QChar/QString: centralize case folding in qchar.cpp

There are (at least) two implementations of the low-level case-folding
algorithm, one of which (for QChar::toLower()) seems to be wrong (it
doesn't deal with special cases which expand to more than one code
point).

The algoithm hidden in QString and entangled with the QString
detaching code makes reusing the code much harder.

At the same time, the dependency of the algorithm on the unicode
tables makes exposing a non-allocating result type in the public API
hard. std::u16string would be an alternative if we can assure that all
implementations use SSO with at least four characters.

So, for the time being, leave this as internal API for use in an
upcoming QStringView::toLower() as well as case-insensitive hashing.

Change-Id: Iabb2611846f6176776aa20e634f44d8464f3305c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qchar.cpp

@@ -1550,6 +1550,29 @@ QChar::UnicodeVersion QChar::currentUnicodeVersion() noexcept
     return UNICODE_DATA_VERSION;
 }
 
+using FullConvertCaseResult = std::array<char16_t, MaxSpecialCaseLength + 1>;
+static FullConvertCaseResult fullConvertCase(char32_t uc, QUnicodeTables::Case which) noexcept
+{
+    FullConvertCaseResult result = {};
+    auto pp = result.begin();
+
+    const auto fold = qGetProp(uc)->cases[which];
+    const auto caseDiff = fold.diff;
+
+    if (Q_UNLIKELY(fold.special)) {
+        const auto *specialCase = specialCaseMap + caseDiff;
+        auto length = *specialCase++;
+        while (length--)
+            *pp++ = *specialCase++;
+    } else if (Q_UNLIKELY(QChar::requiresSurrogates(uc))) {
+        // so far, case convertion never changes planes (guaranteed by the qunicodetables generator)
+        *pp++ = QChar::highSurrogate(uc);
+        *pp++ = QChar::lowSurrogate(uc + caseDiff);
+    } else {
+        *pp++ = uc + caseDiff;
+    }
+    return result;
+}
 
 template <typename T>
 Q_DECL_CONST_FUNCTION static inline T convertCase_helper(T uc, QUnicodeTables::Case which) noexcept

src/corelib/text/qstring.cpp

@@ -6533,36 +6533,32 @@ static QString detachAndConvertCase(T &str, QStringIterator it, QUnicodeTables::
     QChar *pp = s.begin() + it.index(); // will detach if necessary
 
     do {
-        auto uc = it.nextUnchecked();
-
-        const auto fold = qGetProp(uc)->cases[which];
-        signed short caseDiff = fold.diff;
-
-        if (Q_UNLIKELY(fold.special)) {
-            const ushort *specialCase = specialCaseMap + caseDiff;
-            ushort length = *specialCase++;
-
-            if (Q_LIKELY(length == 1)) {
-                *pp++ = QChar(*specialCase);
+        const auto folded = fullConvertCase(it.nextUnchecked(), which);
+        if (Q_UNLIKELY(folded[1])) {
+            if (folded[0] == *pp && !folded[2]) {
+                // special case: only second actually changed (e.g. surrogate pairs),
+                // avoid slow case
+                ++pp;
+                *pp++ = folded[1];
             } else {
                 // slow path: the string is growing
                 int inpos = it.index() - 1;
                 int outpos = pp - s.constBegin();
 
-                s.replace(outpos, 1, reinterpret_cast<const QChar *>(specialCase), length);
-                pp = const_cast<QChar *>(s.constBegin()) + outpos + length;
+                int foldedSize = 2; // must be at least 2, b/c folded[1] != NUL
+                while (folded[foldedSize])
+                    ++foldedSize;
+
+                s.replace(outpos, 1, reinterpret_cast<const QChar *>(folded.data()), foldedSize);
+                pp = const_cast<QChar *>(s.constBegin()) + outpos + foldedSize;
 
                 // do we need to adjust the input iterator too?
                 // if it is pointing to s's data, str is empty
                 if (str.isEmpty())
-                    it = QStringIterator(s.constBegin(), inpos + length, s.constEnd());
+                    it = QStringIterator(s.constBegin(), inpos + foldedSize, s.constEnd());
             }
-        } else if (Q_UNLIKELY(QChar::requiresSurrogates(uc))) {
-            // so far, case convertion never changes planes (guaranteed by the qunicodetables generator)
-            pp++;
-            *pp++ = QChar(QChar::lowSurrogate(uc + caseDiff));
         } else {
-            *pp++ = QChar(uc + caseDiff);
+            *pp++ = folded[0];
         }
     } while (it.hasNext());
 

src/corelib/text/qunicodetables.cpp

@@ -9946,6 +9946,8 @@ static const unsigned short specialCaseMap[] = {
     0x1, 0xa64b
 };
 
+const unsigned int MaxSpecialCaseLength = 3;
+
 
 static const unsigned short uc_decomposition_trie[] = {
     // 0 - 0x3400

util/unicode/main.cpp

@@ -2581,6 +2581,7 @@ static QByteArray createSpecialCaseMap()
     out += "static const unsigned short specialCaseMap[] = {\n"
            "    0x0, // placeholder";
     int i = 1;
+    int maxN = 0;
     while (i < specialCaseMap.size()) {
         out += "\n   ";
         int n = specialCaseMap.at(i);
@@ -2589,9 +2590,12 @@ static QByteArray createSpecialCaseMap()
             out += ",";
         }
         i += n + 1;
+        maxN = std::max(maxN, n);
     }
     out.chop(1);
-    out += "\n};\n\n";
+    out += "\n};\n\nconst unsigned int MaxSpecialCaseLength = ";
+    out += QByteArray::number(maxN);
+    out += ";\n\n\n";
 
     qDebug("    memory usage: %ld bytes", specialCaseMap.size()*sizeof(unsigned short));