util: Add a index fifo

doc/changes/auxiliary/mr.359.1.md

@@ -0,0 +1,2 @@
+util: Add a very simple fifo for indices, this is used to keep track of
+swapchain in order of age (oldness).

src/xrt/auxiliary/util/u_index_fifo.h

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+// Copyright 2020, Collabora, Ltd.
+// SPDX-License-Identifier: BSL-1.0
+/*!
+ * @file
+ * @brief  A FIFO for indices.
+ * @author Jakob Bornecrantz <jakob@collabora.com>
+ * @ingroup aux_util
+ */
+
+#pragma once
+
+
+#define U_MAX_FIFO_INDICES 16
+
+struct u_index_fifo
+{
+	uint32_t indices[U_MAX_FIFO_INDICES];
+	size_t start;
+	size_t end;
+};
+
+static inline int
+u_index_fifo_is_empty(struct u_index_fifo *uif)
+{
+	if (uif->start == uif->end) {
+		return 1;
+	} else {
+		return 0;
+	}
+}
+
+static inline int
+u_index_fifo_is_full(struct u_index_fifo *uif)
+{
+	if (((uif->end + 1) % U_MAX_FIFO_INDICES) == uif->start) {
+		return 1;
+	} else {
+		return 0;
+	}
+}
+
+static inline int
+u_index_fifo_peek(struct u_index_fifo *uif, uint32_t *out_index)
+{
+	if (u_index_fifo_is_empty(uif)) {
+		return -1;
+	}
+
+	*out_index = uif->indices[uif->start];
+	return 0;
+}
+
+static inline int
+u_index_fifo_pop(struct u_index_fifo *uif, uint32_t *out_index)
+{
+	if (u_index_fifo_is_empty(uif)) {
+		return -1;
+	}
+
+	*out_index = uif->indices[uif->start];
+	uif->start = (uif->start + 1) % U_MAX_FIFO_INDICES;
+	return 0;
+}
+
+static inline int
+u_index_fifo_push(struct u_index_fifo *uif, uint32_t index)
+{
+	if (u_index_fifo_is_full(uif)) {
+		return -1;
+	}
+
+	uif->indices[uif->end] = index;
+	uif->end = (uif->end + 1) % U_MAX_FIFO_INDICES;
+
+	return 0;
+}