st/oxr: Add oxr_chain.h header

This should replace all manual poking of the next chain.
Prefer the macros over the functions since they'll do your casting for you.

src/xrt/state_trackers/oxr/CMakeLists.txt

@@ -18,6 +18,7 @@ set(OXR_SOURCE_FILES
 	oxr_api_swapchain.c
 	oxr_api_system.c
 	oxr_api_verify.h
+	oxr_chain.h
 	oxr_event.cpp
 	oxr_handle_base.c
 	oxr_instance.c

src/xrt/state_trackers/oxr/oxr_chain.h

@@ -0,0 +1,94 @@
+// Copyright 2019, Collabora, Ltd.
+// SPDX-License-Identifier: BSL-1.0
+/*!
+ * @file
+ * @brief  Utilities for accessing members in an OpenXR structure chain.
+ * @author Ryan Pavlik <ryan.pavlik@collabora.com>
+ */
+
+#pragma once
+
+#include <xrt/xrt_openxr_includes.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*!
+ * @ingroup oxr_api
+ * @{
+ */
+
+/*!
+ * Finds an input struct of the given type in a next-chain.
+ *
+ * Returns NULL if nothing matching is found.
+ *
+ * Prefer using OXR_GET_INPUT_FROM_CHAIN() instead, since it includes the
+ * casting.
+ */
+static inline XrBaseInStructure const*
+oxr_find_input_in_chain(const void* ptr, XrStructureType desired)
+{
+	while (ptr != NULL) {
+		XrBaseInStructure const* base = (XrBaseInStructure const*)ptr;
+		if (base->type == desired) {
+			return base;
+		}
+		ptr = base->next;
+	}
+	return NULL;
+}
+
+/*!
+ * Finds an input struct of the given type in a next-chain and casts it as
+ * desired.
+ *
+ * Returns NULL if nothing matching is found.
+ *
+ * Note: There is no protection here to ensure that STRUCTURE_TYPE_ENUM (an
+ * XrStructureType value) and TYPE (a type name) actually match!
+ */
+#define OXR_GET_INPUT_FROM_CHAIN(PTR, STRUCTURE_TYPE_ENUM, TYPE)               \
+	((TYPE const*)oxr_find_input_in_chain(PTR, STRUCTURE_TYPE_ENUM))
+
+/*!
+ * Finds an output struct of the given type in a next-chain.
+ *
+ * Returns NULL if nothing matching is found.
+ *
+ * Prefer using OXR_GET_OUTPUT_FROM_CHAIN() instead, since it includes the
+ * casting.
+ */
+static inline XrBaseOutStructure*
+oxr_find_output_in_chain(void* ptr, XrStructureType desired)
+{
+	while (ptr != NULL) {
+		XrBaseOutStructure* base = (XrBaseOutStructure*)ptr;
+		if (base->type == desired) {
+			return base;
+		}
+		ptr = base->next;
+	}
+	return NULL;
+}
+
+/*!
+ * Finds an output struct of the given type in a next-chain and casts it as
+ * desired.
+ *
+ * Returns NULL if nothing matching is found.
+ *
+ * Note: There is no protection here to ensure that STRUCTURE_TYPE_ENUM (an
+ * XrStructureType value) and TYPE (a type name) actually match!
+ */
+#define OXR_GET_OUTPUT_FROM_CHAIN(PTR, STRUCTURE_TYPE_ENUM, TYPE)              \
+	((TYPE*)oxr_find_output_in_chain(PTR, STRUCTURE_TYPE_ENUM))
+
+/*!
+ * @}
+ */
+
+#ifdef __cplusplus
+}
+#endif