corelib/tools: Add QOffsetStringArray API

This function allows to generate an array of offsets of substrings in
a string at compile time. This produces less verbose and easier to maintain
source code.

Change-Id: I5774b8b91906f6191f2b1244a676c07e7eb15b47
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

2 files changed, 212 insertions, 0 deletions

diff --git a/src/corelib/tools/qoffsetstringarray_p.h b/src/corelib/tools/qoffsetstringarray_p.h
new file mode 100644
index 0000000000..a1117b1d47
--- /dev/null
+++ b/src/corelib/tools/qoffsetstringarray_p.h
@@ -0,0 +1,211 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://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.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#ifndef QOFFSETSTRINGARRAY_P_H
+#define QOFFSETSTRINGARRAY_P_H
+
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API.  It exists purely as an
+// implementation detail.  This header file may change from version to
+// version without notice, or even be removed.
+//
+// We mean it.
+//
+
+#include "private/qglobal_p.h"
+
+#include <tuple>
+#include <array>
+
+QT_BEGIN_NAMESPACE
+
+namespace QtPrivate {
+template<int N, int O, int I, int ... Idx>
+struct OffsetSequenceHelper : OffsetSequenceHelper<N - 1, O + I, Idx..., O> { };
+
+template<int O, int I, int ... Idx>
+struct OffsetSequenceHelper<0, O, I, Idx...> : IndexesList<O, Idx...>
+{
+    static const constexpr auto Length = O;
+};
+
+template<int I, int ... Idx>
+struct OffsetSequence : OffsetSequenceHelper<sizeof ... (Idx) + 1, 0, I, Idx..., 0>
+{
+    static const constexpr auto Count = sizeof ... (Idx) + 1;
+    using Type = typename QConditional<
+        Count <= std::numeric_limits<quint8>::max() + 1,
+        quint8,
+        typename QConditional<
+            Count <= std::numeric_limits<quint16>::max() + 1,
+            quint16,
+            int>::Type
+        >::Type;
+};
+
+template<int N>
+struct StaticString
+{
+    const char data[N];
+
+    constexpr StaticString(const char (&literal)[N]) noexcept
+        : StaticString(literal, makeIndexSequence<N> {})
+    { }
+
+    template<int ... Idx>
+    constexpr StaticString(const char (&literal)[N],
+                           IndexesList<Idx...>) noexcept
+        : data{literal[Idx]...}
+    { }
+
+    template<typename ... T>
+    constexpr StaticString(const T ... c) noexcept : data{c...} { }
+
+    constexpr char operator[](int i) const noexcept
+    {
+        return data[i];
+    }
+
+    template<int N2>
+    constexpr StaticString<N + N2> operator+(const StaticString<N2> &rs) const noexcept
+    {
+        return concatenate(rs, makeIndexSequence<N>{}, makeIndexSequence<N2>{});
+    }
+
+    template<int N2, int ... I1, int ... I2>
+    constexpr StaticString<N + N2> concatenate(const StaticString<N2> &rs,
+                                               IndexesList<I1...>,
+                                               IndexesList<I2...>) const noexcept
+    {
+        return StaticString<N + N2>(data[I1]..., rs[I2]...);
+    }
+
+    constexpr int size() const noexcept
+    {
+        return N;
+    }
+};
+
+template<>
+struct StaticString<0>
+{
+};
+
+template<int Sum>
+constexpr StaticString<0> staticString() noexcept
+{
+    return StaticString<0>{};
+}
+
+template<int Sum, int I, int ... Ix>
+constexpr StaticString<Sum> staticString(const char (&s)[I], const char (&...sx)[Ix]) noexcept
+{
+    return StaticString<I>(s) + staticString<Sum - I>(sx...);
+}
+} // namespace QtPrivate
+
+template<typename T, int SizeString, int SizeOffsets>
+class QOffsetStringArray
+{
+public:
+    using Type = T;
+
+    template<int ... Cx, int ... Ox>
+    constexpr QOffsetStringArray(const QtPrivate::StaticString<SizeString> &str,
+                                 QtPrivate::IndexesList<Cx...>,
+                                 QtPrivate::IndexesList<SizeString, Ox...>) noexcept
+        : m_string{str[Cx]...},
+          m_offsets{Ox...}
+    { }
+
+    constexpr inline const char *operator[](const int index) const noexcept
+    {
+        return m_string + m_offsets[qBound(int(0), index, SizeOffsets - 1)];
+    }
+
+    constexpr inline const char *at(const int index) const noexcept
+    {
+        return m_string + m_offsets[index];
+    }
+
+    constexpr inline const char *str() const { return m_string; }
+    constexpr inline const int *offsets() const { return m_offsets; }
+    constexpr inline int count() const { return SizeOffsets; };
+
+    static constexpr const auto sizeString = SizeString;
+    static constexpr const auto sizeOffsets = SizeOffsets;
+
+private:
+    const char m_string[SizeString];
+    const T m_offsets[SizeOffsets];
+};
+
+template<typename T, int N, int ... Ox>
+constexpr QOffsetStringArray<T, N, sizeof ... (Ox)> qOffsetStringArray(
+    const QtPrivate::StaticString<N> &string,
+    QtPrivate::IndexesList<N, Ox...> offsets) noexcept
+{
+    return QOffsetStringArray<T, N, sizeof ... (Ox)>(
+               string,
+               QtPrivate::makeIndexSequence<N> {},
+               offsets);
+}
+
+template<int ... Nx>
+struct QOffsetStringArrayRet
+{
+    using Offsets = QtPrivate::OffsetSequence<Nx...>;
+    using Type = QOffsetStringArray<typename Offsets::Type, Offsets::Length, sizeof ... (Nx)>;
+};
+
+template<int ... Nx>
+constexpr auto qOffsetStringArray(const char (&...strings)[Nx]) noexcept -> typename QOffsetStringArrayRet<Nx...>::Type
+{
+    using Offsets = QtPrivate::OffsetSequence<Nx...>;
+    return qOffsetStringArray<typename Offsets::Type>(
+            QtPrivate::staticString<Offsets::Length>(strings...), Offsets{});
+}
+
+
+QT_END_NAMESPACE
+
+#endif // QOFFSETSTRINGARRAY_P_H
diff --git a/src/corelib/tools/tools.pri b/src/corelib/tools/tools.pri
index 9ec6ea4894..6b180602c2 100644
--- a/src/corelib/tools/tools.pri
+++ b/src/corelib/tools/tools.pri
@@ -39,6 +39,7 @@ HEADERS +=  \
         tools/qmargins.h \
         tools/qmessageauthenticationcode.h \
         tools/qcontiguouscache.h \
+        tools/qoffsetstringarray_p.h \
         tools/qpair.h \
         tools/qpoint.h \
         tools/qqueue.h \