Utils: Add SmallString

SmallString is a very simple utf8 string class. It's purpose is performance.
It uses a short string opimization which provides 31 bytes of heap free
memory to save a 30 bytes long string plus null terminator. If the string
gets larger heap is allocated. The grow strategy is 1.5 to improve reuse
of allocated memory.

It uses optionally constexpr to provide string literals.

Change-Id: I7757fb51abfeca200d074cbfce2f1d99edc0ecb0
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/libs/utils/smallstringview.h

@@ -0,0 +1,140 @@
+/****************************************************************************
+**
+** Copyright (C) 2016 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** 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 General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** 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-3.0.html.
+**
+****************************************************************************/
+
+#pragma once
+
+#include "smallstringiterator.h"
+
+#include <QtGlobal>
+
+#include <cstring>
+
+#pragma push_macro("constexpr")
+#ifndef __cpp_constexpr
+#define constexpr
+#endif
+
+#pragma push_macro("noexcept")
+#ifndef __cpp_noexcept
+#define noexcept
+#endif
+
+namespace Utils {
+
+class SmallStringView
+{
+public:
+    using const_iterator = Internal::SmallStringIterator<std::random_access_iterator_tag, const char>;
+    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+    using size_type = std::size_t;
+
+    template<size_type Size>
+    constexpr
+    SmallStringView(const char(&string)[Size]) noexcept
+        : m_pointer(string),
+          m_size(Size - 1)
+    {
+        static_assert(Size >= 1, "Invalid string literal! Length is zero!");
+    }
+
+    template<typename Type,
+             typename = typename std::enable_if<std::is_pointer<Type>::value>::type
+             >
+    SmallStringView(Type characterPointer) noexcept
+        : m_pointer(characterPointer),
+          m_size(std::strlen(characterPointer))
+    {
+        static_assert(!std::is_array<Type>::value, "Input type is array and not char pointer!");
+    }
+
+    constexpr
+    SmallStringView(const char *const string, const size_type size) noexcept
+        : m_pointer(string),
+          m_size(size)
+    {
+    }
+
+    static
+    SmallStringView fromUtf8(const char *const characterPointer)
+    {
+        return SmallStringView(characterPointer, std::strlen(characterPointer));
+    }
+
+    constexpr
+    const char *data() const
+    {
+        return m_pointer;
+    }
+
+    constexpr
+    size_type size() const
+    {
+        return m_size;
+    }
+
+    const_iterator begin() const noexcept
+    {
+        return data();
+    }
+
+    const_iterator end() const noexcept
+    {
+        return data() + size();
+    }
+
+    const_reverse_iterator rbegin() const noexcept
+    {
+        return const_reverse_iterator(end() - 1l);
+    }
+
+    const_reverse_iterator rend() const noexcept
+    {
+        return const_reverse_iterator(begin() - 1l);
+    }
+
+private:
+    const char *m_pointer;
+    size_type m_size;
+};
+
+inline
+bool operator==(const SmallStringView& first, const SmallStringView& second) noexcept
+{
+    if (Q_LIKELY(first.size() != second.size()))
+        return false;
+
+    return !std::memcmp(first.data(), second.data(), first.size());
+}
+
+inline
+bool operator!=(const SmallStringView& first, const SmallStringView& second) noexcept
+{
+    return !(first == second);
+}
+
+} // namespace Utils
+
+#pragma pop_macro("noexcept")
+#pragma pop_macro("constexpr")