/****************************************************************************
**
** Copyright (C) 2020 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Marc Mutz <marc.mutz@kdab.com>
** Copyright (C) 2019 Mail.ru Group.
** Contact: http://www.qt.io/licensing/
**
** This file is part of the QtCore module of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see https://www.qt.io/terms-conditions. For further
** information use the contact form at https://www.qt.io/contact-us.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 3 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL3 included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 3 requirements
** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 2.0 or (at your option) the GNU General
** Public license version 3 or any later version approved by the KDE Free
** Qt Foundation. The licenses are as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
** included in the packaging of this file. Please review the following
** information to ensure the GNU General Public License requirements will
** be met: https://www.gnu.org/licenses/gpl-2.0.html and
** https://www.gnu.org/licenses/gpl-3.0.html.
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** $QT_END_LICENSE$
**
****************************************************************************/
#ifndef QSTRINGVIEW_H
#define QSTRINGVIEW_H

/*
    Obsolete.
*/
#ifndef QT_STRINGVIEW_LEVEL
#  define QT_STRINGVIEW_LEVEL 1
#endif

#include <QtCore/qchar.h>
#include <QtCore/qbytearray.h>
#include <QtCore/qstringliteral.h>
#include <QtCore/qstringalgorithms.h>

#include <string>

#if defined(Q_OS_DARWIN) || defined(Q_QDOC)
Q_FORWARD_DECLARE_CF_TYPE(CFString);
Q_FORWARD_DECLARE_OBJC_CLASS(NSString);
#endif

QT_BEGIN_NAMESPACE

class QString;
class QStringView;
class QRegularExpression;
class QRegularExpressionMatch;

namespace QtPrivate {
template <typename Char>
struct IsCompatibleCharTypeHelper
    : std::integral_constant<bool,
                             std::is_same<Char, QChar>::value ||
                             std::is_same<Char, ushort>::value ||
                             std::is_same<Char, char16_t>::value ||
                             (std::is_same<Char, wchar_t>::value && sizeof(wchar_t) == sizeof(QChar))> {};
template <typename Char>
struct IsCompatibleCharType
    : IsCompatibleCharTypeHelper<typename std::remove_cv<typename std::remove_reference<Char>::type>::type> {};

template <typename Pointer>
struct IsCompatiblePointerHelper : std::false_type {};
template <typename Char>
struct IsCompatiblePointerHelper<Char*>
    : IsCompatibleCharType<Char> {};
template <typename Pointer>
struct IsCompatiblePointer
    : IsCompatiblePointerHelper<typename std::remove_cv<typename std::remove_reference<Pointer>::type>::type> {};

template <typename T, typename Enable = void>
struct IsContainerCompatibleWithQStringView : std::false_type {};

template <typename T>
struct IsContainerCompatibleWithQStringView<T, std::enable_if_t<std::conjunction_v<
            // lacking concepts and ranges, we accept any T whose std::data yields a suitable pointer ...
            IsCompatiblePointer<decltype( std::data(std::declval<const T &>()) )>,
            // ... and that has a suitable size ...
            std::is_convertible<decltype( std::size(std::declval<const T &>()) ), qsizetype>,
            // ... and it's a range as it defines an iterator-like API
            IsCompatibleCharType<typename std::iterator_traits<decltype( std::begin(std::declval<const T &>()) )>::value_type>,
            std::is_convertible<
                decltype( std::begin(std::declval<const T &>()) != std::end(std::declval<const T &>()) ),
                bool>,

            // These need to be treated specially due to the empty vs null distinction
            std::negation<std::is_same<std::decay_t<T>, QString>>,

            // Don't make an accidental copy constructor
            std::negation<std::is_same<std::decay_t<T>, QStringView>>
        >>> : std::true_type {};

} // namespace QtPrivate

class QStringView
{
public:
    typedef char16_t storage_type;
    typedef const QChar value_type;
    typedef std::ptrdiff_t difference_type;
    typedef qsizetype size_type;
    typedef value_type &reference;
    typedef value_type &const_reference;
    typedef value_type *pointer;
    typedef value_type *const_pointer;

    typedef pointer iterator;
    typedef const_pointer const_iterator;
    typedef std::reverse_iterator<iterator> reverse_iterator;
    typedef std::reverse_iterator<const_iterator> const_reverse_iterator;

private:
    template <typename Char>
    using if_compatible_char = typename std::enable_if<QtPrivate::IsCompatibleCharType<Char>::value, bool>::type;

    template <typename Pointer>
    using if_compatible_pointer = typename std::enable_if<QtPrivate::IsCompatiblePointer<Pointer>::value, bool>::type;

    template <typename T>
    using if_compatible_qstring_like = typename std::enable_if<std::is_same<T, QString>::value, bool>::type;

    template <typename T>
    using if_compatible_container = typename std::enable_if<QtPrivate::IsContainerCompatibleWithQStringView<T>::value, bool>::type;

    template <typename Char>
    static constexpr qsizetype lengthHelperPointer(const Char *str) noexcept
    {
#if defined(__cpp_lib_is_constant_evaluated)
        if (std::is_constant_evaluated())
            return std::char_traits<Char>::length(str);
#elif defined(Q_CC_GNU) && !defined(Q_CC_CLANG) && !defined(Q_CC_INTEL)
        if (__builtin_constant_p(*str))
            return std::char_traits<Char>::length(str);
#endif
        return QtPrivate::qustrlen(reinterpret_cast<const char16_t *>(str));
    }
    static qsizetype lengthHelperPointer(const QChar *str) noexcept
    {
        return QtPrivate::qustrlen(reinterpret_cast<const char16_t *>(str));
    }

    template <typename Container>
    static constexpr qsizetype lengthHelperContainer(const Container &c) noexcept
    {
        return qsizetype(std::size(c));
    }

    template <typename Char, size_t N>
    static constexpr qsizetype lengthHelperContainer(const Char (&str)[N]) noexcept
    {
        const auto it = std::char_traits<Char>::find(str, N, Char(0));
        const auto end = it ? it : std::end(str);
        return qsizetype(std::distance(str, end));
    }

    template <typename Char>
    static const storage_type *castHelper(const Char *str) noexcept
    { return reinterpret_cast<const storage_type*>(str); }
    static constexpr const storage_type *castHelper(const storage_type *str) noexcept
    { return str; }

public:
    constexpr QStringView() noexcept
        : m_size(0), m_data(nullptr) {}
    constexpr QStringView(std::nullptr_t) noexcept
        : QStringView() {}

    template <typename Char, if_compatible_char<Char> = true>
    constexpr QStringView(const Char *str, qsizetype len)
        : m_size((Q_ASSERT(len >= 0), Q_ASSERT(str || !len), len)),
          m_data(castHelper(str)) {}

    template <typename Char, if_compatible_char<Char> = true>
    constexpr QStringView(const Char *f, const Char *l)
        : QStringView(f, l - f) {}

#ifdef Q_CLANG_QDOC
    template <typename Char, size_t N>
    constexpr QStringView(const Char (&array)[N]) noexcept;

    template <typename Char>
    constexpr QStringView(const Char *str) noexcept;
#else

    template <typename Pointer, if_compatible_pointer<Pointer> = true>
    constexpr QStringView(const Pointer &str) noexcept
        : QStringView(str, str ? lengthHelperPointer(str) : 0) {}
#endif

#ifdef Q_CLANG_QDOC
    QStringView(const QString &str) noexcept;
#else
    template <typename String, if_compatible_qstring_like<String> = true>
    QStringView(const String &str) noexcept
        : QStringView(str.isNull() ? nullptr : str.data(), qsizetype(str.size())) {}
#endif

    template <typename Container, if_compatible_container<Container> = true>
    constexpr QStringView(const Container &c) noexcept
        : QStringView(std::data(c), lengthHelperContainer(c)) {}

    template <typename Char, size_t Size, if_compatible_char<Char> = true>
    [[nodiscard]] constexpr static QStringView fromArray(const Char (&string)[Size]) noexcept
    { return QStringView(string, Size); }

    [[nodiscard]] inline QString toString() const; // defined in qstring.h
#if defined(Q_OS_DARWIN) || defined(Q_QDOC)
    // defined in qcore_foundation.mm
    [[nodiscard]] Q_CORE_EXPORT CFStringRef toCFString() const Q_DECL_CF_RETURNS_RETAINED;
    [[nodiscard]] Q_CORE_EXPORT NSString *toNSString() const Q_DECL_NS_RETURNS_AUTORELEASED;
#endif

    [[nodiscard]] constexpr qsizetype size() const noexcept { return m_size; }
    [[nodiscard]] const_pointer data() const noexcept { return reinterpret_cast<const_pointer>(m_data); }
    [[nodiscard]] const_pointer constData() const noexcept { return data(); }
    [[nodiscard]] constexpr const storage_type *utf16() const noexcept { return m_data; }

    [[nodiscard]] constexpr QChar operator[](qsizetype n) const
    { return Q_ASSERT(n >= 0), Q_ASSERT(n < size()), QChar(m_data[n]); }

    //
    // QString API
    //

    template <typename...Args>
    [[nodiscard]] inline QString arg(Args &&...args) const; // defined in qstring.h

    [[nodiscard]] QByteArray toLatin1() const { return QtPrivate::convertToLatin1(*this); }
    [[nodiscard]] QByteArray toUtf8() const { return QtPrivate::convertToUtf8(*this); }
    [[nodiscard]] QByteArray toLocal8Bit() const { return QtPrivate::convertToLocal8Bit(*this); }
    [[nodiscard]] inline QList<uint> toUcs4() const; // defined in qlist.h ### Qt 7 char32_t

    [[nodiscard]] constexpr QChar at(qsizetype n) const noexcept { return (*this)[n]; }

    [[nodiscard]] constexpr QStringView mid(qsizetype pos, qsizetype n = -1) const noexcept
    {
        using namespace QtPrivate;
        auto result = QContainerImplHelper::mid(size(), &pos, &n);
        return result == QContainerImplHelper::Null ? QStringView() : QStringView(m_data + pos, n);
    }
    [[nodiscard]] constexpr QStringView left(qsizetype n) const noexcept
    {
        if (size_t(n) >= size_t(size()))
            n = size();
        return QStringView(m_data, n);
    }
    [[nodiscard]] constexpr QStringView right(qsizetype n) const noexcept
    {
        if (size_t(n) >= size_t(size()))
            n = size();
        return QStringView(m_data + m_size - n, n);
    }

    [[nodiscard]] constexpr QStringView first(qsizetype n) const noexcept
    { Q_ASSERT(n >= 0); Q_ASSERT(n <= size()); return QStringView(m_data, n); }
    [[nodiscard]] constexpr QStringView last(qsizetype n) const noexcept
    { Q_ASSERT(n >= 0); Q_ASSERT(n <= size()); return QStringView(m_data + size() - n, n); }
    [[nodiscard]] constexpr QStringView sliced(qsizetype pos) const noexcept
    { Q_ASSERT(pos >= 0); Q_ASSERT(pos <= size()); return QStringView(m_data + pos, size() - pos); }
    [[nodiscard]] constexpr QStringView sliced(qsizetype pos, qsizetype n) const noexcept
    { Q_ASSERT(pos >= 0); Q_ASSERT(n >= 0); Q_ASSERT(size_t(pos) + size_t(n) <= size_t(size())); return QStringView(m_data + pos, n); }
    [[nodiscard]] constexpr QStringView chopped(qsizetype n) const noexcept
    { return Q_ASSERT(n >= 0), Q_ASSERT(n <= size()), QStringView(m_data, m_size - n); }

    constexpr void truncate(qsizetype n) noexcept
    { Q_ASSERT(n >= 0); Q_ASSERT(n <= size()); m_size = n; }
    constexpr void chop(qsizetype n) noexcept
    { Q_ASSERT(n >= 0); Q_ASSERT(n <= size()); m_size -= n; }

    [[nodiscard]] QStringView trimmed() const noexcept { return QtPrivate::trimmed(*this); }

    template <typename Needle, typename...Flags>
    [[nodiscard]] constexpr inline auto tokenize(Needle &&needle, Flags...flags) const
        noexcept(noexcept(qTokenize(std::declval<const QStringView&>(), std::forward<Needle>(needle), flags...)))
            -> decltype(qTokenize(*this, std::forward<Needle>(needle), flags...))
    { return qTokenize(*this, std::forward<Needle>(needle), flags...); }

    [[nodiscard]] int compare(QStringView other, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept
    { return QtPrivate::compareStrings(*this, other, cs); }
    [[nodiscard]] inline int compare(QLatin1String other, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept;
    [[nodiscard]] constexpr int compare(QChar c) const noexcept
    { return size() >= 1 ? compare_single_char_helper(*utf16() - c.unicode()) : -1; }
    [[nodiscard]] int compare(QChar c, Qt::CaseSensitivity cs) const noexcept
    { return QtPrivate::compareStrings(*this, QStringView(&c, 1), cs); }

    [[nodiscard]] bool startsWith(QStringView s, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept
    { return QtPrivate::startsWith(*this, s, cs); }
    [[nodiscard]] inline bool startsWith(QLatin1String s, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept;
    [[nodiscard]] bool startsWith(QChar c) const noexcept
    { return !empty() && front() == c; }
    [[nodiscard]] bool startsWith(QChar c, Qt::CaseSensitivity cs) const noexcept
    { return QtPrivate::startsWith(*this, QStringView(&c, 1), cs); }

    [[nodiscard]] bool endsWith(QStringView s, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept
    { return QtPrivate::endsWith(*this, s, cs); }
    [[nodiscard]] inline bool endsWith(QLatin1String s, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept;
    [[nodiscard]] bool endsWith(QChar c) const noexcept
    { return !empty() && back() == c; }
    [[nodiscard]] bool endsWith(QChar c, Qt::CaseSensitivity cs) const noexcept
    { return QtPrivate::endsWith(*this, QStringView(&c, 1), cs); }

    [[nodiscard]] qsizetype indexOf(QChar c, qsizetype from = 0, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept
    { return QtPrivate::findString(*this, from, QStringView(&c, 1), cs); }
    [[nodiscard]] qsizetype indexOf(QStringView s, qsizetype from = 0, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept
    { return QtPrivate::findString(*this, from, s, cs); }
    [[nodiscard]] inline qsizetype indexOf(QLatin1String s, qsizetype from = 0, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept;

    [[nodiscard]] bool contains(QChar c, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept
    { return indexOf(QStringView(&c, 1), 0, cs) != qsizetype(-1); }
    [[nodiscard]] bool contains(QStringView s, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept
    { return indexOf(s, 0, cs) != qsizetype(-1); }
    [[nodiscard]] inline bool contains(QLatin1String s, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept;

    [[nodiscard]] qsizetype count(QChar c, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept
    { return QtPrivate::count(*this, c, cs); }
    [[nodiscard]] qsizetype count(QStringView s, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept
    { return QtPrivate::count(*this, s, cs); }

    [[nodiscard]] qsizetype lastIndexOf(QChar c, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept
    { return lastIndexOf(c, -1, cs); }
    [[nodiscard]] qsizetype lastIndexOf(QChar c, qsizetype from, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept
    { return QtPrivate::lastIndexOf(*this, from, QStringView(&c, 1), cs); }
    [[nodiscard]] qsizetype lastIndexOf(QStringView s, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept
    { return lastIndexOf(s, size(), cs); }
    [[nodiscard]] qsizetype lastIndexOf(QStringView s, qsizetype from, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept
    { return QtPrivate::lastIndexOf(*this, from, s, cs); }
    [[nodiscard]] inline qsizetype lastIndexOf(QLatin1String s, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept;
    [[nodiscard]] inline qsizetype lastIndexOf(QLatin1String s, qsizetype from, Qt::CaseSensitivity cs = Qt::CaseSensitive) const noexcept;

#if QT_CONFIG(regularexpression)
    [[nodiscard]] qsizetype indexOf(const QRegularExpression &re, qsizetype from = 0, QRegularExpressionMatch *rmatch = nullptr) const
    {
        return QtPrivate::indexOf(*this, re, from, rmatch);
    }
#ifdef Q_QDOC
    [[nodiscard]] qsizetype lastIndexOf(const QRegularExpression &re, QRegularExpressionMatch *rmatch = nullptr) const;
#else
    // prevent an ambiguity when called like this: lastIndexOf(re, 0)
    template <typename T = QRegularExpressionMatch, std::enable_if_t<std::is_same_v<T, QRegularExpressionMatch>, bool> = false>
    [[nodiscard]] qsizetype lastIndexOf(const QRegularExpression &re, T *rmatch = nullptr) const
    {
        return QtPrivate::lastIndexOf(*this, re, size(), rmatch);
    }
#endif
    [[nodiscard]] qsizetype lastIndexOf(const QRegularExpression &re, qsizetype from, QRegularExpressionMatch *rmatch = nullptr) const
    {
        return QtPrivate::lastIndexOf(*this, re, from, rmatch);
    }
    [[nodiscard]] bool contains(const QRegularExpression &re, QRegularExpressionMatch *rmatch = nullptr) const
    {
        return QtPrivate::contains(*this, re, rmatch);
    }
    [[nodiscard]] qsizetype count(const QRegularExpression &re) const
    {
        return QtPrivate::count(*this, re);
    }
#endif

    [[nodiscard]] bool isRightToLeft() const noexcept
    { return QtPrivate::isRightToLeft(*this); }
    [[nodiscard]] bool isValidUtf16() const noexcept
    { return QtPrivate::isValidUtf16(*this); }

    [[nodiscard]] inline short toShort(bool *ok = nullptr, int base = 10) const;
    [[nodiscard]] inline ushort toUShort(bool *ok = nullptr, int base = 10) const;
    [[nodiscard]] inline int toInt(bool *ok = nullptr, int base = 10) const;
    [[nodiscard]] inline uint toUInt(bool *ok = nullptr, int base = 10) const;
    [[nodiscard]] inline long toLong(bool *ok = nullptr, int base = 10) const;
    [[nodiscard]] inline ulong toULong(bool *ok = nullptr, int base = 10) const;
    [[nodiscard]] inline qlonglong toLongLong(bool *ok = nullptr, int base = 10) const;
    [[nodiscard]] inline qulonglong toULongLong(bool *ok = nullptr, int base = 10) const;
    [[nodiscard]] Q_CORE_EXPORT float toFloat(bool *ok = nullptr) const;
    [[nodiscard]] Q_CORE_EXPORT double toDouble(bool *ok = nullptr) const;

    [[nodiscard]] inline qsizetype toWCharArray(wchar_t *array) const; // defined in qstring.h


    [[nodiscard]] Q_CORE_EXPORT
    QList<QStringView> split(QStringView sep,
                             Qt::SplitBehavior behavior = Qt::KeepEmptyParts,
                             Qt::CaseSensitivity cs = Qt::CaseSensitive) const;
    [[nodiscard]] Q_CORE_EXPORT
    QList<QStringView> split(QChar sep, Qt::SplitBehavior behavior = Qt::KeepEmptyParts,
                             Qt::CaseSensitivity cs = Qt::CaseSensitive) const;

#if QT_CONFIG(regularexpression)
    [[nodiscard]] Q_CORE_EXPORT
    QList<QStringView> split(const QRegularExpression &sep,
                             Qt::SplitBehavior behavior = Qt::KeepEmptyParts) const;
#endif

    // QStringView <> QStringView
    friend bool operator==(QStringView lhs, QStringView rhs) noexcept { return lhs.size() == rhs.size() && QtPrivate::equalStrings(lhs, rhs); }
    friend bool operator!=(QStringView lhs, QStringView rhs) noexcept { return !(lhs == rhs); }
    friend bool operator< (QStringView lhs, QStringView rhs) noexcept { return QtPrivate::compareStrings(lhs, rhs) <  0; }
    friend bool operator<=(QStringView lhs, QStringView rhs) noexcept { return QtPrivate::compareStrings(lhs, rhs) <= 0; }
    friend bool operator> (QStringView lhs, QStringView rhs) noexcept { return QtPrivate::compareStrings(lhs, rhs) >  0; }
    friend bool operator>=(QStringView lhs, QStringView rhs) noexcept { return QtPrivate::compareStrings(lhs, rhs) >= 0; }

    // QStringView <> QChar
    friend bool operator==(QStringView lhs, QChar rhs) noexcept { return lhs == QStringView(&rhs, 1); }
    friend bool operator!=(QStringView lhs, QChar rhs) noexcept { return lhs != QStringView(&rhs, 1); }
    friend bool operator< (QStringView lhs, QChar rhs) noexcept { return lhs <  QStringView(&rhs, 1); }
    friend bool operator<=(QStringView lhs, QChar rhs) noexcept { return lhs <= QStringView(&rhs, 1); }
    friend bool operator> (QStringView lhs, QChar rhs) noexcept { return lhs >  QStringView(&rhs, 1); }
    friend bool operator>=(QStringView lhs, QChar rhs) noexcept { return lhs >= QStringView(&rhs, 1); }

    friend bool operator==(QChar lhs, QStringView rhs) noexcept { return QStringView(&lhs, 1) == rhs; }
    friend bool operator!=(QChar lhs, QStringView rhs) noexcept { return QStringView(&lhs, 1) != rhs; }
    friend bool operator< (QChar lhs, QStringView rhs) noexcept { return QStringView(&lhs, 1) <  rhs; }
    friend bool operator<=(QChar lhs, QStringView rhs) noexcept { return QStringView(&lhs, 1) <= rhs; }
    friend bool operator> (QChar lhs, QStringView rhs) noexcept { return QStringView(&lhs, 1) >  rhs; }
    friend bool operator>=(QChar lhs, QStringView rhs) noexcept { return QStringView(&lhs, 1) >= rhs; }

    //
    // STL compatibility API:
    //
    [[nodiscard]] const_iterator begin()   const noexcept { return data(); }
    [[nodiscard]] const_iterator end()     const noexcept { return data() + size(); }
    [[nodiscard]] const_iterator cbegin()  const noexcept { return begin(); }
    [[nodiscard]] const_iterator cend()    const noexcept { return end(); }
    [[nodiscard]] const_reverse_iterator rbegin()  const noexcept { return const_reverse_iterator(end()); }
    [[nodiscard]] const_reverse_iterator rend()    const noexcept { return const_reverse_iterator(begin()); }
    [[nodiscard]] const_reverse_iterator crbegin() const noexcept { return rbegin(); }
    [[nodiscard]] const_reverse_iterator crend()   const noexcept { return rend(); }

    [[nodiscard]] constexpr bool empty() const noexcept { return size() == 0; }
    [[nodiscard]] constexpr QChar front() const { return Q_ASSERT(!empty()), QChar(m_data[0]); }
    [[nodiscard]] constexpr QChar back()  const { return Q_ASSERT(!empty()), QChar(m_data[m_size - 1]); }

    //
    // Qt compatibility API:
    //
    [[nodiscard]] const_iterator constBegin() const noexcept { return begin(); }
    [[nodiscard]] const_iterator constEnd() const noexcept { return end(); }
    [[nodiscard]] constexpr bool isNull() const noexcept { return !m_data; }
    [[nodiscard]] constexpr bool isEmpty() const noexcept { return empty(); }
    [[nodiscard]] constexpr qsizetype length() const noexcept
    { return size(); }
    [[nodiscard]] constexpr QChar first() const { return front(); }
    [[nodiscard]] constexpr QChar last()  const { return back(); }
private:
    qsizetype m_size;
    const storage_type *m_data;

    constexpr int compare_single_char_helper(int diff) const noexcept
    { return diff ? diff : size() > 1 ? 1 : 0; }
};
Q_DECLARE_TYPEINFO(QStringView, Q_PRIMITIVE_TYPE);

template <typename QStringLike, typename std::enable_if<
    std::is_same<QStringLike, QString>::value,
    bool>::type = true>
inline QStringView qToStringViewIgnoringNull(const QStringLike &s) noexcept
{ return QStringView(s.data(), s.size()); }

// QChar inline functions:

[[nodiscard]] constexpr auto QChar::fromUcs4(char32_t c) noexcept
{
    struct R {
        char16_t chars[2];
        [[nodiscard]] constexpr operator QStringView() const noexcept { return {begin(), end()}; }
        [[nodiscard]] constexpr qsizetype size() const noexcept { return chars[1] ? 2 : 1; }
        [[nodiscard]] constexpr const char16_t *begin() const noexcept { return chars; }
        [[nodiscard]] constexpr const char16_t *end() const noexcept { return begin() + size(); }
    };
    return requiresSurrogates(c) ? R{{QChar::highSurrogate(c),
                                      QChar::lowSurrogate(c)}} :
                                   R{{char16_t(c), u'\0'}} ;
}

QT_END_NAMESPACE

#endif /* QSTRINGVIEW_H */
