AK: Add FlyString, a simple flyweight string class

FlyString is a flyweight string class that wraps a RefPtr<StringImpl>
known to be unique among the set of FlyStrings. The class is very
unoptimized at the moment.

When to use FlyString:

- When you want O(1) string comparison
- When you want to deduplicate a lot of identical strings

When not to use FlyString:

- For strings that don't need either of the above features
- For strings that are likely to be unique

AK/FlyString.h

@@ -0,0 +1,64 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <AK/String.h>
+
+namespace AK {
+
+class FlyString {
+public:
+    FlyString() {}
+    FlyString(const String&);
+    FlyString(const StringView&);
+    FlyString(const char*);
+
+    bool operator==(const FlyString& other) const { return m_impl == other.m_impl; }
+    bool operator!=(const FlyString& other) const { return m_impl != other.m_impl; }
+
+    const StringImpl* impl() const { return m_impl; }
+    const char* characters() const { return m_impl ? m_impl->characters() : nullptr; }
+    size_t length() const { return m_impl ? m_impl->length() : 0; }
+
+    StringView view() const { return { characters(), length() }; }
+
+    int to_int(bool& ok) const;
+
+    static void did_destroy_impl(Badge<StringImpl>, StringImpl&);
+
+private:
+    RefPtr<StringImpl> m_impl;
+};
+
+template<>
+struct Traits<FlyString> : public GenericTraits<FlyString> {
+    static unsigned hash(const FlyString& s) { return s.impl() ? s.impl()->hash() : 0; }
+};
+
+}
+
+using AK::FlyString;