QChar: make fullConvertCase()'s result type more usable

Move it into the function, give it an explicit size and make it
iterable and indicate to QStringView that it's a compatible container.

Change-Id: I483d9225ac73ad93f2037489f2d32473c377e8b7
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/text/qchar.cpp

@@ -1590,11 +1590,21 @@ QChar::UnicodeVersion QChar::currentUnicodeVersion() noexcept
     return UNICODE_DATA_VERSION;
 }
 
-using FullConvertCaseResult = std::array<char16_t, MaxSpecialCaseLength + 1>;
+static auto fullConvertCase(char32_t uc, QUnicodeTables::Case which) noexcept
-static FullConvertCaseResult fullConvertCase(char32_t uc, QUnicodeTables::Case which) noexcept
 {
-    FullConvertCaseResult result = {};
+    struct R {
-    auto pp = result.begin();
+        char16_t chars[MaxSpecialCaseLength + 1];
+        qint8 sz;
+
+        // iterable
+        auto begin() const { return chars; }
+        auto end() const { return chars + sz; }
+        // QStringView-compatible
+        auto data() const { return chars; }
+        auto size() const { return sz; }
+    } result;
+
+    auto pp = result.chars;
 
     const auto fold = qGetProp(uc)->cases[which];
     const auto caseDiff = fold.diff;
@@ -1609,6 +1619,7 @@ static FullConvertCaseResult fullConvertCase(char32_t uc, QUnicodeTables::Case w
         for (char16_t c : QChar::fromUcs4(uc + caseDiff))
             *pp++ = c;
     }
+    result.sz = pp - result.chars;
     return result;
 }
 

src/corelib/text/qstring.cpp

@@ -6148,31 +6148,27 @@ static QString detachAndConvertCase(T &str, QStringIterator it, QUnicodeTables::
 
     do {
         const auto folded = fullConvertCase(it.nextUnchecked(), which);
-        if (Q_UNLIKELY(folded[1])) {
+        if (Q_UNLIKELY(folded.size() > 1)) {
-            if (folded[0] == *pp && !folded[2]) {
+            if (folded.chars[0] == *pp && folded.size() == 2) {
                 // special case: only second actually changed (e.g. surrogate pairs),
                 // avoid slow case
                 ++pp;
-                *pp++ = folded[1];
+                *pp++ = folded.chars[1];
             } else {
                 // slow path: the string is growing
                 int inpos = it.index() - 1;
                 int outpos = pp - s.constBegin();
 
-                int foldedSize = 2; // must be at least 2, b/c folded[1] != NUL
+                s.replace(outpos, 1, reinterpret_cast<const QChar *>(folded.data()), folded.size());
-                while (folded[foldedSize])
+                pp = const_cast<QChar *>(s.constBegin()) + outpos + folded.size();
-                    ++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 + foldedSize, s.constEnd());
+                    it = QStringIterator(s.constBegin(), inpos + folded.size(), s.constEnd());
             }
         } else {
-            *pp++ = folded[0];
+            *pp++ = folded.chars[0];
         }
     } while (it.hasNext());