCompare)