Cleanup String::mapArg() implementation details

Avoids redundant ptr addition at return, uses more idomatic isEmpty()
and utilizes static decoders (since they are stateless and an error
would cause termination, they can be reused), though they need to be
thead_local to ensure multiple threads can use this function.

Author: oblivioncth

lib/core/src/qx-string.cpp

@@ -345,27 +345,30 @@ QString String::mapArg(QAnyStringView s, const QMap<QString, QString>& args, Qt:
 
     for(const QAnyStringView& view : resultBp.views())
     {
-        // QString::arg() uses size() here instead of isEmpty(), so we keep consistent
         resultRaw = view.visit(qxFuncAggregate{
             [resultRaw](QLatin1StringView v){
-                if(v.size())
-                {
-                    auto fromLatin1 = QStringDecoder(QStringDecoder::Latin1, QStringDecoder::Flag::Stateless);
-                    auto postAppend = fromLatin1.appendToBuffer(resultRaw, v);
-                    Q_ASSERT(!fromLatin1.hasError());
-                    return postAppend;
-                }
-                return resultRaw + v.size();
+                if(v.isEmpty())
+                    return resultRaw;
+
+                thread_local static auto fromLatin1 = QStringDecoder(QStringDecoder::Latin1, QStringDecoder::Flag::Stateless);
+                auto postAppend = fromLatin1.appendToBuffer(resultRaw, v);
+                Q_ASSERT(!fromLatin1.hasError());
+                return postAppend;
             },
             [resultRaw](QUtf8StringView v){
-                auto fromUtf8 = QStringDecoder(QStringDecoder::Utf8, QStringDecoder::Flag::Stateless);
+                if(v.isEmpty())
+                    return resultRaw;
+
+                thread_local static auto fromUtf8 = QStringDecoder(QStringDecoder::Utf8, QStringDecoder::Flag::Stateless);
                 auto postAppend = fromUtf8.appendToBuffer(resultRaw, v);
                 Q_ASSERT(!fromUtf8.hasError());
                 return postAppend;
             },
             [resultRaw](QStringView v){
-                if(v.size())
-                    memcpy(resultRaw, v.data(), v.size() * sizeof(QChar));
+                if(v.isEmpty())
+                    return resultRaw;
+
+                memcpy(resultRaw, v.data(), v.size() * sizeof(QChar));
                 return resultRaw + v.size();
             }
         });