AK: Add a prefix tree implementation

`AK::Trie` can be keyed by any given hashable type, and can store any
metadata (including nothing at all).
Also adds a test.

AK/Tests/CMakeLists.txt

@@ -33,6 +33,7 @@ set(AK_TEST_SOURCES
     TestString.cpp
     TestStringUtils.cpp
     TestStringView.cpp
+    TestTrie.cpp
     TestTypedTransfer.cpp
     TestURL.cpp
     TestUtf8.cpp

AK/Tests/TestTrie.cpp

@@ -0,0 +1,83 @@
+/*
+ * Copyright (c) 2020, the SerenityOS developers.
+ * 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/TestSuite.h>
+
+#include <AK/Trie.h>
+
+TEST_CASE(normal_behaviour)
+{
+    Trie<char, String> dictionary('/', "");
+    constexpr static const char* data[] { "test", "example", "foo", "foobar" };
+    constexpr static const size_t total_chars = 18; // root (1), 'test' (4), 'example' (7), 'foo' (3), 'foobar' (3, "foo" already stored).
+    for (auto& entry : data) {
+        StringView view { entry };
+        auto it = view.begin();
+        dictionary.insert(it, view.end(), view, [](auto& parent, auto& it) -> Optional<String> { return String::formatted("{}{}", parent.metadata_value(), *it); });
+    }
+
+    size_t i = 0;
+    for ([[maybe_unused]] auto& node : dictionary)
+        ++i;
+    EXPECT_EQ(i, total_chars);
+
+    for (auto& entry : data) {
+        StringView view { entry };
+        auto it = view.begin();
+        auto& node = dictionary.traverse_until_last_accessible_node(it, view.end());
+        EXPECT(it.is_end());
+        EXPECT(node.metadata().has_value());
+        EXPECT_EQ(view, node.metadata_value());
+    }
+
+    constexpr static const char* test_data_with_prefix_in_dict[] { "testx", "exampley", "fooa", "foobarb", "fox", "text" };
+    for (auto& entry : test_data_with_prefix_in_dict) {
+        StringView view { entry };
+        auto it = view.begin();
+        auto& node = dictionary.traverse_until_last_accessible_node(it, view.end());
+        EXPECT(!it.is_end());
+        EXPECT(node.metadata().has_value());
+        EXPECT(view.starts_with(node.metadata_value()));
+    }
+}
+
+TEST_CASE(iterate)
+{
+    Trie<int> bunch_of_numbers { 0 };
+    Array<int, 64> input;
+    for (size_t i = 0; i < input.size(); ++i)
+        input[i] = i;
+
+    // Iteration order is preorder (order between adjacent nodes is not defined, but parents come before children)
+    // in this case, the tree is linear.
+    size_t i = 0;
+    for (auto& node : bunch_of_numbers) {
+        EXPECT_EQ(input[i], node.value());
+        ++i;
+    }
+}
+
+TEST_MAIN(AllOf)

AK/Trie.h

@@ -0,0 +1,232 @@
+/*
+ * Copyright (c) 2020, the SerenityOS developers.
+ * 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/Forward.h>
+#include <AK/HashMap.h>
+#include <AK/Types.h>
+
+namespace AK {
+
+namespace Detail {
+
+template<typename TypeA, typename Default>
+struct SubstituteIfVoid {
+    using Type = TypeA;
+};
+
+template<typename Default>
+struct SubstituteIfVoid<void, Default> {
+    using Type = Default;
+};
+
+template<typename DeclaredBaseType, typename DefaultBaseType, typename ValueType, typename MetadataT, typename ValueTraits>
+class Trie {
+    using BaseType = typename SubstituteIfVoid<DeclaredBaseType, DefaultBaseType>::Type;
+
+    class ConstIterator {
+
+    public:
+        static ConstIterator end() { return {}; }
+
+        bool operator==(const ConstIterator& other) const { return m_current_node == other.m_current_node; }
+
+        const BaseType& operator*() const { return static_cast<const BaseType&>(*m_current_node); }
+        const BaseType* operator->() const { return static_cast<const BaseType*>(m_current_node); }
+        void operator++() { skip_to_next(); }
+
+        explicit ConstIterator(const Trie& node)
+        {
+            m_current_node = &node;
+            m_state.empend(false, node.m_children.begin(), node.m_children.end());
+        }
+
+    private:
+        void skip_to_next()
+        {
+            auto& current_state = m_state.last();
+            if (current_state.did_generate_root)
+                ++current_state.it;
+            else
+                current_state.did_generate_root = true;
+            if (current_state.it == current_state.end)
+                return pop_and_get_next();
+
+            m_current_node = &*(*current_state.it).value;
+            m_state.empend(false, m_current_node->m_children.begin(), m_current_node->m_children.end());
+        }
+        void pop_and_get_next()
+        {
+            m_state.take_last();
+            if (m_state.is_empty()) {
+                m_current_node = nullptr;
+                return;
+            }
+
+            skip_to_next();
+        }
+
+        ConstIterator() = default;
+
+        struct State {
+            bool did_generate_root { false };
+            typename HashMap<ValueType, NonnullOwnPtr<Trie>, ValueTraits>::ConstIteratorType it;
+            typename HashMap<ValueType, NonnullOwnPtr<Trie>, ValueTraits>::ConstIteratorType end;
+        };
+        Vector<State> m_state;
+        const Trie* m_current_node { nullptr };
+    };
+
+public:
+    using MetadataType = MetadataT;
+
+    Trie(ValueType value, Optional<MetadataType> metadata)
+        : m_value(move(value))
+        , m_metadata(move(metadata))
+    {
+    }
+
+    template<typename It>
+    BaseType& traverse_until_last_accessible_node(It& it, const It& end)
+    {
+        Trie* node = this;
+        for (; it < end; ++it) {
+            auto next_it = node->m_children.find(*it);
+            if (next_it == node->m_children.end())
+                return static_cast<BaseType&>(*node);
+            node = &*(*next_it).value;
+        }
+        return static_cast<BaseType&>(*node);
+    }
+
+    template<typename It>
+    const BaseType& traverse_until_last_accessible_node(It& it, const It& end) const { return const_cast<Trie*>(this)->traverse_until_last_accessible_node(it, end); }
+
+    template<typename It>
+    BaseType& traverse_until_last_accessible_node(const It& begin, const It& end)
+    {
+        auto it = begin;
+        return const_cast<Trie*>(this)->traverse_until_last_accessible_node(it, end);
+    }
+
+    template<typename It>
+    const BaseType& traverse_until_last_accessible_node(const It& begin, const It& end) const
+    {
+        auto it = begin;
+        return const_cast<Trie*>(this)->traverse_until_last_accessible_node(it, end);
+    }
+
+    Optional<MetadataType> metadata() const requires(!IsSame<MetadataType, decltype(nullptr)>::value) { return m_metadata; }
+    void set_metadata(MetadataType metadata) requires(!IsSame<MetadataType, decltype(nullptr)>::value) { m_metadata = move(metadata); }
+    const MetadataType& metadata_value() const requires(!IsSame<MetadataType, decltype(nullptr)>::value) { return m_metadata.value(); }
+
+    const ValueType& value() const { return m_value; }
+    ValueType& value() { return m_value; }
+
+    Trie& ensure_child(ValueType value, Optional<MetadataType> metadata = {})
+    {
+        auto it = m_children.find(value);
+        if (it == m_children.end()) {
+            auto node = make<Trie>(value, move(metadata));
+            auto& node_ref = *node;
+            m_children.set(move(value), move(node));
+            return static_cast<BaseType&>(node_ref);
+        }
+
+        auto& node_ref = *it->value;
+        if (metadata.has_value())
+            node_ref.m_metadata = move(metadata);
+        return static_cast<BaseType&>(node_ref);
+    }
+
+    template<typename It, typename ProvideMetadataFunction>
+    BaseType& insert(
+        It& it, const It& end, MetadataType metadata, ProvideMetadataFunction provide_missing_metadata) requires(!IsSame<MetadataType, decltype(nullptr)>::value)
+    {
+        Trie* last_root_node = &traverse_until_last_accessible_node(it, end);
+        for (; it != end; ++it)
+            last_root_node = static_cast<Trie*>(&last_root_node->ensure_child(*it, provide_missing_metadata(static_cast<BaseType&>(*last_root_node), it)));
+        last_root_node->set_metadata(move(metadata));
+        return static_cast<BaseType&>(*last_root_node);
+    }
+
+    template<typename It>
+    BaseType& insert(It& it, const It& end) requires(IsSame<MetadataType, decltype(nullptr)>::value)
+    {
+        Trie* last_root_node = &traverse_until_last_accessible_node(it, end);
+        for (; it != end; ++it)
+            last_root_node = static_cast<Trie*>(&last_root_node->ensure_child(*it, {}));
+        return static_cast<BaseType&>(*last_root_node);
+    }
+
+    ConstIterator begin() const { return ConstIterator(*this); }
+    ConstIterator end() const { return ConstIterator::end(); }
+
+    bool is_empty() const { return m_children.is_empty(); }
+    void clear() { m_children.clear(); }
+
+    BaseType deep_copy()
+    {
+        Trie root(m_value, m_metadata);
+        for (auto& it : m_children)
+            root.m_children.set(it.key, make<Trie>(it.value->deep_copy()));
+        return static_cast<BaseType&&>(move(root));
+    }
+
+private:
+    ValueType m_value;
+    Optional<MetadataType> m_metadata;
+    HashMap<ValueType, NonnullOwnPtr<Trie>, ValueTraits> m_children;
+};
+
+template<typename BaseType, typename DefaultBaseType, typename ValueType, typename ValueTraits>
+class Trie<BaseType, DefaultBaseType, ValueType, void, ValueTraits> : public Trie<BaseType, DefaultBaseType, ValueType, decltype(nullptr), ValueTraits> {
+    using Trie<BaseType, DefaultBaseType, ValueType, decltype(nullptr), ValueTraits>::Trie;
+};
+
+}
+
+template<typename ValueType, typename MetadataT = void, typename ValueTraits = Traits<ValueType>, typename BaseT = void>
+class Trie : public Detail::Trie<BaseT, Trie<ValueType, MetadataT, ValueTraits>, ValueType, MetadataT, ValueTraits> {
+public:
+    using DetailTrie = Detail::Trie<BaseT, Trie<ValueType, MetadataT, ValueTraits>, ValueType, MetadataT, ValueTraits>;
+    using MetadataType = typename DetailTrie::MetadataType;
+
+    Trie(ValueType value, MetadataType metadata) requires(!IsSame<MetadataType, void>::value && !IsSame<MetadataType, decltype(nullptr)>::value)
+        : DetailTrie(move(value), move(metadata))
+    {
+    }
+
+    explicit Trie(ValueType value)
+        : DetailTrie(move(value), Optional<MetadataType> {})
+    {
+    }
+};
+
+}
+
+using AK::Trie;