Provide QStringConverter equivalent of QTextCodec::availableCodecs

A text editor commonly wants to display a list of codecs that are
supported. With the introduction of the ICU based QStringConverter, that
list is no longer statically known. So provide the necessary
functionality.

Fixes: QTBUG-109104
Change-Id: I9ecf59aa6bcc6fe65c8872cab84affafec4fa362
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/corelib/text/qstringconverter.cpp

@@ -10,6 +10,7 @@
 #include "private/qstringiterator_p.h"
 #include "private/qtools_p.h"
 #include "qbytearraymatcher.h"
+#include <QtCore/qbytearraylist.h>
 
 #if QT_CONFIG(icu)
 #include <unicode/ucnv.h>
@@ -2032,6 +2033,7 @@ const char *QStringConverter::name() const noexcept
 
     Returns the canonical name of the encoding this QStringConverter can encode or decode.
     Returns a nullptr if the converter is not valid.
+    The returned name is UTF-8 encoded.
 
     \sa isValid()
 */
@@ -2043,6 +2045,8 @@ const char *QStringConverter::name() const noexcept
     \c{std::nullopt} is returned. Such a name may, none the less, be accepted by
     the QStringConverter constructor when Qt is built with ICU, if ICU provides a
     converter with the given name.
+
+    \a name is expected to be UTF-8 encoded.
 */
 std::optional<QStringConverter::Encoding> QStringConverter::encodingForName(const char *name) noexcept
 {
@@ -2163,6 +2167,60 @@ std::optional<QStringConverter::Encoding> QStringConverter::encodingForHtml(QByt
     return Utf8;
 }
 
+static qsizetype availableCodecCount()
+{
+#if !QT_CONFIG(icu)
+    return QStringConverter::Encoding::LastEncoding;
+#else
+    /* icu contains also the names of what Qt provides
+       except for the special Locale one (so add one for it)
+    */
+    return 1 + ucnv_countAvailable();
+#endif
+}
+
+/*!
+    Returns a list of names of supported codecs. The names returned
+    by this function can be passed to QStringEncoder's and
+    QStringDecoder's constructor to create a en- or decoder for
+    the given codec.
+
+    \note The order of codecs is an internal implementation detail
+    and not guaranteed to be stable.
+ */
+QStringList QStringConverter::availableCodecs()
+{
+    auto availableCodec = [](qsizetype index) -> QString
+    {
+    #if !QT_CONFIG(icu)
+        return QString::fromLatin1(encodingInterfaces[index].name);
+    #else
+        if (index == 0) // "Locale", not provided by icu
+            return QString::fromLatin1(
+                        encodingInterfaces[QStringConverter::Encoding::System].name);
+        // this mirrors the setup we do to set a converters name
+        UErrorCode status = U_ZERO_ERROR;
+        auto icuName = ucnv_getAvailableName(int32_t(index - 1));
+        const char *standardName = ucnv_getStandardName(icuName, "MIME", &status);
+        if (U_FAILURE(status) || !standardName) {
+            status = U_ZERO_ERROR;
+            standardName = ucnv_getStandardName(icuName, "IANA", &status);
+        }
+        if (!standardName)
+            standardName = icuName;
+        return QString::fromLatin1(standardName);
+    #endif
+    };
+
+    qsizetype codecCount = availableCodecCount();
+    QStringList result;
+    result.reserve(codecCount);
+    for (qsizetype i = 0; i < codecCount; ++i)
+        result.push_back(availableCodec(i));
+    return result;
+}
+
+
 /*!
     Tries to determine the encoding of the HTML in \a data by looking at leading byte
     order marks or a charset specifier in the HTML meta tag and returns a QStringDecoder

src/corelib/text/qstringconverter_base.h

@@ -13,6 +13,7 @@
 
 #include <QtCore/qglobal.h> // QT_{BEGIN,END}_NAMESPACE
 #include <QtCore/qflags.h> // Q_DECLARE_FLAGS
+#include <QtCore/qcontainerfwd.h>
 
 #include <cstring>
 
@@ -159,6 +160,8 @@ public:
     encodingForData(QByteArrayView data, char16_t expectedFirstCharacter = 0) noexcept;
     Q_CORE_EXPORT static std::optional<Encoding> encodingForHtml(QByteArrayView data);
 
+    Q_CORE_EXPORT static QStringList availableCodecs();
+
 protected:
     const Interface *iface;
     State state;

tests/auto/corelib/text/qstringconverter/tst_qstringconverter.cpp

@@ -178,6 +178,8 @@ private slots:
 
     void encodingForHtml_data();
     void encodingForHtml();
+
+    void availableCodesAreAvailable();
 };
 
 void tst_QStringConverter::constructByName()
@@ -2426,6 +2428,13 @@ void tst_QStringConverter::encodingForHtml()
     }
 }
 
+void tst_QStringConverter::availableCodesAreAvailable()
+{
+    auto codecs = QStringConverter::availableCodecs();
+    for (const auto &codecName: codecs)
+        QVERIFY(QStringEncoder(codecName.toLatin1()).isValid());
+}
+
 class LoadAndConvert: public QRunnable
 {
 public: