From f27cb146691781b916c8e13764a6d3dafde71c15 Mon Sep 17 00:00:00 2001
From: Jakob Bornecrantz <jakob@collabora.com>
Date: Mon, 18 Sep 2023 12:28:19 +0100
Subject: [PATCH] u/limited_unique_id: Add process unique id generator

---
 src/xrt/auxiliary/util/CMakeLists.txt         |  2 +
 .../auxiliary/util/u_limited_unique_id.cpp    | 26 ++++++++++++
 src/xrt/auxiliary/util/u_limited_unique_id.h  | 40 +++++++++++++++++++
 3 files changed, 68 insertions(+)
 create mode 100644 src/xrt/auxiliary/util/u_limited_unique_id.cpp
 create mode 100644 src/xrt/auxiliary/util/u_limited_unique_id.h

diff --git a/src/xrt/auxiliary/util/CMakeLists.txt b/src/xrt/auxiliary/util/CMakeLists.txt
index e541ea66c..426cd2300 100644
--- a/src/xrt/auxiliary/util/CMakeLists.txt
+++ b/src/xrt/auxiliary/util/CMakeLists.txt
@@ -59,6 +59,8 @@ add_library(
 	u_json.c
 	u_json.h
 	u_json.hpp
+	u_limited_unique_id.cpp
+	u_limited_unique_id.h
 	u_logging.c
 	u_logging.h
 	u_metrics.c
diff --git a/src/xrt/auxiliary/util/u_limited_unique_id.cpp b/src/xrt/auxiliary/util/u_limited_unique_id.cpp
new file mode 100644
index 000000000..53f242966
--- /dev/null
+++ b/src/xrt/auxiliary/util/u_limited_unique_id.cpp
@@ -0,0 +1,26 @@
+// Copyright 2023, Collabora, Ltd.
+// SPDX-License-Identifier: BSL-1.0
+/*!
+ * @file
+ * @brief  A very simple generator to create process unique ids.
+ * @author Jakob Bornecrantz <jakob@collabora.com>
+ * @ingroup aux_util
+ */
+
+#include "u_limited_unique_id.h"
+
+#include <atomic>
+
+
+/*
+ * This code is in C++ and not C because MSVC doesn't implement C atomics yet,
+ * but to make things even more fun the C++11 standard defines atomic_uint64_t
+ * as optional, but atomic_uint_fast64_t is listed as required, so here we are.
+ */
+static std::atomic_uint_fast64_t generator;
+
+extern "C" xrt_limited_unique_id_t
+u_limited_unique_id_get(void)
+{
+	return xrt_limited_unique_id_t{static_cast<uint64_t>(++generator)};
+}
diff --git a/src/xrt/auxiliary/util/u_limited_unique_id.h b/src/xrt/auxiliary/util/u_limited_unique_id.h
new file mode 100644
index 000000000..fca7c579c
--- /dev/null
+++ b/src/xrt/auxiliary/util/u_limited_unique_id.h
@@ -0,0 +1,40 @@
+// Copyright 2023, Collabora, Ltd.
+// SPDX-License-Identifier: BSL-1.0
+/*!
+ * @file
+ * @brief  A very simple generator to create process unique ids.
+ * @author Jakob Bornecrantz <jakob@collabora.com>
+ * @ingroup aux_util
+ */
+
+
+#pragma once
+
+#include "xrt/xrt_defines.h"
+
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
+/*!
+ * This function returns a unsigned 64 bit value that is guaranteed to be unique
+ * within the current running process, and not zero. There is of course the
+ * limit of running out of those ID once all values has been returned, but the
+ * value is 64 bit so that should not be a practical limit. The value is useful
+ * when needing to implement caching of a complex object, this lets us not use
+ * memory addresses as keys which may be reused by underlying alloc
+ * implementation and could lead to false hits.
+ *
+ * The current implementation is naive and is a simple monotonic counter.
+ *
+ * @ingroup aux_util
+ */
+xrt_limited_unique_id_t
+u_limited_unique_id_get(void);
+
+
+#ifdef __cplusplus
+}
+#endif