Imported Upstream version 0.13.2+dsfg1

plugins/mac-avcapture/left-right.hpp

@@ -0,0 +1,65 @@
+#pragma once
+
+#include <atomic>
+#include <mutex>
+#include <thread>
+
+#include <cinttypes>
+
+// left_right based on
+// https://github.com/pramalhe/ConcurrencyFreaks/blob/master/papers/left-right-2014.pdf
+// see concurrencyfreaks.com
+
+namespace left_right {
+
+template <typename T>
+struct left_right {
+	template <typename Func>
+	void update(Func &&f)
+	{
+		std::lock_guard<std::mutex> lock(write_mutex);
+		auto cur = current.load();
+		auto next = (cur + 1) & 1;
+
+		while (readers[next] != 0)
+			std::this_thread::yield();
+
+		f(data[next]);
+		current = next;
+
+		while (readers[cur] != 0)
+			std::this_thread::yield();
+
+		f(data[cur]);
+	}
+
+	T read()
+	{
+		auto cur = current.load();
+		for (;;) {
+			readers[cur] += 1;
+
+			auto cur_ = current.load();
+			if (cur_ == cur)
+				break;
+
+			readers[cur] -= 1;
+			cur = cur_;
+		}
+
+		auto val = data[cur];
+
+		readers[cur] -= 1;
+
+		return val;
+	}
+
+private:
+	std::atomic_uint_fast8_t current;
+	std::atomic_long         readers[2];
+	std::mutex               write_mutex;
+
+	T data[2] = {{}, {}};
+};
+
+}