QTextOdfWriter: store all bulletChar()s as char16_t's

... and return them as QStringView from a file-static function so that
the conversion to QString is centralized in just one place (and we can
think of skipping the QString conversion once QXmlStreamWriter can
write QStringViews and not just QStrings).

Because the Style enum is ... weird (negative values), plaster the
code with static_asserts so that we get to detect breakage when the
enum values change.

Change-Id: I4ca89b6c2601c6a1153e202de966356bb4f51651
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/gui/text/qtextodfwriter.cpp

@@ -159,29 +159,32 @@ private:
     QString manifestNS;
 };
 
+static QStringView bullet_char(QTextListFormat::Style style)
+{
+    static_assert(int(QTextListFormat::ListDisc) == -1);
+    static_assert(int(QTextListFormat::ListUpperRoman) == -8);
+    static const char16_t chars[] = {
+        u'\x25cf', // bullet character
+        u'\x25cb', // white circle
+        u'\x25a1', // white square
+        u'1',
+        u'a',
+        u'A',
+        u'i',
+        u'I',
+    };
+    const auto map = [](QTextListFormat::Style s) { return -int(s) - 1; };
+    static_assert(uint(map(QTextListFormat::ListUpperRoman)) == std::size(chars) - 1);
+    const auto idx = map(style);
+    if (idx < 0)
+        return nullptr;
+    else
+        return {chars + idx, 1};
+}
+
 static QString bulletChar(QTextListFormat::Style style)
 {
-    switch(style) {
-    case QTextListFormat::ListDisc:
-        return QChar(0x25cf); // bullet character
-    case QTextListFormat::ListCircle:
-        return QChar(0x25cb); // white circle
-    case QTextListFormat::ListSquare:
-        return QChar(0x25a1); // white square
-    case QTextListFormat::ListDecimal:
-        return QString::fromLatin1("1");
-    case QTextListFormat::ListLowerAlpha:
-        return QString::fromLatin1("a");
-    case QTextListFormat::ListUpperAlpha:
-        return QString::fromLatin1("A");
-    case QTextListFormat::ListLowerRoman:
-        return QString::fromLatin1("i");
-    case QTextListFormat::ListUpperRoman:
-        return QString::fromLatin1("I");
-    default:
-    case QTextListFormat::ListStyleUndefined:
-        return QString();
-    }
+    return bullet_char(style).toString();
 }
 
 static QString borderStyleName(QTextFrameFormat::BorderStyle style)