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.cpp

@@ -0,0 +1,91 @@
+/*
+ * 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.
+ */
+
+#include <AK/FlyString.h>
+#include <AK/HashTable.h>
+#include <AK/String.h>
+#include <AK/StringUtils.h>
+
+namespace AK {
+
+struct FlyStringImplTraits : public AK::Traits<StringImpl*> {
+    static unsigned hash(const StringImpl* s) { return s ? s->hash() : 0; }
+    static bool equals(const StringImpl* a, const StringImpl* b)
+    {
+        ASSERT(a);
+        ASSERT(b);
+        if (a == b)
+            return true;
+        if (a->length() != b->length())
+            return false;
+        return !__builtin_memcmp(a->characters(), b->characters(), a->length());
+    }
+};
+
+static HashTable<StringImpl*, FlyStringImplTraits>& fly_impls()
+{
+    static HashTable<StringImpl*, FlyStringImplTraits>* table;
+    if (!table)
+        table = new HashTable<StringImpl*, FlyStringImplTraits>;
+    return *table;
+}
+
+void FlyString::did_destroy_impl(Badge<StringImpl>, StringImpl& impl)
+{
+    fly_impls().remove(&impl);
+}
+
+FlyString::FlyString(const String& string)
+{
+    if (string.is_null())
+        return;
+    auto it = fly_impls().find(const_cast<StringImpl*>(string.impl()));
+    if (it == fly_impls().end()) {
+        fly_impls().set(const_cast<StringImpl*>(string.impl()));
+        string.impl()->set_fly({}, true);
+        m_impl = string.impl();
+    } else {
+        ASSERT((*it)->is_fly());
+        m_impl = *it;
+    }
+}
+
+FlyString::FlyString(const StringView& string)
+    : FlyString(static_cast<String>(string))
+{
+}
+
+FlyString::FlyString(const char* string)
+    : FlyString(static_cast<String>(string))
+{
+}
+
+int FlyString::to_int(bool& ok) const
+{
+    return StringUtils::convert_to_int(view(), ok);
+}
+
+}