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include/util/reference_counter.hpp

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+#pragma once
+
+#include <cinttypes>
+#include <atomic>
+
+/*
+ * Code taken Daniel Anderson's Talk "When Lock-Free Still Isn't Enough" Slide 28:
+ * https://github.com/CppCon/CppCon2024/blob/main/Presentations/When_Lock-Free_Still_Isn't_Enough.pdf
+ */
+
+struct reference_counter
+{
+	using size_type = std::uint64_t;
+
+	static constexpr size_type bit_count = sizeof(size_type) * 8;
+	static constexpr size_type was_zero_flag = size_type{ 1 } << (bit_count - 1);
+	static constexpr size_type has_helped_flag = size_type{ 1 } << (bit_count - 2);
+
+	bool increment()
+	{
+		const auto prev_count = m_counter.fetch_add(1);
+		return (prev_count & was_zero_flag) == 0;
+	}
+
+	bool decrement()
+	{
+		if (m_counter.fetch_sub(1) == 1)
+		{
+			auto e = size_type{};
+			if (m_counter.compare_exchange_strong(e, was_zero_flag))
+			{
+				return true;
+			}
+
+			if ((e & has_helped_flag) and (m_counter.exchange(was_zero_flag) & has_helped_flag))
+			{
+				return true;
+			}
+		}
+		return false;
+	}
+
+	size_type count()
+	{
+		auto value = m_counter.load();
+		if (value == 0 and m_counter.compare_exchange_strong(value, was_zero_flag | has_helped_flag))
+		{
+			return 0;
+		}
+		return value & was_zero_flag ? 0 : value;
+	}
+
+private:
+	std::atomic<size_type> m_counter{ 1 };
+};