Custom data structure for active objects to get performance *and* safety (#13880)

src/util/container.h

@@ -28,9 +28,11 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include <map>
 #include <set>
 #include <queue>
+#include <cassert>
+#include <limits>
 
 /*
-Queue with unique values with fast checking of value existence
+	Queue with unique values with fast checking of value existence
 */
 
 template<typename Value>
@@ -75,6 +77,10 @@ private:
 	std::queue<Value> m_queue;
 };
 
+/*
+	Thread-safe map
+*/
+
 template<typename Key, typename Value>
 class MutexedMap
 {
@@ -116,7 +122,9 @@ private:
 };
 
 
-// Thread-safe Double-ended queue
+/*
+	Thread-safe double-ended queue
+*/
 
 template<typename T>
 class MutexedQueue
@@ -237,6 +245,10 @@ protected:
 	Semaphore m_signal;
 };
 
+/*
+	LRU cache
+*/
+
 template<typename K, typename V>
 class LRUCache
 {
@@ -305,3 +317,206 @@ private:
 	// we can't use std::deque here, because its iterators get invalidated
 	std::list<K> m_queue;
 };
+
+/*
+	Map that can be safely modified (insertion, deletion) during iteration
+	Caveats:
+	- you cannot insert null elements
+	- you have to check for null elements during iteration, those are ones already deleted
+	- size() and empty() don't work during iteration
+	- not thread-safe in any way
+
+	How this is implemented:
+	- there are two maps: new and real
+	- if inserting duration iteration, the value is inserted into the "new" map
+	- if deleting during iteration, the value is set to null (to be GC'd later)
+	- when iteration finishes the "new" map is merged into the "real" map
+*/
+
+template<typename K, typename V>
+class ModifySafeMap
+{
+public:
+	// this allows bare pointers but also e.g. std::unique_ptr
+	static_assert(std::is_default_constructible<V>::value,
+		"Value type must be default constructible");
+	static_assert(std::is_constructible<bool, V>::value,
+		"Value type must be convertible to bool");
+	static_assert(std::is_move_assignable<V>::value,
+		"Value type must be move-assignable");
+
+	typedef K key_type;
+	typedef V mapped_type;
+
+	ModifySafeMap() {
+		// the null value must convert to false and all others to true, but
+		// we can't statically check the latter.
+		sanity_check(!null_value);
+	}
+	~ModifySafeMap() {
+		assert(!m_iterating);
+	}
+
+	// possible to implement but we don't need it
+	DISABLE_CLASS_COPY(ModifySafeMap)
+	ALLOW_CLASS_MOVE(ModifySafeMap)
+
+	const V &get(const K &key) const {
+		if (m_iterating) {
+			auto it = m_new.find(key);
+			if (it != m_new.end())
+				return it->second;
+		}
+		auto it = m_values.find(key);
+		return it == m_values.end() ? null_value : it->second;
+	}
+
+	void put(const K &key, const V &value) {
+		if (!value) {
+			assert(false);
+			return;
+		}
+		if (m_iterating)
+			m_new.emplace(key, value);
+		else
+			m_values.emplace(key, value);
+	}
+
+	void put(const K &key, V &&value) {
+		if (!value) {
+			assert(false);
+			return;
+		}
+		if (m_iterating)
+			m_new.emplace(key, std::move(value));
+		else
+			m_values.emplace(key, std::move(value));
+	}
+
+	V take(const K &key) {
+		V ret = V();
+		if (m_iterating) {
+			auto it = m_new.find(key);
+			if (it != m_new.end()) {
+				ret = std::move(it->second);
+				m_new.erase(it);
+			}
+		}
+		auto it = m_values.find(key);
+		if (it == m_values.end())
+			return ret;
+		ret = std::move(it->second);
+		if (m_iterating) {
+			it->second = V();
+			m_garbage++;
+		} else {
+			m_values.erase(it);
+		}
+		return ret;
+	}
+
+	bool remove(const K &key) {
+		return !!take(key);
+	}
+
+	// Warning: not constant-time!
+	size_t size() const {
+		if (m_iterating) {
+			// This is by no means impossible to determine, it's just annoying
+			// to code and we happen to not need this.
+			return unknown;
+		}
+		assert(m_new.empty());
+		if (m_garbage == 0)
+			return m_values.size();
+		size_t n = 0;
+		for (auto &it : m_values)
+			n += !it.second ? 0 : 1;
+		return n;
+	}
+
+	// Warning: not constant-time!
+	bool empty() const {
+		if (m_iterating)
+			return false; // maybe
+		if (m_garbage == 0)
+			return m_values.empty();
+		for (auto &it : m_values) {
+			if (!!it.second)
+				return false;
+		}
+		return true;
+	}
+
+	auto iter() { return IterationHelper(this); }
+
+	void clear() {
+		if (m_iterating) {
+			for (auto &it : m_values)
+				it.second = V();
+			m_garbage = m_values.size();
+		} else {
+			m_values.clear();
+			m_garbage = 0;
+		}
+	}
+
+	static inline const V null_value = V();
+
+	// returned by size() if called during iteration
+	static constexpr size_t unknown = static_cast<size_t>(-1);
+
+protected:
+	void merge_new() {
+		assert(!m_iterating);
+		if (!m_new.empty()) {
+			m_new.merge(m_values); // entries in m_new take precedence
+			m_values.clear();
+			std::swap(m_values, m_new);
+		}
+	}
+
+	void collect_garbage() {
+		assert(!m_iterating);
+		if (m_values.size() < GC_MIN_SIZE || m_garbage < m_values.size() / 2)
+			return;
+		for (auto it = m_values.begin(); it != m_values.end(); ) {
+			if (!it->second)
+				it = m_values.erase(it);
+			else
+				++it;
+		}
+		m_garbage = 0;
+	}
+
+	struct IterationHelper {
+		friend class ModifySafeMap<K, V>;
+		~IterationHelper() {
+			assert(m->m_iterating);
+			m->m_iterating--;
+			if (!m->m_iterating) {
+				m->merge_new();
+				m->collect_garbage();
+			}
+		}
+
+		auto begin() { return m->m_values.cbegin(); }
+		auto end() { return m->m_values.cend(); }
+
+	private:
+		IterationHelper(ModifySafeMap<K, V> *parent) : m(parent) {
+			assert(m->m_iterating < std::numeric_limits<decltype(m_iterating)>::max());
+			m->m_iterating++;
+		}
+
+		ModifySafeMap<K, V> *m;
+	};
+
+private:
+	std::map<K, V> m_values;
+	std::map<K, V> m_new;
+	unsigned int m_iterating = 0;
+	size_t m_garbage = 0; // upper bound for null-placeholders in m_values
+
+	static constexpr size_t GC_MIN_SIZE = 30;
+};